diff --git a/.changeset/new-oranges-knock.md b/.changeset/new-oranges-knock.md
new file mode 100644
index 000000000..3b5091d84
--- /dev/null
+++ b/.changeset/new-oranges-knock.md
@@ -0,0 +1,5 @@
+---
+'@lion/localize': minor
+---
+
+Add types for localize package. JSDocs types in the .js files, TSC generates type definition files. Mixins type definition files are hand-typed.
diff --git a/.changeset/pretty-mice-wonder.md b/.changeset/pretty-mice-wonder.md
new file mode 100644
index 000000000..33971dbc4
--- /dev/null
+++ b/.changeset/pretty-mice-wonder.md
@@ -0,0 +1,5 @@
+---
+'singleton-manager': patch
+---
+
+Added basic JSDocs types to SingletonManager, in order for localize to be able to be typed correctly.
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 22dfc3e20..63c12aa1c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -45,4 +45,4 @@ jobs:
           publish: yarn release
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}
+          NPM_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
diff --git a/packages/localize/src/LocalizeManager.js b/packages/localize/src/LocalizeManager.js
index 86610800c..faa9840a6 100644
--- a/packages/localize/src/LocalizeManager.js
+++ b/packages/localize/src/LocalizeManager.js
@@ -2,22 +2,36 @@ import MessageFormat from '@bundled-es-modules/message-format/MessageFormat.js';
 import { LionSingleton } from '@lion/core';
 import isLocalizeESModule from './isLocalizeESModule.js';
 
+/**
+ * @typedef {import('../types/LocalizeMixinTypes').NamespaceObject} NamespaceObject
+ */
+
 /**
  * `LocalizeManager` manages your translations (includes loading)
  */
 export class LocalizeManager extends LionSingleton {
   // eslint-disable-line no-unused-vars
-  constructor(params = {}) {
-    super(params);
-    this._fakeExtendsEventTarget();
+  constructor({ autoLoadOnLocaleChange = false, fallbackLocale = '' } = {}) {
+    super();
+    this.__delegationTarget = document.createDocumentFragment();
+    this._autoLoadOnLocaleChange = !!autoLoadOnLocaleChange;
+    this._fallbackLocale = fallbackLocale;
 
-    this._autoLoadOnLocaleChange = !!params.autoLoadOnLocaleChange;
-    this._fallbackLocale = params.fallbackLocale;
+    /** @type {Object.<string, Object.<string, Object>>} */
     this.__storage = {};
+
+    /** @type {Map.<RegExp|string, function>} */
     this.__namespacePatternsMap = new Map();
+
+    /** @type {Object.<string, function|null>} */
     this.__namespaceLoadersCache = {};
+
+    /** @type {Object.<string, Object.<string, Promise.<Object>>>} */
     this.__namespaceLoaderPromisesCache = {};
-    this.formatNumberOptions = { returnIfNaN: '' };
+
+    this.formatNumberOptions = {
+      returnIfNaN: '',
+    };
 
     /**
      * Via html[data-localize-lang], developers are allowed to set the initial locale, without
@@ -73,18 +87,24 @@ export class LocalizeManager extends LionSingleton {
     this._teardownHtmlLangAttributeObserver();
   }
 
-  // eslint-disable-next-line class-methods-use-this
+  /**
+   * @returns {string}
+   */
   get locale() {
     if (this._supportExternalTranslationTools) {
-      return this.__locale;
+      return this.__locale || '';
     }
     return document.documentElement.lang;
   }
 
+  /**
+   * @param {string} value
+   */
   set locale(value) {
+    /** @type {string} */
     let oldLocale;
     if (this._supportExternalTranslationTools) {
-      oldLocale = this.__locale;
+      oldLocale = /** @type {string} */ (this.__locale);
       this.__locale = value;
       if (this._langAttrSetByTranslationTool === null) {
         this._setHtmlLangAttribute(value);
@@ -101,12 +121,19 @@ export class LocalizeManager extends LionSingleton {
     this._onLocaleChanged(value, oldLocale);
   }
 
+  /**
+   * @param {string} locale
+   */
   _setHtmlLangAttribute(locale) {
     this._teardownHtmlLangAttributeObserver();
     document.documentElement.lang = locale;
     this._setupHtmlLangAttributeObserver();
   }
 
+  /**
+   * @param {string} value
+   * @throws {Error} Language only locales are not allowed(Use 'en-GB' instead of 'en')
+   */
   // eslint-disable-next-line class-methods-use-this
   __handleLanguageOnly(value) {
     throw new Error(`
@@ -116,6 +143,9 @@ export class LocalizeManager extends LionSingleton {
     `);
   }
 
+  /**
+   * @returns {Promise.<Object>}
+   */
   get loadingComplete() {
     return Promise.all(Object.values(this.__namespaceLoaderPromisesCache[this.locale]));
   }
@@ -127,6 +157,12 @@ export class LocalizeManager extends LionSingleton {
     this.__namespaceLoaderPromisesCache = {};
   }
 
+  /**
+   * @param {string} locale
+   * @param {string} namespace
+   * @param {object} data
+   * @throws {Error} Namespace can be added only once, for a given locale
+   */
   addData(locale, namespace, data) {
     if (this._isNamespaceInCache(locale, namespace)) {
       throw new Error(
@@ -138,18 +174,41 @@ export class LocalizeManager extends LionSingleton {
     this.__storage[locale][namespace] = data;
   }
 
+  /**
+   * @param {RegExp|string} pattern
+   * @param {function} loader
+   */
   setupNamespaceLoader(pattern, loader) {
     this.__namespacePatternsMap.set(pattern, loader);
   }
 
+  /**
+   * @param {NamespaceObject[]} namespaces
+   * @param {Object} [options]
+   * @param {string} [options.locale]
+   * @returns {Promise.<Object>}
+   */
   loadNamespaces(namespaces, { locale } = {}) {
-    return Promise.all(namespaces.map(namespace => this.loadNamespace(namespace, { locale })));
+    return Promise.all(
+      namespaces.map(
+        /** @param {NamespaceObject} namespace */
+        namespace => this.loadNamespace(namespace, { locale }),
+      ),
+    );
   }
 
+  /**
+   * @param {NamespaceObject} namespaceObj
+   * @param {Object} [options]
+   * @param {string} [options.locale]
+   * @returns {Promise.<Object|void>}
+   */
   loadNamespace(namespaceObj, { locale = this.locale } = { locale: this.locale }) {
     const isDynamicImport = typeof namespaceObj === 'object';
 
-    const namespace = isDynamicImport ? Object.keys(namespaceObj)[0] : namespaceObj;
+    const namespace = /** @type {string} */ (isDynamicImport
+      ? Object.keys(namespaceObj)[0]
+      : namespaceObj);
 
     if (this._isNamespaceInCache(locale, namespace)) {
       return Promise.resolve();
@@ -163,6 +222,13 @@ export class LocalizeManager extends LionSingleton {
     return this._loadNamespaceData(locale, namespaceObj, isDynamicImport, namespace);
   }
 
+  /**
+   * @param {string | string[]} keys
+   * @param {Object.<string,?>} [vars]
+   * @param {Object} [opts]
+   * @param {string} [opts.locale]
+   * @returns {string}
+   */
   msg(keys, vars, opts = {}) {
     const locale = opts.locale ? opts.locale : this.locale;
     const message = this._getMessageForKeys(keys, locale);
@@ -186,7 +252,7 @@ export class LocalizeManager extends LionSingleton {
               this._langAttrSetByTranslationTool = document.documentElement.lang;
             }
           } else {
-            this._onLocaleChanged(document.documentElement.lang, mutation.oldValue);
+            this._onLocaleChanged(document.documentElement.lang, mutation.oldValue || '');
           }
         });
       });
@@ -199,13 +265,23 @@ export class LocalizeManager extends LionSingleton {
   }
 
   _teardownHtmlLangAttributeObserver() {
-    this._htmlLangAttributeObserver.disconnect();
+    if (this._htmlLangAttributeObserver) {
+      this._htmlLangAttributeObserver.disconnect();
+    }
   }
 
+  /**
+   * @param {string} locale
+   * @param {string} namespace
+   */
   _isNamespaceInCache(locale, namespace) {
     return !!(this.__storage[locale] && this.__storage[locale][namespace]);
   }
 
+  /**
+   * @param {string} locale
+   * @param {string} namespace
+   */
   _getCachedNamespaceLoaderPromise(locale, namespace) {
     if (this.__namespaceLoaderPromisesCache[locale]) {
       return this.__namespaceLoaderPromisesCache[locale][namespace];
@@ -213,22 +289,41 @@ export class LocalizeManager extends LionSingleton {
     return null;
   }
 
+  /**
+   * @param {string} locale
+   * @param {NamespaceObject} namespaceObj
+   * @param {boolean} isDynamicImport
+   * @param {string} namespace
+   * @returns {Promise.<Object|void>}
+   */
   _loadNamespaceData(locale, namespaceObj, isDynamicImport, namespace) {
     const loader = this._getNamespaceLoader(namespaceObj, isDynamicImport, namespace);
     const loaderPromise = this._getNamespaceLoaderPromise(loader, locale, namespace);
     this._cacheNamespaceLoaderPromise(locale, namespace, loaderPromise);
-    return loaderPromise.then(obj => {
-      const data = isLocalizeESModule(obj) ? obj.default : obj;
-      this.addData(locale, namespace, data);
-    });
+    return loaderPromise.then(
+      /**
+       * @param {Object} obj
+       * @param {Object} obj.default
+       */
+      obj => {
+        const data = isLocalizeESModule(obj) ? obj.default : obj;
+        this.addData(locale, namespace, data);
+      },
+    );
   }
 
+  /**
+   * @param {NamespaceObject} namespaceObj
+   * @param {boolean} isDynamicImport
+   * @param {string} namespace
+   * @throws {Error} Namespace shall setup properly. Check loader!
+   */
   _getNamespaceLoader(namespaceObj, isDynamicImport, namespace) {
     let loader = this.__namespaceLoadersCache[namespace];
-
     if (!loader) {
       if (isDynamicImport) {
-        loader = namespaceObj[namespace];
+        const _namespaceObj = /** @type {Object.<string,function>} */ (namespaceObj);
+        loader = _namespaceObj[namespace];
         this.__namespaceLoadersCache[namespace] = loader;
       } else {
         loader = this._lookupNamespaceLoader(namespace);
@@ -245,12 +340,20 @@ export class LocalizeManager extends LionSingleton {
     return loader;
   }
 
+  /**
+   * @param {function} loader
+   * @param {string} locale
+   * @param {string} namespace
+   * @param {string} [fallbackLocale]
+   * @returns {Promise.<any>}
+   * @throws {Error} Data for namespace and (locale or fallback locale) could not be loaded.
+   */
   _getNamespaceLoaderPromise(loader, locale, namespace, fallbackLocale = this._fallbackLocale) {
     return loader(locale, namespace).catch(() => {
       const lang = this._getLangFromLocale(locale);
       return loader(lang, namespace).catch(() => {
         if (fallbackLocale) {
-          return this._getNamespaceLoaderPromise(loader, fallbackLocale, namespace, false).catch(
+          return this._getNamespaceLoaderPromise(loader, fallbackLocale, namespace, '').catch(
             () => {
               const fallbackLang = this._getLangFromLocale(fallbackLocale);
               throw new Error(
@@ -268,6 +371,11 @@ export class LocalizeManager extends LionSingleton {
     });
   }
 
+  /**
+   * @param {string} locale
+   * @param {string} namespace
+   * @param {Promise.<Object>} promise
+   */
   _cacheNamespaceLoaderPromise(locale, namespace, promise) {
     if (!this.__namespaceLoaderPromisesCache[locale]) {
       this.__namespaceLoaderPromisesCache[locale] = {};
@@ -275,6 +383,10 @@ export class LocalizeManager extends LionSingleton {
     this.__namespaceLoaderPromisesCache[locale][namespace] = promise;
   }
 
+  /**
+   * @param {string} namespace
+   * @returns {function|null}
+   */
   _lookupNamespaceLoader(namespace) {
     /* eslint-disable no-restricted-syntax */
     for (const [key, value] of this.__namespacePatternsMap) {
@@ -289,18 +401,45 @@ export class LocalizeManager extends LionSingleton {
     /* eslint-enable no-restricted-syntax */
   }
 
+  /**
+   * @param {string} locale
+   * @returns {string}
+   */
   // eslint-disable-next-line class-methods-use-this
   _getLangFromLocale(locale) {
     return locale.substring(0, 2);
   }
 
-  _fakeExtendsEventTarget() {
-    const delegate = document.createDocumentFragment();
-    ['addEventListener', 'dispatchEvent', 'removeEventListener'].forEach(funcName => {
-      this[funcName] = (...args) => delegate[funcName](...args);
-    });
+  /**
+   * @param {string} type
+   * @param {EventListener} listener
+   * @param {...Object} options
+   */
+  addEventListener(type, listener, ...options) {
+    this.__delegationTarget.addEventListener(type, listener, ...options);
   }
 
+  /**
+   * @param {string} type
+   * @param {EventListener} listener
+   * @param {...Object} options
+   */
+  removeEventListener(type, listener, ...options) {
+    this.__delegationTarget.removeEventListener(type, listener, ...options);
+  }
+
+  /**
+   *  @param {CustomEvent} event
+   */
+  dispatchEvent(event) {
+    this.__delegationTarget.dispatchEvent(event);
+  }
+
+  /**
+   * @param {string} newLocale
+   * @param {string} oldLocale
+   * @returns {undefined}
+   */
   _onLocaleChanged(newLocale, oldLocale) {
     if (newLocale === oldLocale) {
       return;
@@ -311,19 +450,34 @@ export class LocalizeManager extends LionSingleton {
     this.dispatchEvent(new CustomEvent('localeChanged', { detail: { newLocale, oldLocale } }));
   }
 
+  /**
+   * @param {string} newLocale
+   * @param {string} oldLocale
+   * @returns {Promise.<Object>}
+   */
   _loadAllMissing(newLocale, oldLocale) {
     const oldLocaleNamespaces = this.__storage[oldLocale] || {};
     const newLocaleNamespaces = this.__storage[newLocale] || {};
+    /** @type {Promise<Object|void>[]} */
     const promises = [];
     Object.keys(oldLocaleNamespaces).forEach(namespace => {
       const newNamespaceData = newLocaleNamespaces[namespace];
       if (!newNamespaceData) {
-        promises.push(this.loadNamespace(namespace, { locale: newLocale }));
+        promises.push(
+          this.loadNamespace(namespace, {
+            locale: newLocale,
+          }),
+        );
       }
     });
     return Promise.all(promises);
   }
 
+  /**
+   * @param {string | string[]} keys
+   * @param {string} locale
+   * @returns {string | undefined}
+   */
   _getMessageForKeys(keys, locale) {
     if (typeof keys === 'string') {
       return this._getMessageForKey(keys, locale);
@@ -341,16 +495,33 @@ export class LocalizeManager extends LionSingleton {
     return undefined;
   }
 
+  /**
+   * @param {string | undefined} key
+   * @param {string} locale
+   * @returns {string}
+   * @throws {Error} `key`is missing namespace. The format for `key` is "namespace:name"
+   *
+   */
   _getMessageForKey(key, locale) {
-    if (key.indexOf(':') === -1) {
+    if (!key || key.indexOf(':') === -1) {
       throw new Error(
         `Namespace is missing in the key "${key}". The format for keys is "namespace:name".`,
       );
     }
     const [ns, namesString] = key.split(':');
     const namespaces = this.__storage[locale];
-    const messages = namespaces ? namespaces[ns] : null;
+    const messages = namespaces ? namespaces[ns] : {};
     const names = namesString.split('.');
-    return names.reduce((message, n) => (message ? message[n] : null), messages);
+    const result = names.reduce(
+      /**
+       * @param {Object.<string, any> | string} message
+       * @param {string} name
+       * @returns {string}
+       */
+      (message, name) => (typeof message === 'object' ? message[name] : message),
+      messages,
+    );
+
+    return String(result || '');
   }
 }
diff --git a/packages/localize/src/LocalizeMixin.js b/packages/localize/src/LocalizeMixin.js
index 3abd1cbec..aa74f8be7 100644
--- a/packages/localize/src/LocalizeMixin.js
+++ b/packages/localize/src/LocalizeMixin.js
@@ -2,111 +2,154 @@ import { dedupeMixin, until, nothing } from '@lion/core';
 import { localize } from './localize.js';
 
 /**
- * # LocalizeMixin - for self managed templates
- *
- * @polymerMixin
- * @mixinFunction
+ * @typedef {import('../types/LocalizeMixinTypes').LocalizeMixin} LocalizeMixin
  */
-export const LocalizeMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line
-    class LocalizeMixin extends superclass {
-      static get localizeNamespaces() {
-        return [];
-      }
 
-      static get waitForLocalizeNamespaces() {
-        return true;
-      }
+/**
+ * # LocalizeMixin - for self managed templates
+ * @type {LocalizeMixin}
+ */
+const LocalizeMixinImplementation = superclass =>
+  // eslint-disable-next-line
+  class LocalizeMixin extends superclass {
+    /**
+     * @returns {Object.<string,function>[]}
+     */
+    static get localizeNamespaces() {
+      return [];
+    }
 
-      constructor() {
-        super();
+    /**
+     * @returns {boolean}
+     */
+    static get waitForLocalizeNamespaces() {
+      return true;
+    }
 
-        this.__boundLocalizeOnLocaleChanged = (...args) => this.__localizeOnLocaleChanged(...args);
+    constructor() {
+      super();
 
-        // should be loaded in advance
-        this.__localizeStartLoadingNamespaces();
+      this.__boundLocalizeOnLocaleChanged =
+        /** @param {...Object} args */
+        (...args) => {
+          const event = /** @type {CustomEvent} */ (Array.from(args)[0]);
+          this.__localizeOnLocaleChanged(event);
+        };
+
+      // should be loaded in advance
+      this.__localizeStartLoadingNamespaces();
+
+      if (this.localizeNamespacesLoaded) {
         this.localizeNamespacesLoaded.then(() => {
           this.__localizeMessageSync = true;
         });
       }
+    }
 
-      /**
-       * hook into LitElement to only render once all translations are loaded
-       */
-      async performUpdate() {
-        if (this.constructor.waitForLocalizeNamespaces) {
-          await this.localizeNamespacesLoaded;
-        }
-        super.performUpdate();
+    /**
+     * hook into LitElement to only render once all translations are loaded
+     * @returns {Promise.<void>}
+     */
+    async performUpdate() {
+      if (Object.getPrototypeOf(this).constructor.waitForLocalizeNamespaces) {
+        await this.localizeNamespacesLoaded;
+      }
+      super.performUpdate();
+    }
+
+    connectedCallback() {
+      if (super.connectedCallback) {
+        super.connectedCallback();
       }
 
-      connectedCallback() {
-        if (super.connectedCallback) {
-          super.connectedCallback();
-        }
-
+      if (this.localizeNamespacesLoaded) {
         this.localizeNamespacesLoaded.then(() => this.onLocaleReady());
-        this.__localizeAddLocaleChangedListener();
+      }
+      this.__localizeAddLocaleChangedListener();
+    }
+
+    disconnectedCallback() {
+      if (super.disconnectedCallback) {
+        super.disconnectedCallback();
       }
 
-      disconnectedCallback() {
-        if (super.disconnectedCallback) {
-          super.disconnectedCallback();
-        }
+      this.__localizeRemoveLocaleChangedListener();
+    }
 
-        this.__localizeRemoveLocaleChangedListener();
+    /**
+     * @param {string | string[]} keys
+     * @param {Object.<string,?>} variables
+     * @param {Object} [options]
+     * @param {string} [options.locale]
+     * @return {string | function}
+     */
+    msgLit(keys, variables, options) {
+      if (this.__localizeMessageSync) {
+        return localize.msg(keys, variables, options);
       }
 
-      msgLit(...args) {
-        if (this.__localizeMessageSync) {
-          return localize.msg(...args);
-        }
-        return until(
-          this.localizeNamespacesLoaded.then(() => localize.msg(...args)),
-          nothing,
-        );
+      if (!this.localizeNamespacesLoaded) {
+        return '';
       }
 
-      __getUniqueNamespaces() {
-        const uniqueNamespaces = [];
+      return until(
+        this.localizeNamespacesLoaded.then(() => localize.msg(keys, variables, options)),
+        nothing,
+      );
+    }
 
-        // IE11 does not support iterable in the constructor
-        const s = new Set();
-        this.constructor.localizeNamespaces.forEach(s.add.bind(s));
-        s.forEach(uniqueNamespace => {
-          uniqueNamespaces.push(uniqueNamespace);
-        });
-        return uniqueNamespaces;
-      }
+    /**
+     * @returns {string[]}
+     */
+    __getUniqueNamespaces() {
+      /** @type {string[]} */
+      const uniqueNamespaces = [];
 
-      __localizeStartLoadingNamespaces() {
-        this.localizeNamespacesLoaded = localize.loadNamespaces(this.__getUniqueNamespaces());
-      }
+      // IE11 does not support iterable in the constructor
+      const s = new Set();
+      Object.getPrototypeOf(this).constructor.localizeNamespaces.forEach(s.add.bind(s));
+      s.forEach(uniqueNamespace => {
+        uniqueNamespaces.push(uniqueNamespace);
+      });
+      return uniqueNamespaces;
+    }
 
-      __localizeAddLocaleChangedListener() {
-        localize.addEventListener('localeChanged', this.__boundLocalizeOnLocaleChanged);
-      }
+    __localizeStartLoadingNamespaces() {
+      this.localizeNamespacesLoaded = localize.loadNamespaces(this.__getUniqueNamespaces());
+    }
 
-      __localizeRemoveLocaleChangedListener() {
-        localize.removeEventListener('localeChanged', this.__boundLocalizeOnLocaleChanged);
-      }
+    __localizeAddLocaleChangedListener() {
+      localize.addEventListener('localeChanged', this.__boundLocalizeOnLocaleChanged);
+    }
 
-      __localizeOnLocaleChanged(event) {
-        this.onLocaleChanged(event.detail.newLocale, event.detail.oldLocale);
-      }
+    __localizeRemoveLocaleChangedListener() {
+      localize.removeEventListener('localeChanged', this.__boundLocalizeOnLocaleChanged);
+    }
 
-      onLocaleReady() {
-        this.onLocaleUpdated();
-      }
+    /**
+     * @param {CustomEvent} event
+     */
+    __localizeOnLocaleChanged(event) {
+      this.onLocaleChanged(event.detail.newLocale, event.detail.oldLocale);
+    }
 
-      onLocaleChanged() {
-        this.localizeNamespacesLoaded = localize.loadNamespaces(this.__getUniqueNamespaces());
-        this.onLocaleUpdated();
-        this.requestUpdate();
-      }
+    onLocaleReady() {
+      this.onLocaleUpdated();
+    }
 
-      // eslint-disable-next-line class-methods-use-this
-      onLocaleUpdated() {}
-    },
-);
+    /**
+     * @param {string} newLocale
+     * @param {string} oldLocale
+     */
+    // eslint-disable-next-line no-unused-vars
+    onLocaleChanged(newLocale, oldLocale) {
+      this.__localizeStartLoadingNamespaces();
+      this.onLocaleUpdated();
+      this.requestUpdate();
+    }
+
+    // eslint-disable-next-line class-methods-use-this
+    onLocaleUpdated() {}
+  };
+
+export const LocalizeMixin = dedupeMixin(LocalizeMixinImplementation);
diff --git a/packages/localize/src/date/addLeadingZero.js b/packages/localize/src/date/addLeadingZero.js
index 67aecbd7a..dc05496ee 100644
--- a/packages/localize/src/date/addLeadingZero.js
+++ b/packages/localize/src/date/addLeadingZero.js
@@ -4,8 +4,8 @@ import { pad } from './pad.js';
 /**
  * To add a leading zero to a single number
  *
- * @param dateString
- * @returns {*}
+ * @param {string} dateString
+ * @returns {string}
  */
 export function addLeadingZero(dateString) {
   const dateParts = splitDate(dateString);
diff --git a/packages/localize/src/date/clean.js b/packages/localize/src/date/clean.js
index fe817867a..a5e65ac7c 100644
--- a/packages/localize/src/date/clean.js
+++ b/packages/localize/src/date/clean.js
@@ -3,8 +3,8 @@ import { trim } from './trim.js';
 /**
  * To clean date from added characters from IE
  *
- * @param dateAsString
- * @returns {string|XML}
+ * @param {string} dateAsString
+ * @returns {string}
  */
 export function clean(dateAsString) {
   // list of separators is from wikipedia https://www.wikiwand.com/en/Date_format_by_country
diff --git a/packages/localize/src/date/formatDate.js b/packages/localize/src/date/formatDate.js
index a0239eeb8..ed191435c 100644
--- a/packages/localize/src/date/formatDate.js
+++ b/packages/localize/src/date/formatDate.js
@@ -4,14 +4,31 @@ import { normalizeIntlDate } from './normalizeIntlDate.js';
 /**
  * Formats date based on locale and options
  *
- * @param date
- * @param options
- * @returns {*}
+ * @param {Date} date
+ * @param {Object} [options] Intl options are available
+ * @param {string} [options.locale]
+ * @param {string} [options.localeMatcher]
+ * @param {string} [options.formatMatcher]
+ * @param {boolean}[options.hour12]
+ * @param {string} [options.numberingSystem]
+ * @param {string} [options.calendar]
+ * @param {string} [options.timeZone]
+ * @param {string} [options.timeZoneName]
+ * @param {string} [options.weekday]
+ * @param {string} [options.era]
+ * @param {string} [options.year]
+ * @param {string} [options.month]
+ * @param {string} [options.day]
+ * @param {string} [options.hour]
+ * @param {string} [options.minute]
+ * @param {string} [options.second]
+ * @returns {string}
  */
 export function formatDate(date, options) {
   if (!(date instanceof Date)) {
     return '';
   }
+  /** @type {options} */
   const formatOptions = options || {};
   /**
    * Set smart defaults if:
diff --git a/packages/localize/src/date/getDateFormatBasedOnLocale.js b/packages/localize/src/date/getDateFormatBasedOnLocale.js
index a4e44cdd1..6e216b53d 100644
--- a/packages/localize/src/date/getDateFormatBasedOnLocale.js
+++ b/packages/localize/src/date/getDateFormatBasedOnLocale.js
@@ -3,13 +3,28 @@ import { splitDate } from './splitDate.js';
 
 /**
  * To compute the localized date format
- *
  * @returns {string}
  */
 export function getDateFormatBasedOnLocale() {
+  /**
+   *
+   * @param {ArrayLike.<string>} dateParts
+   * @returns {string[]}
+   */
   function computePositions(dateParts) {
+    /**
+     * @param {number} index
+     * @returns {string}
+     */
     function getPartByIndex(index) {
-      return { 2012: 'year', 12: 'month', 20: 'day' }[dateParts[index]];
+      /** @type {Object.<string,string>} */
+      const template = {
+        '2012': 'year',
+        '12': 'month',
+        '20': 'day',
+      };
+      const key = dateParts[index];
+      return template[key];
     }
 
     return [1, 3, 5].map(getPartByIndex);
@@ -28,6 +43,8 @@ export function getDateFormatBasedOnLocale() {
   const dateParts = splitDate(formattedDate);
 
   const dateFormat = {};
-  dateFormat.positions = computePositions(dateParts);
+  if (dateParts) {
+    dateFormat.positions = computePositions(dateParts);
+  }
   return `${dateFormat.positions[0]}-${dateFormat.positions[1]}-${dateFormat.positions[2]}`;
 }
diff --git a/packages/localize/src/date/getMonthNames.js b/packages/localize/src/date/getMonthNames.js
index 66841bb24..716db16c7 100644
--- a/packages/localize/src/date/getMonthNames.js
+++ b/packages/localize/src/date/getMonthNames.js
@@ -1,12 +1,14 @@
 import { normalizeIntlDate } from './normalizeIntlDate.js';
 
+/** @type {Object.<string, Object.<string,string[]>>} */
 const monthsLocaleCache = {};
 
 /**
  * @desc Returns month names for locale
- * @param {string} options.locale locale
+ * @param {Object} [options]
+ * @param {string} [options.locale] locale
  * @param {string} [options.style=long] long, short or narrow
- * @returns {Array} like: ['January', 'February', ...etc].
+ * @returns {string[]} like: ['January', 'February', ...etc].
  */
 export function getMonthNames({ locale, style = 'long' } = {}) {
   let months = monthsLocaleCache[locale] && monthsLocaleCache[locale][style];
diff --git a/packages/localize/src/date/getWeekdayNames.js b/packages/localize/src/date/getWeekdayNames.js
index a48d5bcaf..d96f8dd14 100644
--- a/packages/localize/src/date/getWeekdayNames.js
+++ b/packages/localize/src/date/getWeekdayNames.js
@@ -1,24 +1,32 @@
 import { normalizeIntlDate } from './normalizeIntlDate.js';
 
+/** @type {Object.<string, Object.<string,string[]>>} */
 const weekdayNamesCache = {};
 
 /**
  * @desc Return cached weekday names for locale for all styles ('long', 'short', 'narrow')
  * @param {string} locale locale
- * @returns {Object} like { long: ['Sunday', 'Monday'...], short: ['Sun', ...], narrow: ['S', ...] }
+ * @returns {Object.<string,string[]>} - like { long: ['Sunday', 'Monday'...], short: ['Sun', ...], narrow: ['S', ...] }
  */
 function getCachedWeekdayNames(locale) {
-  let weekdays = weekdayNamesCache[locale];
+  const cachedWeekdayNames = weekdayNamesCache[locale];
+  let weekdays;
 
-  if (weekdays) {
-    return weekdays;
+  if (cachedWeekdayNames) {
+    return cachedWeekdayNames;
   }
 
-  weekdayNamesCache[locale] = { long: [], short: [], narrow: [] };
+  weekdayNamesCache[locale] = {
+    long: [],
+    short: [],
+    narrow: [],
+  };
 
   ['long', 'short', 'narrow'].forEach(style => {
     weekdays = weekdayNamesCache[locale][style];
-    const formatter = new Intl.DateTimeFormat(locale, { weekday: style });
+    const formatter = new Intl.DateTimeFormat(locale, {
+      weekday: style,
+    });
 
     const date = new Date('2019/04/07'); // start from Sunday
     for (let i = 0; i < 7; i += 1) {
@@ -34,10 +42,11 @@ function getCachedWeekdayNames(locale) {
 
 /**
  * @desc Returns weekday names for locale
- * @param {string} options.locale locale
+ * @param {Object} [options]
+ * @param {string} [options.locale] locale
  * @param {string} [options.style=long] long, short or narrow
  * @param {number} [options.firstDayOfWeek=0] 0 (Sunday), 1 (Monday), etc...
- * @returns {Array} like: ['Sunday', 'Monday', 'Tuesday', ...etc].
+ * @returns {string[]} like: ['Sunday', 'Monday', 'Tuesday', ...etc].
  */
 export function getWeekdayNames({ locale, style = 'long', firstDayOfWeek = 0 } = {}) {
   const weekdays = getCachedWeekdayNames(locale)[style];
diff --git a/packages/localize/src/date/normalizeDateTime.js b/packages/localize/src/date/normalizeDateTime.js
index 290b79df1..d5420dbb6 100644
--- a/packages/localize/src/date/normalizeDateTime.js
+++ b/packages/localize/src/date/normalizeDateTime.js
@@ -1,8 +1,8 @@
 /**
  * @desc Makes suitable for date comparisons
- * @param {Date} d
+ * @param {Date} date
  * @returns {Date}
  */
-export function normalizeDateTime(d) {
-  return new Date(d.getFullYear(), d.getMonth(), d.getDate());
+export function normalizeDateTime(date) {
+  return new Date(date.getFullYear(), date.getMonth(), date.getDate());
 }
diff --git a/packages/localize/src/date/normalizeIntlDate.js b/packages/localize/src/date/normalizeIntlDate.js
index c606f44f9..e3573724f 100644
--- a/packages/localize/src/date/normalizeIntlDate.js
+++ b/packages/localize/src/date/normalizeIntlDate.js
@@ -1,7 +1,7 @@
 /**
  * To filter out some added characters in IE
  *
- * @param str
+ * @param {string} str
  * @returns {string}
  */
 export function normalizeIntlDate(str) {
diff --git a/packages/localize/src/date/pad.js b/packages/localize/src/date/pad.js
index f618bedeb..54d1b5911 100644
--- a/packages/localize/src/date/pad.js
+++ b/packages/localize/src/date/pad.js
@@ -1,12 +1,11 @@
 /**
  * To get the absolute value of a number.
  *
- * @param n
+ * @param  {string} n - number in string format
  * @returns {string}
  */
 export function pad(n) {
   const digitRegex = /^\d+$/;
-  const v = digitRegex.test(n) ? Math.abs(n) : n;
-
+  const v = digitRegex.test(String(n)) ? Math.abs(Number(n)) : n;
   return String(v < 10 ? `0${v}` : v);
 }
diff --git a/packages/localize/src/date/parseDate.js b/packages/localize/src/date/parseDate.js
index 0ea60cda7..5aa6eefef 100644
--- a/packages/localize/src/date/parseDate.js
+++ b/packages/localize/src/date/parseDate.js
@@ -2,9 +2,14 @@ import { localize } from '../localize.js';
 import { getDateFormatBasedOnLocale } from './getDateFormatBasedOnLocale.js';
 import { addLeadingZero } from './addLeadingZero.js';
 
+/**
+ * @param {function} fn
+ */
 const memoize = fn => {
+  /** @type {Object.<any, any>} */
   const cache = {};
-  return parm => {
+
+  return /** @param {any} parm */ parm => {
     const n = parm;
     if (n in cache) {
       return cache[n];
@@ -20,11 +25,11 @@ const memoizedGetDateFormatBasedOnLocale = memoize(getDateFormatBasedOnLocale);
 /**
  * To parse a date into the right format
  *
- * @param date
- * @returns {Date}
+ * @param {string} dateString
+ * @returns {Date | undefined}
  */
-export function parseDate(date) {
-  const stringToParse = addLeadingZero(date);
+export function parseDate(dateString) {
+  const stringToParse = addLeadingZero(dateString);
   let parsedString;
   switch (memoizedGetDateFormatBasedOnLocale(localize.locale)) {
     case 'day-month-year':
@@ -51,7 +56,7 @@ export function parseDate(date) {
   const parsedDate = new Date(parsedString);
   // Check if parsedDate is not `Invalid Date`
   // eslint-disable-next-line no-restricted-globals
-  if (!isNaN(parsedDate)) {
+  if (!isNaN(parsedDate.getTime())) {
     return parsedDate;
   }
   return undefined;
diff --git a/packages/localize/src/date/sanitizedDateTimeFormat.js b/packages/localize/src/date/sanitizedDateTimeFormat.js
index 4c734e10a..c31a79b63 100644
--- a/packages/localize/src/date/sanitizedDateTimeFormat.js
+++ b/packages/localize/src/date/sanitizedDateTimeFormat.js
@@ -4,8 +4,8 @@ import { clean } from './clean.js';
 /**
  * To sanitize a date from IE11 handling
  *
- * @param date
- * @returns {string|XML}
+ * @param {Date} date
+ * @returns {string}
  */
 export function sanitizedDateTimeFormat(date) {
   const fDate = formatDate(date);
diff --git a/packages/localize/src/date/splitDate.js b/packages/localize/src/date/splitDate.js
index 6c09d9429..94ce81692 100644
--- a/packages/localize/src/date/splitDate.js
+++ b/packages/localize/src/date/splitDate.js
@@ -1,9 +1,9 @@
 /**
  * To split a date into days, months, years, etc
  *
- * @param date
- * @returns {Array|{index: number, input: string}|*}
+ * @param {string} dateAsString
+ * @returns {ArrayLike.<string> | null}
  */
-export function splitDate(date) {
-  return date.match(/(\d{1,4})([^\d]+)(\d{1,4})([^\d]+)(\d{1,4})/);
+export function splitDate(dateAsString) {
+  return dateAsString.match(/(\d{1,4})([^\d]+)(\d{1,4})([^\d]+)(\d{1,4})/);
 }
diff --git a/packages/localize/src/date/trim.js b/packages/localize/src/date/trim.js
index b6add67b2..e20304267 100644
--- a/packages/localize/src/date/trim.js
+++ b/packages/localize/src/date/trim.js
@@ -1,8 +1,8 @@
 /**
  * To trim the date
  *
- * @param dateAsString
- * @returns {string|XML}
+ * @param {string} dateAsString
+ * @returns {string}
  */
 export function trim(dateAsString) {
   return dateAsString.replace(/^[^\d]*/g, '').replace(/[^\d]*$/g, '');
diff --git a/packages/localize/src/isLocalizeESModule.js b/packages/localize/src/isLocalizeESModule.js
index e894f5aed..bbe631ea1 100644
--- a/packages/localize/src/isLocalizeESModule.js
+++ b/packages/localize/src/isLocalizeESModule.js
@@ -1,3 +1,7 @@
+/**
+ * @param {Object.<string, Object>} obj
+ * @returns {boolean}
+ */
 export default function isLocalizeESModule(obj) {
   return !!(obj && obj.default && typeof obj.default === 'object' && Object.keys(obj).length === 1);
 }
diff --git a/packages/localize/src/localize.js b/packages/localize/src/localize.js
index 6f0d4ab62..ead5dac76 100644
--- a/packages/localize/src/localize.js
+++ b/packages/localize/src/localize.js
@@ -1,6 +1,7 @@
 import { singletonManager } from 'singleton-manager';
 import { LocalizeManager } from './LocalizeManager.js';
 
+/** @type {LocalizeManager} */
 // eslint-disable-next-line import/no-mutable-exports
 export let localize =
   singletonManager.get('@lion/localize::localize::0.10.x') ||
@@ -9,6 +10,9 @@ export let localize =
     fallbackLocale: 'en-GB',
   });
 
+/**
+ * @param {LocalizeManager} newLocalize
+ */
 export function setLocalize(newLocalize) {
   localize.teardown();
   localize = newLocalize;
diff --git a/packages/localize/src/number/emptyStringWhenNumberNan.js b/packages/localize/src/number/emptyStringWhenNumberNan.js
index 427a72ef1..c077e89f4 100644
--- a/packages/localize/src/number/emptyStringWhenNumberNan.js
+++ b/packages/localize/src/number/emptyStringWhenNumberNan.js
@@ -4,9 +4,8 @@ import { localize } from '../localize.js';
  * When number is NaN we should return an empty string or returnIfNaN param
  *
  * @param {string} returnIfNaN
- * @returns {*}
+ * @returns {string}
  */
 export function emptyStringWhenNumberNan(returnIfNaN) {
-  const stringToReturn = returnIfNaN || localize.formatNumberOptions.returnIfNaN;
-  return stringToReturn;
+  return returnIfNaN || localize.formatNumberOptions.returnIfNaN;
 }
diff --git a/packages/localize/src/number/forceAddGroupSeparators.js b/packages/localize/src/number/forceAddGroupSeparators.js
index ae198871c..b3528d150 100644
--- a/packages/localize/src/number/forceAddGroupSeparators.js
+++ b/packages/localize/src/number/forceAddGroupSeparators.js
@@ -1,11 +1,13 @@
 /**
  * Add separators when they are not present
  *
- * @param {Array} formattedParts
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
  * @param {string} groupSeparator
- * @returns {Array}
+ * @returns {FormatNumberPart[]}
  */
 export function forceAddGroupSeparators(formattedParts, groupSeparator) {
+  /** @type {FormatNumberPart[]} */
   let concatArray = [];
   let firstPart;
   let integerPart;
@@ -29,24 +31,41 @@ export function forceAddGroupSeparators(formattedParts, groupSeparator) {
       numberPart += integerPart[0].value[i];
       // Create first grouping which is < 3
       if (numberPart.length === mod3 && firstGroup === false) {
-        numberArray.push({ type: 'integer', value: numberPart });
+        numberArray.push({
+          type: 'integer',
+          value: numberPart,
+        });
         if (numberOfDigits > 3) {
-          numberArray.push({ type: 'group', value: groupSeparator });
+          numberArray.push({
+            type: 'group',
+            value: groupSeparator,
+          });
         }
         numberPart = '';
         firstGroup = true;
         // Create groupings of 3
       } else if (numberPart.length === 3 && i < numberOfDigits - 1) {
         numberOfGroups += 1;
-        numberArray.push({ type: 'integer', value: numberPart });
+        numberArray.push({
+          type: 'integer',
+          value: numberPart,
+        });
         if (numberOfGroups !== groups) {
-          numberArray.push({ type: 'group', value: groupSeparator });
+          numberArray.push({
+            type: 'group',
+            value: groupSeparator,
+          });
         }
         numberPart = '';
       }
     }
-    numberArray.push({ type: 'integer', value: numberPart });
-    concatArray = firstPart.concat(numberArray, formattedParts);
+    numberArray.push({
+      type: 'integer',
+      value: numberPart,
+    });
+    if (firstPart) {
+      concatArray = firstPart.concat(numberArray, formattedParts);
+    }
   }
   return concatArray;
 }
diff --git a/packages/localize/src/number/forceCurrencyToEnd.js b/packages/localize/src/number/forceCurrencyToEnd.js
index 101cf9dcd..d4f02f5b4 100644
--- a/packages/localize/src/number/forceCurrencyToEnd.js
+++ b/packages/localize/src/number/forceCurrencyToEnd.js
@@ -1,8 +1,9 @@
 /**
  * For Dutch and Belgian amounts the currency should be at the end of the string
  *
- * @param {Array} formattedParts
- * @returns {Array}
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @returns {FormatNumberPart[]}
  */
 export function forceCurrencyToEnd(formattedParts) {
   if (formattedParts[0].type === 'currency') {
diff --git a/packages/localize/src/number/forceENAUSymbols.js b/packages/localize/src/number/forceENAUSymbols.js
index 14a583cef..4c9822492 100644
--- a/packages/localize/src/number/forceENAUSymbols.js
+++ b/packages/localize/src/number/forceENAUSymbols.js
@@ -1,19 +1,25 @@
-export function forceENAUSymbols(formattedParts, options) {
+/** @type {Object.<string,string>} */
+const CURRENCY_CODE_SYMBOL_MAP = {
+  EUR: '€',
+  USD: '$',
+  JPY: '¥',
+};
+
+/**
+ * Change the symbols for locale 'en-AU', due to bug in Chrome
+ *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @param {Object} [options]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @returns {FormatNumberPart[]}
+ */
+export function forceENAUSymbols(formattedParts, { currency, currencyDisplay } = {}) {
   const result = formattedParts;
-  const numberOfParts = result.length;
-  // Change the symbols for locale 'en-AU', due to bug in Chrome
-  if (numberOfParts > 1 && options && options.currencyDisplay === 'symbol') {
-    switch (options.currency) {
-      case 'EUR':
-        result[0].value = '€';
-        break;
-      case 'USD':
-        result[0].value = '$';
-        break;
-      case 'JPY':
-        result[0].value = '¥';
-        break;
-      /* no default */
+  if (formattedParts.length > 1 && currencyDisplay === 'symbol') {
+    if (Object.keys(CURRENCY_CODE_SYMBOL_MAP).includes(currency)) {
+      result[0].value = CURRENCY_CODE_SYMBOL_MAP[currency];
     }
     result[1].value = '';
   }
diff --git a/packages/localize/src/number/forceNormalSpaces.js b/packages/localize/src/number/forceNormalSpaces.js
index 444d824f7..b8d083f47 100644
--- a/packages/localize/src/number/forceNormalSpaces.js
+++ b/packages/localize/src/number/forceNormalSpaces.js
@@ -1,10 +1,14 @@
 import { normalSpaces } from './normalSpaces.js';
 
 /**
- * @param {Array} formattedParts
- * @return {Array} parts with forced "normal" spaces
+ * Parts with forced "normal" spaces
+ *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @returns {FormatNumberPart[]}
  */
 export function forceNormalSpaces(formattedParts) {
+  /** @type {FormatNumberPart[]} */
   const result = [];
   formattedParts.forEach(part => {
     result.push({
diff --git a/packages/localize/src/number/forceSpaceBetweenCurrencyCodeAndNumber.js b/packages/localize/src/number/forceSpaceBetweenCurrencyCodeAndNumber.js
index cbe9fbde6..0fa2a1271 100644
--- a/packages/localize/src/number/forceSpaceBetweenCurrencyCodeAndNumber.js
+++ b/packages/localize/src/number/forceSpaceBetweenCurrencyCodeAndNumber.js
@@ -1,14 +1,20 @@
 /**
  * When in some locales there is no space between currency and amount it is added
  *
- * @param {Array} formattedParts
- * @param {Object} options
- * @returns {*}
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @param {Object} [options]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @returns {FormatNumberPart[]}
  */
-export function forceSpaceBetweenCurrencyCodeAndNumber(formattedParts, options) {
+export function forceSpaceBetweenCurrencyCodeAndNumber(
+  formattedParts,
+  { currency, currencyDisplay } = {},
+) {
   const numberOfParts = formattedParts.length;
   const literalObject = { type: 'literal', value: ' ' };
-  if (numberOfParts > 1 && options && options.currency && options.currencyDisplay === 'code') {
+  if (numberOfParts > 1 && currency && currencyDisplay === 'code') {
     if (formattedParts[0].type === 'currency' && formattedParts[1].type !== 'literal') {
       // currency in front of a number: EUR 1.00
       formattedParts.splice(1, 0, literalObject);
diff --git a/packages/localize/src/number/forceSpaceInsteadOfZeroForGroup.js b/packages/localize/src/number/forceSpaceInsteadOfZeroForGroup.js
index e968d8958..18c2acc74 100644
--- a/packages/localize/src/number/forceSpaceInsteadOfZeroForGroup.js
+++ b/packages/localize/src/number/forceSpaceInsteadOfZeroForGroup.js
@@ -1,8 +1,10 @@
 /**
  * @desc Intl uses 0 as group separator for bg-BG locale.
  * This should be a ' '
- * @param {{type,value}[]} formattedParts
- * @returns {{type,value}[]} corrected formatted parts
+ *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @returns {FormatNumberPart[]} corrected formatted parts
  */
 export function forceSpaceInsteadOfZeroForGroup(formattedParts) {
   return formattedParts.map(p => {
diff --git a/packages/localize/src/number/forceTryCurrencyCode.js b/packages/localize/src/number/forceTryCurrencyCode.js
index a9aba4465..54a24875f 100644
--- a/packages/localize/src/number/forceTryCurrencyCode.js
+++ b/packages/localize/src/number/forceTryCurrencyCode.js
@@ -1,7 +1,15 @@
-export function forceTryCurrencyCode(formattedParts, options) {
+/**
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @param {Object} [options]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @returns {FormatNumberPart[]}
+ */
+export function forceTryCurrencyCode(formattedParts, { currency, currencyDisplay } = {}) {
   const result = formattedParts;
-  // Chage the currencycode from TRY to TL, for Turkey
-  if (options.currency === 'TRY' && options.currencyDisplay === 'code') {
+  // Change the currency code from TRY to TL, for Turkey
+  if (currency === 'TRY' && currencyDisplay === 'code') {
     if (result[0].value === 'TRY') {
       result[0].value = 'TL';
     }
diff --git a/packages/localize/src/number/forceYenSymbol.js b/packages/localize/src/number/forceYenSymbol.js
index ac01af76f..eee6d0eef 100644
--- a/packages/localize/src/number/forceYenSymbol.js
+++ b/packages/localize/src/number/forceYenSymbol.js
@@ -1,13 +1,16 @@
-export function forceYenSymbol(formattedParts, options) {
+/**
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @param {Object} [options]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @returns {FormatNumberPart[]}
+ */
+export function forceYenSymbol(formattedParts, { currency, currencyDisplay } = {}) {
   const result = formattedParts;
   const numberOfParts = result.length;
   // Change the symbol from JPY to ¥, due to bug in Chrome
-  if (
-    numberOfParts > 1 &&
-    options &&
-    options.currency === 'JPY' &&
-    options.currencyDisplay === 'symbol'
-  ) {
+  if (numberOfParts > 1 && currency === 'JPY' && currencyDisplay === 'symbol') {
     result[numberOfParts - 1].value = '¥';
   }
   return result;
diff --git a/packages/localize/src/number/formatNumber.js b/packages/localize/src/number/formatNumber.js
index 60f227155..1da70f2cf 100644
--- a/packages/localize/src/number/formatNumber.js
+++ b/packages/localize/src/number/formatNumber.js
@@ -4,25 +4,41 @@ import { formatNumberToParts } from './formatNumberToParts.js';
 /**
  * Formats a number based on locale and options. It uses Intl for the formatting.
  *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
  * @param {number} number Number to be formatted
- * @param {Object} options Intl options are available extended by roundMode
- * @returns {*} Formatted number
+ * @param {Object} [options] Intl options are available extended by roundMode and returnIfNaN
+ * @param {string} [options.roundMode]
+ * @param {string} [options.returnIfNaN]
+ * @param {string} [options.locale]
+ * @param {string} [options.localeMatcher]
+ * @param {string} [options.numberingSystem]
+ * @param {string} [options.style]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @param {boolean}[options.useGrouping]
+ * @param {number} [options.minimumIntegerDigits]
+ * @param {number} [options.minimumFractionDigits]
+ * @param {number} [options.maximumFractionDigits]
+ * @param {number} [options.minimumSignificantDigits]
+ * @param {number} [options.maximumSignificantDigits]
+ * @returns {string}
  */
-export function formatNumber(number, options) {
+export function formatNumber(number, options = {}) {
   if (number === undefined || number === null) return '';
   const formattedToParts = formatNumberToParts(number, options);
   // If number is not a number
   if (
-    formattedToParts === (options && options.returnIfNaN) ||
+    formattedToParts === options.returnIfNaN ||
     formattedToParts === localize.formatNumberOptions.returnIfNaN
   ) {
-    return formattedToParts;
+    return /** @type {string} */ (formattedToParts);
   }
   let printNumberOfParts = '';
   // update numberOfParts because there may be some parts added
   const numberOfParts = formattedToParts && formattedToParts.length;
   for (let i = 0; i < numberOfParts; i += 1) {
-    printNumberOfParts += formattedToParts[i].value;
+    const part = /** @type {FormatNumberPart} */ (formattedToParts[i]);
+    printNumberOfParts += part.value;
   }
   return printNumberOfParts;
 }
diff --git a/packages/localize/src/number/formatNumberToParts.js b/packages/localize/src/number/formatNumberToParts.js
index d109694b9..704e110e0 100644
--- a/packages/localize/src/number/formatNumberToParts.js
+++ b/packages/localize/src/number/formatNumberToParts.js
@@ -9,11 +9,26 @@ import { roundNumber } from './roundNumber.js';
 /**
  * Splits a number up in parts for integer, fraction, group, literal, decimal and currency.
  *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
  * @param {number} number Number to split up
- * @param {Object} options Intl options are available extended by roundMode
- * @returns {Array} Array with parts
+ * @param {Object} [options] Intl options are available extended by roundMode,returnIfNaN
+ * @param {string} [options.roundMode]
+ * @param {string} [options.returnIfNaN]
+ * @param {string} [options.locale]
+ * @param {string} [options.localeMatcher]
+ * @param {string} [options.numberingSystem]
+ * @param {string} [options.style]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
+ * @param {boolean}[options.useGrouping]
+ * @param {number} [options.minimumIntegerDigits]
+ * @param {number} [options.minimumFractionDigits]
+ * @param {number} [options.maximumFractionDigits]
+ * @param {number} [options.minimumSignificantDigits]
+ * @param {number} [options.maximumSignificantDigits]
+ * @returns {string | FormatNumberPart[]} Array with parts or (an empty string or returnIfNaN if not a number)
  */
-export function formatNumberToParts(number, options) {
+export function formatNumberToParts(number, options = {}) {
   let parsedNumber = typeof number === 'string' ? parseFloat(number) : number;
   const computedLocale = getLocale(options && options.locale);
   // when parsedNumber is not a number we should return an empty string or returnIfNaN
@@ -25,6 +40,7 @@ export function formatNumberToParts(number, options) {
     parsedNumber = roundNumber(number, options.roundMode);
   }
   let formattedParts = [];
+
   const formattedNumber = Intl.NumberFormat(computedLocale, options).format(parsedNumber);
   const regexCurrency = /[.,\s0-9]/;
   const regexMinusSign = /[-]/; // U+002D, Hyphen-Minus, &#45;
@@ -95,7 +111,7 @@ export function formatNumberToParts(number, options) {
       if (numberPart) {
         formattedParts.push({ type: 'fraction', value: numberPart });
       }
-      // If there are no fractions but we reached the end write the numberpart as integer
+      // If there are no fractions but we reached the end write the number part as integer
     } else if (i === formattedNumber.length - 1 && numberPart) {
       formattedParts.push({ type: 'integer', value: numberPart });
     }
diff --git a/packages/localize/src/number/getCurrencyName.js b/packages/localize/src/number/getCurrencyName.js
index b2cc8e3dc..45bc00df0 100644
--- a/packages/localize/src/number/getCurrencyName.js
+++ b/packages/localize/src/number/getCurrencyName.js
@@ -3,17 +3,18 @@ import { formatNumberToParts } from './formatNumberToParts.js';
 /**
  * Based on number, returns currency name like 'US dollar'
  *
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
  * @param {string} currencyIso iso code like USD
  * @param {Object} options Intl options are available extended by roundMode
  * @returns {string} currency name like 'US dollar'
  */
 export function getCurrencyName(currencyIso, options) {
-  const parts = formatNumberToParts(1, {
+  const parts = /** @type {FormatNumberPart[]} */ (formatNumberToParts(1, {
     ...options,
     style: 'currency',
     currency: currencyIso,
     currencyDisplay: 'name',
-  });
+  }));
   const currencyName = parts
     .filter(p => p.type === 'currency')
     .map(o => o.value)
diff --git a/packages/localize/src/number/getDecimalSeparator.js b/packages/localize/src/number/getDecimalSeparator.js
index 8f2e730a6..8cd0a3400 100644
--- a/packages/localize/src/number/getDecimalSeparator.js
+++ b/packages/localize/src/number/getDecimalSeparator.js
@@ -4,13 +4,13 @@ import { getLocale } from './getLocale.js';
  * To get the decimal separator
  *
  * @param {string} locale To override the browser locale
- * @returns {Object} the separator
+ * @returns {string} The separator
  */
 export function getDecimalSeparator(locale) {
   const computedLocale = getLocale(locale);
   const formattedNumber = Intl.NumberFormat(computedLocale, {
     style: 'decimal',
     minimumFractionDigits: 1,
-  }).format('1');
+  }).format(1);
   return formattedNumber[1];
 }
diff --git a/packages/localize/src/number/getFractionDigits.js b/packages/localize/src/number/getFractionDigits.js
index c2331b0c9..cd14c5ee0 100644
--- a/packages/localize/src/number/getFractionDigits.js
+++ b/packages/localize/src/number/getFractionDigits.js
@@ -4,14 +4,15 @@ import { formatNumberToParts } from './formatNumberToParts.js';
  * @example
  * getFractionDigits('JOD'); // return 3
  *
- * @param {string} currency Currency code e.g. EUR
- * @return {number} fraction for the given currency
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {string} [currency="EUR"] Currency code e.g. EUR
+ * @returns {number} fraction for the given currency
  */
 export function getFractionDigits(currency = 'EUR') {
-  const parts = formatNumberToParts(123, {
+  const parts = /** @type {FormatNumberPart[]} */ (formatNumberToParts(123, {
     style: 'currency',
     currency,
-  });
+  }));
   const [fractionPart] = parts.filter(part => part.type === 'fraction');
   return fractionPart ? fractionPart.value.length : 0;
 }
diff --git a/packages/localize/src/number/getGroupSeparator.js b/packages/localize/src/number/getGroupSeparator.js
index 987bea89e..5f97eca6d 100644
--- a/packages/localize/src/number/getGroupSeparator.js
+++ b/packages/localize/src/number/getGroupSeparator.js
@@ -2,16 +2,16 @@ import { getLocale } from './getLocale.js';
 import { normalSpaces } from './normalSpaces.js';
 
 /**
- * To get the group separator
+ * Gets the group separator
  *
- * @param {string} locale To override the browser locale
- * @returns {Object} the separator
+ * @param {string} [locale] To override the browser locale
+ * @returns {string}
  */
 export function getGroupSeparator(locale) {
   const computedLocale = getLocale(locale);
   const formattedNumber = Intl.NumberFormat(computedLocale, {
     style: 'decimal',
     minimumFractionDigits: 0,
-  }).format('10000');
+  }).format(10000);
   return normalSpaces(formattedNumber[2]);
 }
diff --git a/packages/localize/src/number/getLocale.js b/packages/localize/src/number/getLocale.js
index a8e96b988..0725d2f8d 100644
--- a/packages/localize/src/number/getLocale.js
+++ b/packages/localize/src/number/getLocale.js
@@ -3,7 +3,7 @@ import { localize } from '../localize.js';
 /**
  * Gets the locale to use
  *
- * @param {string} locale Locale to override browser locale
+ * @param {string} [locale] Locale to override browser locale
  * @returns {string}
  */
 export function getLocale(locale) {
diff --git a/packages/localize/src/number/normalSpaces.js b/packages/localize/src/number/normalSpaces.js
index 80e8eeef6..8a370f4de 100644
--- a/packages/localize/src/number/normalSpaces.js
+++ b/packages/localize/src/number/normalSpaces.js
@@ -1,6 +1,6 @@
 /**
- * @param {Array} value
- * @return {Array} value with forced "normal" space
+ * @param {string} value
+ * @returns {string} value with forced "normal" space
  */
 export function normalSpaces(value) {
   // If non-breaking space (160) or narrow non-breaking space (8239) then return ' '
diff --git a/packages/localize/src/number/normalizeCurrencyLabel.js b/packages/localize/src/number/normalizeCurrencyLabel.js
index 5054830fc..e8a2e8136 100644
--- a/packages/localize/src/number/normalizeCurrencyLabel.js
+++ b/packages/localize/src/number/normalizeCurrencyLabel.js
@@ -1,13 +1,10 @@
 /**
- * Function that fixes currency label with locale options
+ * For Turkey fixes currency label with locale options
  *
- * @param {String} currency
+ * @param {string} currency
  * @param {string} locale
- * @returns {string} currency
+ * @returns {string}
  */
 export function normalizeCurrencyLabel(currency, locale) {
-  if (currency === 'TRY' && locale === 'tr-TR') {
-    return 'TL';
-  }
-  return currency;
+  return currency === 'TRY' && locale === 'tr-TR' ? 'TL' : currency;
 }
diff --git a/packages/localize/src/number/normalizeIntl.js b/packages/localize/src/number/normalizeIntl.js
index b1859905b..0492be9bd 100644
--- a/packages/localize/src/number/normalizeIntl.js
+++ b/packages/localize/src/number/normalizeIntl.js
@@ -11,15 +11,19 @@ import { forceENAUSymbols } from './forceENAUSymbols.js';
 /**
  * Function with all fixes on localize
  *
- * @param {Array} formattedParts
- * @param {Object} options
+ * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+ * @param {FormatNumberPart[]} formattedParts
+ * @param {Object} [options]
+ * @param {string} [options.style]
+ * @param {string} [options.currency]
+ * @param {string} [options.currencyDisplay]
  * @param {string} _locale
- * @returns {*}
+ * @returns {FormatNumberPart[]}
  */
-export function normalizeIntl(formattedParts, options, _locale) {
-  let normalize = forceNormalSpaces(formattedParts, options);
+export function normalizeIntl(formattedParts, options = {}, _locale) {
+  let normalize = forceNormalSpaces(formattedParts);
   // Dutch and Belgian currency must be moved to end of number
-  if (options && options.style === 'currency') {
+  if (options.style === 'currency') {
     if (options.currencyDisplay === 'code' && _locale.slice(0, 2) === 'nl') {
       normalize = forceCurrencyToEnd(normalize);
     }
diff --git a/packages/localize/src/number/roundNumber.js b/packages/localize/src/number/roundNumber.js
index 154787a65..c7438e94a 100644
--- a/packages/localize/src/number/roundNumber.js
+++ b/packages/localize/src/number/roundNumber.js
@@ -3,7 +3,8 @@
  *
  * @param {number} number
  * @param {string} roundMode
- * @returns {*}
+ * @throws {Error} roundMode can only be round|floor|ceiling
+ * @returns {number}
  */
 export function roundNumber(number, roundMode) {
   switch (roundMode) {
diff --git a/packages/localize/test-helpers/fake-imports.js b/packages/localize/test-helpers/fake-imports.js
index ca1490464..18deac662 100644
--- a/packages/localize/test-helpers/fake-imports.js
+++ b/packages/localize/test-helpers/fake-imports.js
@@ -1,11 +1,22 @@
+/**
+ * @type {Object.<String, Object>}
+ */
 let fakeImports = {};
 
+/**
+ * @param {string} path
+ * @param {Object} data
+ */
 export function setupFakeImport(path, data) {
   const fakeExports = { ...data };
   Object.defineProperty(fakeExports, '__esModule', { value: true });
   fakeImports[path] = fakeExports;
 }
 
+/**
+ * @param {string[]} namespaces
+ * @param {string[]} locales
+ */
 export function setupEmptyFakeImportsFor(namespaces, locales) {
   namespaces.forEach(namespace => {
     locales.forEach(locale => {
@@ -20,6 +31,11 @@ export function resetFakeImport() {
   fakeImports = {};
 }
 
+/**
+ * @param {Object} result
+ * @param {Function} resolve
+ * @param {Function} reject
+ */
 function resolveOrReject(result, resolve, reject) {
   if (result) {
     resolve(result);
@@ -28,6 +44,11 @@ function resolveOrReject(result, resolve, reject) {
   }
 }
 
+/**
+ * @param {string} path
+ * @param {number} [ms=0]
+ * @returns {Promise.<Object>}
+ */
 export async function fakeImport(path, ms = 0) {
   const result = fakeImports[path];
   if (ms > 0) {
diff --git a/packages/localize/test/LocalizeManager.test.js b/packages/localize/test/LocalizeManager.test.js
index bb9c978b5..3b9b6543f 100644
--- a/packages/localize/test/LocalizeManager.test.js
+++ b/packages/localize/test/LocalizeManager.test.js
@@ -1,17 +1,20 @@
 import { expect, oneEvent, aTimeout } from '@open-wc/testing';
 import sinon from 'sinon';
-// eslint-disable-next-line import/no-unresolved
 import { fetchMock } from '@bundled-es-modules/fetch-mock';
 import { setupFakeImport, resetFakeImport, fakeImport } from '../test-helpers.js';
 
 import { LocalizeManager } from '../src/LocalizeManager.js';
 
-// useful for IE11 where LTR and RTL symbols are put by Intl when rendering dates
+/**
+ * @param {string} str
+ * Useful for IE11 where LTR and RTL symbols are put by Intl when rendering dates
+ */
 function removeLtrRtl(str) {
   return str.replace(/(\u200E|\u200E)/g, '');
 }
 
 describe('LocalizeManager', () => {
+  /** @type {LocalizeManager} */
   let manager;
 
   beforeEach(() => {
@@ -43,7 +46,10 @@ describe('LocalizeManager', () => {
 
   it('has teardown() method removing all side effects', () => {
     manager = new LocalizeManager();
-    const disconnectObserverSpy = sinon.spy(manager._htmlLangAttributeObserver, 'disconnect');
+    const disconnectObserverSpy = sinon.spy(
+      manager._htmlLangAttributeObserver,
+      /** @type {never} */ ('disconnect'),
+    );
     manager.teardown();
     expect(disconnectObserverSpy.callCount).to.equal(1);
   });
@@ -54,7 +60,11 @@ describe('LocalizeManager', () => {
       setTimeout(() => {
         manager.locale = 'en-US';
       });
-      const event = await oneEvent(manager, 'localeChanged');
+
+      const event = await oneEvent(
+        /** @type {EventTarget} */ (/** @type {unknown} */ (manager)),
+        'localeChanged',
+      );
       expect(event.detail.newLocale).to.equal('en-US');
       expect(event.detail.oldLocale).to.equal('en-GB');
     });
@@ -62,6 +72,7 @@ describe('LocalizeManager', () => {
     it('does not fire "localeChanged" event if it was set to the same locale', () => {
       manager = new LocalizeManager();
       const eventSpy = sinon.spy();
+
       manager.addEventListener('localeChanged', eventSpy);
       manager.locale = 'en-US';
       manager.locale = 'en-US';
@@ -127,6 +138,7 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager();
 
       await manager.loadNamespace({
+        /** @param {string} locale */
         'my-component': locale => fakeImport(`./my-component/${locale}.js`),
       });
 
@@ -145,6 +157,7 @@ describe('LocalizeManager', () => {
 
       await manager.loadNamespace(
         {
+          /** @param {string} locale */
           'my-component': locale => fakeImport(`./my-component/${locale}.js`),
         },
         { locale: 'nl-NL' },
@@ -164,8 +177,14 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager();
 
       await manager.loadNamespaces([
-        { 'my-defaults': locale => fakeImport(`./my-defaults/${locale}.js`) },
-        { 'my-send-button': locale => fakeImport(`./my-send-button/${locale}.js`) },
+        {
+          /** @param {string} locale */
+          'my-defaults': locale => fakeImport(`./my-defaults/${locale}.js`),
+        },
+        {
+          /** @param {string} locale */
+          'my-send-button': locale => fakeImport(`./my-send-button/${locale}.js`),
+        },
       ]);
 
       expect(manager.__storage).to.deep.equal({
@@ -185,8 +204,14 @@ describe('LocalizeManager', () => {
 
       await manager.loadNamespaces(
         [
-          { 'my-defaults': locale => fakeImport(`./my-defaults/${locale}.js`) },
-          { 'my-send-button': locale => fakeImport(`./my-send-button/${locale}.js`) },
+          {
+            /** @param {string} locale */
+            'my-defaults': locale => fakeImport(`./my-defaults/${locale}.js`),
+          },
+          {
+            /** @param {string} locale */
+            'my-send-button': locale => fakeImport(`./my-send-button/${locale}.js`),
+          },
         ],
         { locale: 'nl-NL' },
       );
@@ -205,6 +230,7 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager();
 
       await manager.loadNamespace({
+        /** @param {string} locale */
         'my-component': locale => fakeImport(`./my-component/${locale}.js`),
       });
 
@@ -220,6 +246,7 @@ describe('LocalizeManager', () => {
 
       try {
         await manager.loadNamespace({
+          /** @param {string} locale */
           'my-component': locale => fakeImport(`./my-component/${locale}.js`),
         });
       } catch (e) {
@@ -242,6 +269,7 @@ describe('LocalizeManager', () => {
         setupFakeImport('./my-component/en-GB.js', { default: { greeting: 'Hello!' } });
 
         await manager.loadNamespace({
+          /** @param {string} locale */
           'my-component': locale => fakeImport(`./my-component/${locale}.js`),
         });
 
@@ -259,6 +287,7 @@ describe('LocalizeManager', () => {
         setupFakeImport('./my-component/en.js', { default: { greeting: 'Hello!' } });
 
         await manager.loadNamespace({
+          /** @param {string} locale */
           'my-component': locale => fakeImport(`./my-component/${locale}.js`),
         });
 
@@ -275,6 +304,7 @@ describe('LocalizeManager', () => {
 
         try {
           await manager.loadNamespace({
+            /** @param {string} locale */
             'my-component': locale => fakeImport(`./my-component/${locale}.js`),
           });
         } catch (e) {
@@ -315,10 +345,14 @@ describe('LocalizeManager', () => {
 
       manager = new LocalizeManager();
 
-      manager.setupNamespaceLoader('my-component', async locale => {
-        const response = await fetch(`./my-component/${locale}.json`);
-        return response.json();
-      });
+      manager.setupNamespaceLoader(
+        'my-component',
+        /** @param {string} locale */
+        async locale => {
+          const response = await fetch(`./my-component/${locale}.json`);
+          return response.json();
+        },
+      );
 
       await manager.loadNamespace('my-component');
 
@@ -330,26 +364,43 @@ describe('LocalizeManager', () => {
     });
 
     it('loads multiple namespaces via loadNamespaces() using string routes', async () => {
-      fetchMock.get('./my-defaults/en-GB.json', { submit: 'Submit' });
-      fetchMock.get('./my-send-button/en-GB.json', { submit: 'Send' });
+      fetchMock.get('./my-defaults/en-GB.json', {
+        submit: 'Submit',
+      });
+      fetchMock.get('./my-send-button/en-GB.json', {
+        submit: 'Send',
+      });
 
       manager = new LocalizeManager();
 
-      manager.setupNamespaceLoader('my-defaults', async locale => {
-        const response = await fetch(`./my-defaults/${locale}.json`);
-        return response.json();
-      });
-      manager.setupNamespaceLoader('my-send-button', async locale => {
-        const response = await fetch(`./my-send-button/${locale}.json`);
-        return response.json();
-      });
+      manager.setupNamespaceLoader(
+        'my-defaults',
+        /** @param {string} locale */
+        async locale => {
+          const response = await fetch(`./my-defaults/${locale}.json`);
+          return response.json();
+        },
+      );
+
+      manager.setupNamespaceLoader(
+        'my-send-button',
+        /** @param {string} locale */
+        async locale => {
+          const response = await fetch(`./my-send-button/${locale}.json`);
+          return response.json();
+        },
+      );
 
       await manager.loadNamespaces(['my-defaults', 'my-send-button']);
 
       expect(manager.__storage).to.deep.equal({
         'en-GB': {
-          'my-send-button': { submit: 'Send' },
-          'my-defaults': { submit: 'Submit' },
+          'my-send-button': {
+            submit: 'Send',
+          },
+          'my-defaults': {
+            submit: 'Submit',
+          },
         },
       });
     });
@@ -359,10 +410,17 @@ describe('LocalizeManager', () => {
 
       manager = new LocalizeManager();
 
-      manager.setupNamespaceLoader(/my-.+/, async (locale, namespace) => {
-        const response = await fetch(`./${namespace}/${locale}.json`);
-        return response.json();
-      });
+      manager.setupNamespaceLoader(
+        /my-.+/,
+        /**
+         * @param {string} locale
+         * @param {string} namespace
+         */
+        async (locale, namespace) => {
+          const response = await fetch(`./${namespace}/${locale}.json`);
+          return response.json();
+        },
+      );
 
       await manager.loadNamespace('my-component');
 
@@ -379,10 +437,17 @@ describe('LocalizeManager', () => {
 
       manager = new LocalizeManager();
 
-      manager.setupNamespaceLoader(/my-.+/, async (locale, namespace) => {
-        const response = await fetch(`./${namespace}/${locale}.json`);
-        return response.json();
-      });
+      manager.setupNamespaceLoader(
+        /my-.+/,
+        /**
+         * @param {string} locale
+         * @param {string} namespace
+         */
+        async (locale, namespace) => {
+          const response = await fetch(`./${namespace}/${locale}.json`);
+          return response.json();
+        },
+      );
 
       await manager.loadNamespaces(['my-defaults', 'my-send-button']);
 
@@ -403,6 +468,7 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager({ autoLoadOnLocaleChange: true });
 
       await manager.loadNamespace({
+        /** @param {string} locale */
         'my-component': locale => fakeImport(`./my-component/${locale}.js`, 25),
       });
 
@@ -426,6 +492,7 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager();
 
       manager.loadNamespace({
+        /** @param {string} locale */
         'my-component': locale => fakeImport(`./my-component/${locale}.js`, 25),
       });
       expect(manager.__storage).to.deep.equal({});
@@ -468,6 +535,7 @@ describe('LocalizeManager', () => {
 
       let called = 0;
       await manager.loadNamespace({
+        /** @param {string} locale */
         'my-component': locale => {
           called += 1;
           return fakeImport(`./my-component/${locale}.js`);
@@ -527,7 +595,7 @@ describe('LocalizeManager', () => {
       manager = new LocalizeManager();
       manager.addData('en-GB', 'my-ns', { greeting: 'Hi!' });
       manager.addData('nl-NL', 'my-ns', { greeting: 'Hey!' });
-      expect(manager.msg('my-ns:greeting', null, { locale: 'nl-NL' })).to.equal('Hey!');
+      expect(manager.msg('my-ns:greeting', undefined, { locale: 'nl-NL' })).to.equal('Hey!');
       manager.reset();
       manager.addData('en-GB', 'my-ns', { greeting: 'Hi {name}!' });
       manager.addData('nl-NL', 'my-ns', { greeting: 'Hey {name}!' });
@@ -561,6 +629,7 @@ describe('When supporting external translation tools like Google Translate', ()
   let manager;
   const originalLang = document.documentElement.lang;
 
+  /** @param {string} lang */
   async function simulateGoogleTranslateOn(lang) {
     document.documentElement.lang = lang;
   }
@@ -569,6 +638,10 @@ describe('When supporting external translation tools like Google Translate', ()
     document.documentElement.lang = 'auto';
   }
 
+  /**
+   *  @param {...*} [cfg]
+   *  @returns {LocalizeManager}
+   */
   function getInstance(cfg) {
     LocalizeManager.resetInstance();
     return LocalizeManager.getInstance(cfg || {});
@@ -660,6 +733,7 @@ describe('When supporting external translation tools like Google Translate', ()
 });
 
 describe('[deprecated] When not supporting external translation tools like Google Translate', () => {
+  /** @type {LocalizeManager} */
   let manager;
 
   beforeEach(() => {
@@ -677,16 +751,20 @@ describe('[deprecated] When not supporting external translation tools like Googl
   });
 
   it('initializes locale from <html> by default', () => {
-    manager = new LocalizeManager({ supportExternalTranslationTools: false });
+    manager = new LocalizeManager({});
     expect(manager.locale).to.equal('en-GB');
   });
 
   it('fires "localeChanged" event if locale was changed via <html lang> attribute', async () => {
-    manager = new LocalizeManager({ supportExternalTranslationTools: false });
+    manager = new LocalizeManager({});
     setTimeout(() => {
       document.documentElement.lang = 'en-US';
     });
-    const event = await oneEvent(manager, 'localeChanged');
+
+    const event = await oneEvent(
+      /** @type {EventTarget} */ (/** @type {unknown} */ (manager)),
+      'localeChanged',
+    );
     expect(event.detail.newLocale).to.equal('en-US');
     expect(event.detail.oldLocale).to.equal('en-GB');
   });
@@ -696,11 +774,11 @@ describe('[deprecated] When not supporting external translation tools like Googl
     setupFakeImport('./my-component/nl-NL.js', { default: { greeting: 'Hallo!' } });
 
     manager = new LocalizeManager({
-      supportExternalTranslationTools: false,
       autoLoadOnLocaleChange: true,
     });
 
     await manager.loadNamespace({
+      /** @param {string} locale */
       'my-component': locale => fakeImport(`./my-component/${locale}.js`, 25),
     });
 
@@ -709,7 +787,7 @@ describe('[deprecated] When not supporting external translation tools like Googl
     });
 
     document.documentElement.lang = 'nl-NL';
-    await aTimeout(); // wait for mutation observer to be called
+    await aTimeout(0); // wait for mutation observer to be called
     await manager.loadingComplete;
 
     expect(manager.__storage).to.deep.equal({
diff --git a/packages/localize/test/LocalizeMixin.test.js b/packages/localize/test/LocalizeMixin.test.js
index 72317a4be..cf4a788e0 100644
--- a/packages/localize/test/LocalizeMixin.test.js
+++ b/packages/localize/test/LocalizeMixin.test.js
@@ -7,6 +7,7 @@ import {
   fixtureSync,
   html,
   nextFrame,
+  unsafeStatic,
 } from '@open-wc/testing';
 import sinon from 'sinon';
 import { localize } from '../src/localize.js';
@@ -19,6 +20,10 @@ import {
   setupFakeImport,
 } from '../test-helpers.js';
 
+/**
+ * @typedef {import('../types/LocalizeMixinTypes').LocalizeMixin} LocalizeMixinHost
+ */
+
 describe('LocalizeMixin', () => {
   afterEach(() => {
     resetFakeImport();
@@ -26,21 +31,26 @@ describe('LocalizeMixin', () => {
   });
 
   it('loads namespaces defined in "get localizeNamespaces()" when created before attached to DOM', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
 
-    class MyElement extends LocalizeMixin(class {}) {
-      static get localizeNamespaces() {
-        return [myElementNs, ...super.localizeNamespaces];
-      }
-
-      requestUpdate() {}
-    }
+    const tagString = defineCE(
+      // @ts-ignore
+      class MyElement extends LocalizeMixin(LitElement) {
+        static get localizeNamespaces() {
+          return [myElementNs, ...super.localizeNamespaces];
+        }
+      },
+    );
+    const tag = unsafeStatic(tagString);
 
     setupEmptyFakeImportsFor(['my-element'], ['en-GB']);
 
     const loadNamespaceSpy = sinon.spy(localize, 'loadNamespace');
+    await fixture(html`<${tag}></${tag}>`);
 
-    new MyElement(); // eslint-disable-line no-new
     expect(loadNamespaceSpy.callCount).to.equal(1);
     expect(loadNamespaceSpy.calledWith(myElementNs)).to.be.true;
 
@@ -48,27 +58,41 @@ describe('LocalizeMixin', () => {
   });
 
   it('ignores duplicates in "get localizeNamespaces()" chain', async () => {
-    const defaultNs = { default: loc => fakeImport(`./default/${loc}.js`) };
-    const parentElementNs = { 'parent-element': loc => fakeImport(`./parent-element/${loc}.js`) };
-    const childElementNs = { 'child-element': loc => fakeImport(`./child-element/${loc}.js`) };
+    const defaultNs = {
+      /** @param {string} loc */
+      default: loc => fakeImport(`./default/${loc}.js`),
+    };
+    const parentElementNs = {
+      /** @param {string} loc */
+      'parent-element': loc => fakeImport(`./parent-element/${loc}.js`),
+    };
+    const childElementNs = {
+      /** @param {string} loc */
+      'child-element': loc => fakeImport(`./child-element/${loc}.js`),
+    };
 
-    class ParentElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class ParentElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [parentElementNs, defaultNs, ...super.localizeNamespaces];
       }
     }
 
-    class ChildElement extends LocalizeMixin(ParentElement) {
-      static get localizeNamespaces() {
-        return [childElementNs, defaultNs, ...super.localizeNamespaces];
-      }
-    }
+    const tagString = defineCE(
+      // @ts-ignore
+      class ChildElement extends LocalizeMixin(ParentElement) {
+        static get localizeNamespaces() {
+          return [childElementNs, defaultNs, ...super.localizeNamespaces];
+        }
+      },
+    );
+    const tag = unsafeStatic(tagString);
 
     setupEmptyFakeImportsFor(['default', 'parent-element', 'child-element'], ['en-GB']);
 
     const loadNamespaceSpy = sinon.spy(localize, 'loadNamespace');
 
-    new ChildElement(); // eslint-disable-line no-new
+    await fixture(html`<${tag}></${tag}>`);
     expect(loadNamespaceSpy.callCount).to.equal(3);
     expect(loadNamespaceSpy.calledWith(childElementNs)).to.be.true;
     expect(loadNamespaceSpy.calledWith(defaultNs)).to.be.true;
@@ -78,49 +102,56 @@ describe('LocalizeMixin', () => {
   });
 
   it('calls "onLocaleReady()" after namespaces were loaded for the first time (only if attached to DOM)', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
 
-      requestUpdate() {}
-
       onLocaleReady() {}
     }
+    const tagString = defineCE(MyElement);
 
     setupEmptyFakeImportsFor(['my-element'], ['en-GB']);
 
-    const element = new MyElement();
-    const onLocaleReadySpy = sinon.spy(element, 'onLocaleReady');
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
+    const wrapper = await fixture('<div></div>');
+    const onLocaleReadySpy = sinon.spy(el, 'onLocaleReady');
 
     await localize.loadingComplete;
     expect(onLocaleReadySpy.callCount).to.equal(0);
 
-    element.connectedCallback();
+    wrapper.appendChild(el);
 
     await localize.loadingComplete;
     expect(onLocaleReadySpy.callCount).to.equal(1);
   });
 
   it('calls "onLocaleChanged(newLocale, oldLocale)" after locale was changed (only if attached to DOM)', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyOtherElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
-
-      requestUpdate() {}
-
-      onLocaleChanged() {}
     }
 
+    const tagString = defineCE(MyOtherElement);
+
     setupEmptyFakeImportsFor(['my-element'], ['en-GB', 'nl-NL', 'ru-RU']);
 
-    const element = new MyElement();
-    const onLocaleChangedSpy = sinon.spy(element, 'onLocaleChanged');
+    const el = /** @type {MyOtherElement} */ (document.createElement(tagString));
+    const wrapper = await fixture('<div></div>');
+    const onLocaleChangedSpy = sinon.spy(el, 'onLocaleChanged');
 
     await localize.loadingComplete;
 
@@ -128,33 +159,40 @@ describe('LocalizeMixin', () => {
     await localize.loadingComplete;
     expect(onLocaleChangedSpy.callCount).to.equal(0);
 
-    element.connectedCallback();
+    wrapper.appendChild(el);
 
     localize.locale = 'ru-RU';
     await localize.loadingComplete;
     expect(onLocaleChangedSpy.callCount).to.equal(1);
-    expect(onLocaleChangedSpy.calledWith('ru-RU', 'nl-NL')).to.be.true;
+    // FIXME: Expected 0 arguments, but got 2. ts(2554) --> not sure why this sinon type is not working
+    // @ts-ignore
+    expect(onLocaleChangedSpy.calledWithExactly('ru-RU', 'nl-NL')).to.be.true;
   });
 
   it('calls "onLocaleUpdated()" after both "onLocaleReady()" and "onLocaleChanged()"', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
 
-      requestUpdate() {}
-
       onLocaleUpdated() {}
     }
 
+    const tagString = defineCE(MyElement);
+
     setupEmptyFakeImportsFor(['my-element'], ['en-GB', 'nl-NL']);
 
-    const el = new MyElement();
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
+    const wrapper = await fixture('<div></div>');
     const onLocaleUpdatedSpy = sinon.spy(el, 'onLocaleUpdated');
 
-    el.connectedCallback();
+    wrapper.appendChild(el);
     await el.localizeNamespacesLoaded;
     expect(onLocaleUpdatedSpy.callCount).to.equal(1);
 
@@ -162,8 +200,11 @@ describe('LocalizeMixin', () => {
     expect(onLocaleUpdatedSpy.callCount).to.equal(2);
   });
 
-  it('should have the localizeNamespacesLoaded avaliable within "onLocaleUpdated()"', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+  it('should have the localizeNamespacesLoaded available within "onLocaleUpdated()"', async () => {
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         label: 'one',
@@ -174,13 +215,13 @@ describe('LocalizeMixin', () => {
         label: 'two',
       },
     });
-    class MyElement extends LocalizeMixin(class {}) {
+
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
 
-      requestUpdate() {}
-
       async onLocaleUpdated() {
         super.onLocaleUpdated();
         await this.localizeNamespacesLoaded;
@@ -188,7 +229,8 @@ describe('LocalizeMixin', () => {
       }
     }
 
-    const el = new MyElement();
+    const tagString = defineCE(MyElement);
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
     el.connectedCallback();
 
     await el.localizeNamespacesLoaded;
@@ -201,19 +243,22 @@ describe('LocalizeMixin', () => {
   });
 
   it('calls "requestUpdate()" after locale was changed', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
-
-      requestUpdate() {}
     }
 
     setupEmptyFakeImportsFor(['my-element'], ['en-GB']);
 
-    const el = new MyElement();
+    const tagString = defineCE(MyElement);
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
     const updateSpy = sinon.spy(el, 'requestUpdate');
 
     el.connectedCallback();
@@ -224,23 +269,29 @@ describe('LocalizeMixin', () => {
   });
 
   it('has msgLit() which integrates with lit-html', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         greeting: 'Hi!',
       },
     });
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
     }
 
-    const element = new MyElement();
+    const tagString = defineCE(MyElement);
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
+    el.connectedCallback();
     const lionLocalizeMessageSpy = sinon.spy(localize, 'msg');
 
-    const messageDirective = element.msgLit('my-element:greeting');
+    const messageDirective = el.msgLit('my-element:greeting');
     expect(lionLocalizeMessageSpy.callCount).to.equal(0);
 
     expect(isDirective(messageDirective)).to.be.true;
@@ -250,7 +301,7 @@ describe('LocalizeMixin', () => {
     expect(lionLocalizeMessageSpy.callCount).to.equal(1);
     expect(lionLocalizeMessageSpy.calledWith('my-element:greeting')).to.be.true;
 
-    const message = element.msgLit('my-element:greeting');
+    const message = el.msgLit('my-element:greeting');
     expect(message).to.equal('Hi!');
     expect(typeof message).to.equal('string');
     expect(lionLocalizeMessageSpy.callCount).to.equal(2);
@@ -259,22 +310,25 @@ describe('LocalizeMixin', () => {
     lionLocalizeMessageSpy.restore();
   });
 
-  it('has a Promise "localizeNamespacesLoaded" which resolves once tranlations are available', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25) };
+  it('has a Promise "localizeNamespacesLoaded" which resolves once translations are available', async () => {
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         greeting: 'Hi!',
       },
     });
 
-    class MyElement extends LocalizeMixin(class {}) {
+    // @ts-ignore
+    class MyElement extends LocalizeMixin(LitElement) {
       static get localizeNamespaces() {
         return [myElementNs, ...super.localizeNamespaces];
       }
-
-      requestUpdate() {}
     }
-    const el = new MyElement();
+    const tagString = defineCE(MyElement);
+    const el = /** @type {MyElement} */ (document.createElement(tagString));
 
     const messageDirective = el.msgLit('my-element:greeting');
     expect(isDirective(messageDirective)).to.be.true;
@@ -284,33 +338,46 @@ describe('LocalizeMixin', () => {
   });
 
   it('renders only once all translations have been loaded (if BaseElement supports it)', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         greeting: 'Hi!',
       },
     });
 
-    const tag = defineCE(
-      class extends LocalizeMixin(LitElement) {
-        static get localizeNamespaces() {
-          return [myElementNs, ...super.localizeNamespaces];
-        }
+    // @ts-ignore
+    class MyLocalizedClass extends LocalizeMixin(LitElement) {
+      static get localizeNamespaces() {
+        return [myElementNs, ...super.localizeNamespaces];
+      }
 
-        render() {
-          return html`<p>${this.msgLit('my-element:greeting')}</p>`;
-        }
-      },
-    );
+      render() {
+        return html`<p>${this.msgLit('my-element:greeting')}</p>`;
+      }
+    }
 
-    const el = await fixtureSync(`<${tag}></${tag}>`);
-    expect(el.shadowRoot.children.length).to.equal(0);
-    await el.updateComplete;
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('Hi!');
+    const tag = defineCE(MyLocalizedClass);
+    const el = /** @type {MyLocalizedClass} */ (await fixtureSync(`<${tag}></${tag}>`));
+    expect(el.shadowRoot).to.exist;
+    if (el.shadowRoot) {
+      expect(el.shadowRoot.children.length).to.equal(0);
+      await el.updateComplete;
+      const pTag = el.shadowRoot.querySelector('p');
+      expect(pTag).to.exist;
+      if (pTag) {
+        expect(pTag.innerText).to.equal('Hi!');
+      }
+    }
   });
 
-  it('rerender on locale change once all translations are loaded (if BaseElement supports it)', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25) };
+  it('re-render on locale change once all translations are loaded (if BaseElement supports it)', async () => {
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`, 25),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         greeting: 'Hi!',
@@ -322,57 +389,69 @@ describe('LocalizeMixin', () => {
       },
     });
 
-    const tag = defineCE(
-      class TestPromise extends LocalizeMixin(LitElement) {
-        static get localizeNamespaces() {
-          return [myElementNs, ...super.localizeNamespaces];
-        }
+    // @ts-ignore
+    class MyLocalizedClass extends LocalizeMixin(LitElement) {
+      static get localizeNamespaces() {
+        return [myElementNs, ...super.localizeNamespaces];
+      }
 
-        render() {
-          return html`<p>${this.msgLit('my-element:greeting')}</p>`;
-        }
-      },
-    );
+      render() {
+        return html`<p>${this.msgLit('my-element:greeting')}</p>`;
+      }
+    }
 
-    const el = await fixture(`<${tag}></${tag}>`);
+    const tag = defineCE(MyLocalizedClass);
+    const el = /** @type {MyLocalizedClass} */ (await fixture(`<${tag}></${tag}>`));
     await el.updateComplete;
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('Hi!');
 
-    localize.locale = 'en-US';
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('Hi!');
-    await el.updateComplete;
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('Howdy!');
+    expect(el.shadowRoot).to.exist;
+    if (el.shadowRoot) {
+      const p = /** @type {HTMLParagraphElement} */ (el.shadowRoot.querySelector('p'));
+      expect(p.innerText).to.equal('Hi!');
+      localize.locale = 'en-US';
+      expect(p.innerText).to.equal('Hi!');
+      await el.updateComplete;
+      expect(p.innerText).to.equal('Howdy!');
+    }
   });
 
   it('it can still render async by setting "static get waitForLocalizeNamespaces() { return false; }" (if BaseElement supports it)', async () => {
-    const myElementNs = { 'my-element': locale => fakeImport(`./my-element/${locale}.js`, 50) };
+    const myElementNs = {
+      /** @param {string} locale */
+      'my-element': locale => fakeImport(`./my-element/${locale}.js`, 50),
+    };
     setupFakeImport('./my-element/en-GB.js', {
       default: {
         greeting: 'Hi!',
       },
     });
 
-    const tag = defineCE(
-      class extends LocalizeMixin(LitElement) {
-        static get waitForLocalizeNamespaces() {
-          return false;
-        }
+    // @ts-ignore
+    class MyLocalizedClass extends LocalizeMixin(LitElement) {
+      static get waitForLocalizeNamespaces() {
+        return false;
+      }
 
-        static get localizeNamespaces() {
-          return [myElementNs, ...super.localizeNamespaces];
-        }
+      static get localizeNamespaces() {
+        return [myElementNs, ...super.localizeNamespaces];
+      }
 
-        render() {
-          return html`<p>${this.msgLit('my-element:greeting')}</p>`;
-        }
-      },
-    );
+      render() {
+        return html`<p>${this.msgLit('my-element:greeting')}</p>`;
+      }
+    }
 
-    const el = await fixture(`<${tag}></${tag}>`);
+    const tag = defineCE(MyLocalizedClass);
+
+    const el = /** @type {MyLocalizedClass} */ (await fixture(`<${tag}></${tag}>`));
     await el.updateComplete;
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('');
-    await el.localizeNamespacesLoaded;
-    await el.updateComplete;
-    expect(el.shadowRoot.querySelector('p').innerText).to.equal('Hi!');
+    expect(el.shadowRoot).to.exist;
+    if (el.shadowRoot) {
+      const p = /** @type {HTMLParagraphElement} */ (el.shadowRoot.querySelector('p'));
+      expect(p.innerText).to.equal('');
+      await el.localizeNamespacesLoaded;
+      await el.updateComplete;
+      expect(p.innerText).to.equal('Hi!');
+    }
   });
 });
diff --git a/packages/localize/test/date/formatDate.test.js b/packages/localize/test/date/formatDate.test.js
index 1eb330f63..8ea6368b7 100644
--- a/packages/localize/test/date/formatDate.test.js
+++ b/packages/localize/test/date/formatDate.test.js
@@ -57,10 +57,10 @@ describe('formatDate', () => {
       locale: 'en-US',
     };
     localize.locale = 'hu-HU';
-    let date = parseDate('2018-5-28');
+    let date = /** @type {Date} */ (parseDate('2018-5-28'));
     expect(formatDate(date)).to.equal('2018. 05. 28.');
 
-    date = parseDate('1970-11-3');
+    date = /** @type {Date} */ (parseDate('1970-11-3'));
     expect(formatDate(date, options)).to.equal('Tuesday, November 03, 1970');
   });
 
@@ -73,13 +73,13 @@ describe('formatDate', () => {
       locale: 'en-US',
     };
     localize.locale = 'bg-BG';
-    let date = parseDate('29-12-2017');
+    let date = /** @type {Date} */ (parseDate('29-12-2017'));
     expect(formatDate(date)).to.equal('29.12.2017 г.');
 
-    date = parseDate('13-1-1940');
+    date = /** @type {Date} */ (parseDate('13-1-1940'));
     expect(formatDate(date)).to.equal('13.01.1940 г.');
 
-    date = parseDate('3-11-1970');
+    date = /** @type {Date} */ (parseDate('3-11-1970'));
     expect(formatDate(date, options)).to.equal('Tuesday, November 03, 1970');
   });
 
@@ -92,13 +92,13 @@ describe('formatDate', () => {
       locale: 'en-US',
     };
     localize.locale = 'en-US';
-    let date = parseDate('12-29-1940');
+    let date = /** @type {Date} */ (parseDate('12-29-1940'));
     expect(formatDate(date)).to.equal('12/29/1940');
 
-    date = parseDate('1-13-1940');
+    date = /** @type {Date} */ (parseDate('1-13-1940'));
     expect(formatDate(date)).to.equal('01/13/1940');
 
-    date = parseDate('11-3-1970');
+    date = /** @type {Date} */ (parseDate('11-3-1970'));
     expect(formatDate(date, options)).to.equal('Tuesday, November 03, 1970');
   });
 
@@ -110,10 +110,10 @@ describe('formatDate', () => {
       day: '2-digit',
       locale: 'en-US',
     };
-    let parsedDate = parseDate('05.11.2017');
+    let parsedDate = /** @type {Date} */ (parseDate('05.11.2017'));
     expect(formatDate(parsedDate, options)).to.equal('Sunday, November 05, 2017');
 
-    parsedDate = parseDate('01-01-1940');
+    parsedDate = /** @type {Date} */ (parseDate('01-01-1940'));
     options = {
       weekday: 'long',
       year: 'numeric',
@@ -130,7 +130,7 @@ describe('formatDate', () => {
       month: 'long',
       day: '2-digit',
     };
-    const parsedDate = parseDate('12.10.2019');
+    const parsedDate = /** @type {Date} */ (parseDate('12.10.2019'));
     expect(formatDate(parsedDate, options)).to.equal('Saturday, 12 October');
   });
 
@@ -140,7 +140,7 @@ describe('formatDate', () => {
       year: 'numeric',
       day: '2-digit',
     };
-    const parsedDate = parseDate('12.10.2019');
+    const parsedDate = /** @type {Date} */ (parseDate('12.10.2019'));
     expect(formatDate(parsedDate, options)).to.equal('Saturday 12 2019');
   });
 
@@ -150,12 +150,13 @@ describe('formatDate', () => {
       year: 'numeric',
       month: 'long',
     };
-    const parsedDate = parseDate('12.10.2019');
+    const parsedDate = /** @type {Date} */ (parseDate('12.10.2019'));
     expect(formatDate(parsedDate, options)).to.equal('October 2019 Saturday');
   });
 
   it('returns empty string when input is not a Date object', async () => {
     const date = '1-1-2016';
+    // @ts-ignore tests what happens if you use a wrong type
     expect(formatDate(date)).to.equal('');
   });
 });
diff --git a/packages/localize/test/date/getMonthNames.test.js b/packages/localize/test/date/getMonthNames.test.js
index 538717013..09063f066 100644
--- a/packages/localize/test/date/getMonthNames.test.js
+++ b/packages/localize/test/date/getMonthNames.test.js
@@ -2,6 +2,9 @@ import { expect } from '@open-wc/testing';
 
 import { getMonthNames } from '../../src/date/getMonthNames.js';
 
+/**
+ * @param {TemplateStringsArray} strings
+ */
 function s(strings) {
   return strings[0].split(' ');
 }
diff --git a/packages/localize/test/date/getWeekdayNames.test.js b/packages/localize/test/date/getWeekdayNames.test.js
index ded662192..3d8ffeecb 100644
--- a/packages/localize/test/date/getWeekdayNames.test.js
+++ b/packages/localize/test/date/getWeekdayNames.test.js
@@ -2,6 +2,9 @@ import { expect } from '@open-wc/testing';
 
 import { getWeekdayNames } from '../../src/date/getWeekdayNames.js';
 
+/**
+ * @param {TemplateStringsArray} strings
+ */
 function s(strings) {
   return strings[0].split(' ');
 }
diff --git a/packages/localize/test/date/parseDate.test.js b/packages/localize/test/date/parseDate.test.js
index 154aaae02..82fec7b5d 100644
--- a/packages/localize/test/date/parseDate.test.js
+++ b/packages/localize/test/date/parseDate.test.js
@@ -3,9 +3,15 @@ import { localizeTearDown } from '../../test-helpers.js';
 
 import { parseDate } from '../../src/date/parseDate.js';
 
+/**
+ *
+ * @param {Date | undefined} value
+ * @param {Date} date
+ */
 function equalsDate(value, date) {
   return (
     Object.prototype.toString.call(value) === '[object Date]' && // is Date Object
+    value &&
     value.getDate() === date.getDate() && // day
     value.getMonth() === date.getMonth() && // month
     value.getFullYear() === date.getFullYear() // year
diff --git a/packages/localize/test/isLocalizeESModule.test.js b/packages/localize/test/isLocalizeESModule.test.js
index 3894a61be..63990ec11 100644
--- a/packages/localize/test/isLocalizeESModule.test.js
+++ b/packages/localize/test/isLocalizeESModule.test.js
@@ -27,6 +27,7 @@ describe('isLocalizeESModule', () => {
   });
 
   it('ignores if not an object', () => {
+    // @ts-ignore passing a non-object is not allowed by ts, but we still want to test the outcome
     expect(isLocalizeESModule(undefined)).to.equal(false);
   });
 });
diff --git a/packages/localize/test/localize.test.js b/packages/localize/test/localize.test.js
index b87b77dd8..f7ea31c6c 100644
--- a/packages/localize/test/localize.test.js
+++ b/packages/localize/test/localize.test.js
@@ -7,7 +7,7 @@ import { LocalizeManager } from '../src/LocalizeManager.js';
 import { localize, setLocalize } from '../src/localize.js';
 
 describe('localize', () => {
-  // this is an importan mindset:
+  // this is an important mindset:
   // we don't test the singleton
   // we check that it is an instance of the right class
   // we test newly created instances of this class separately
@@ -26,9 +26,11 @@ describe('localize', () => {
     const oldLocalizeTeardown = localize.teardown;
     localize.teardown = sinon.spy();
 
-    const newLocalize = { teardown: () => {} };
+    const newLocalize = /** @type {LocalizeManager} */ ({ teardown: () => {} });
     setLocalize(newLocalize);
     expect(localize).to.equal(newLocalize);
+
+    // @ts-ignore since we're testing another reference to the same global instance
     expect(oldLocalize.teardown.callCount).to.equal(1);
 
     setLocalize(oldLocalize);
diff --git a/packages/localize/test/number/formatNumber.test.js b/packages/localize/test/number/formatNumber.test.js
index 17e817e9f..35d6aedcd 100644
--- a/packages/localize/test/number/formatNumber.test.js
+++ b/packages/localize/test/number/formatNumber.test.js
@@ -3,14 +3,22 @@ import { localize } from '../../src/localize.js';
 import { formatNumber } from '../../src/number/formatNumber.js';
 import { localizeTearDown } from '../../test-helpers.js';
 
-const currencyCode = currency => ({ style: 'currency', currencyDisplay: 'code', currency });
-const currencySymbol = currency => ({ style: 'currency', currencyDisplay: 'symbol', currency });
+const currencyCode = /** @param {string} currency */ currency => ({
+  style: 'currency',
+  currencyDisplay: 'code',
+  currency,
+});
+const currencySymbol = /** @param {string} currency */ currency => ({
+  style: 'currency',
+  currencyDisplay: 'symbol',
+  currency,
+});
 
 describe('formatNumber', () => {
   afterEach(localizeTearDown);
 
   it('displays the appropriate amount of decimal places based on currencies spec http://www.currency-iso.org/en/home/tables/table-a1.html', () => {
-    const clean = str => str.replace(/[a-zA-Z]+/g, '').trim();
+    const clean = /** @param {string} str */ str => str.replace(/[a-zA-Z]+/g, '').trim();
     expect(clean(formatNumber(123456.789, currencyCode('JPY')))).to.equal('123,457');
     expect(clean(formatNumber(123456.789, currencyCode('EUR')))).to.equal('123,456.79');
     expect(clean(formatNumber(123456.789, currencyCode('BHD')))).to.equal('123,456.789');
@@ -54,11 +62,10 @@ describe('formatNumber', () => {
     expect(formatNumber(-12.6, { roundMode: 'floor' })).to.equal('−13');
   });
 
-  it('returns empty string when NaN', () => {
+  it('returns empty string when passing wrong type', () => {
+    // @ts-ignore tests what happens if you pass wrong type
     expect(formatNumber('foo')).to.equal('');
-  });
-
-  it('returns empty string when number is undefined', () => {
+    // @ts-ignore tests what happens if you pass wrong type
     expect(formatNumber(undefined)).to.equal('');
   });
 
@@ -66,12 +73,14 @@ describe('formatNumber', () => {
     const savedReturnIfNaN = localize.formatNumberOptions.returnIfNaN;
 
     localize.formatNumberOptions.returnIfNaN = '-';
+    // @ts-ignore
     expect(formatNumber('foo')).to.equal('-');
 
     localize.formatNumberOptions.returnIfNaN = savedReturnIfNaN;
   });
 
   it("can set what to returns when NaN via `returnIfNaN: 'foo'`", () => {
+    // @ts-ignore
     expect(formatNumber('foo', { returnIfNaN: '-' })).to.equal('-');
   });
 
diff --git a/packages/localize/test/number/formatNumberToParts.test.js b/packages/localize/test/number/formatNumberToParts.test.js
index 9e34d63e8..e88649194 100644
--- a/packages/localize/test/number/formatNumberToParts.test.js
+++ b/packages/localize/test/number/formatNumberToParts.test.js
@@ -1,18 +1,26 @@
 import { expect } from '@open-wc/testing';
 import { localize } from '../../src/localize.js';
 import { localizeTearDown } from '../../test-helpers.js';
-
 import { formatNumberToParts } from '../../src/number/formatNumberToParts.js';
 
-const c = v => ({ type: 'currency', value: v });
-const d = v => ({ type: 'decimal', value: v });
-const i = v => ({ type: 'integer', value: v });
-const f = v => ({ type: 'fraction', value: v });
-const g = v => ({ type: 'group', value: v });
-const l = v => ({ type: 'literal', value: v });
+const c = /** @param {string} v */ v => ({
+  type: 'currency',
+  value: v,
+});
+const d = /** @param {string} v */ v => ({ type: 'decimal', value: v });
+const i = /** @param {string} v */ v => ({ type: 'integer', value: v });
+const f = /** @param {string} v */ v => ({ type: 'fraction', value: v });
+const g = /** @param {string} v */ v => ({ type: 'group', value: v });
+const l = /** @param {string} v */ v => ({ type: 'literal', value: v });
 const m = { type: 'minusSign', value: '−' };
 
-const stringifyParts = parts => parts.map(part => part.value).join('');
+const stringifyParts =
+  /**
+   * @typedef {import('../../types/LocalizeMixinTypes').FormatNumberPart} FormatNumberPart
+   * @param {FormatNumberPart[]} parts
+   * @returns {string}
+   */
+  parts => parts.map(part => part.value).join('');
 
 describe('formatNumberToParts', () => {
   afterEach(localizeTearDown);
@@ -32,12 +40,14 @@ describe('formatNumberToParts', () => {
     ];
 
     specs.forEach(([locale, currency, amount, expectedResult]) => {
-      it(`formats ${locale} ${currency} ${amount} as "${stringifyParts(expectedResult)}"`, () => {
+      it(`formats ${locale} ${currency} ${amount} as "${stringifyParts(
+        /** @type {FormatNumberPart[]} */ (expectedResult),
+      )}"`, () => {
         expect(
-          formatNumberToParts(amount, {
-            locale,
+          formatNumberToParts(Number(amount), {
             style: 'currency',
-            currency,
+            locale: String(locale),
+            currency: String(currency),
           }),
         ).to.deep.equal(expectedResult);
       });
@@ -59,13 +69,15 @@ describe('formatNumberToParts', () => {
     ];
 
     specs.forEach(([locale, currency, amount, expectedResult]) => {
-      it(`formats ${locale} ${currency} ${amount} as "${stringifyParts(expectedResult)}"`, () => {
+      it(`formats ${locale} ${currency} ${amount} as "${stringifyParts(
+        /** @type {FormatNumberPart[]} */ (expectedResult),
+      )}"`, () => {
         expect(
-          formatNumberToParts(amount, {
-            locale,
+          formatNumberToParts(Number(amount), {
             style: 'currency',
             currencyDisplay: 'code',
-            currency,
+            locale: String(locale),
+            currency: String(currency),
           }),
         ).to.deep.equal(expectedResult);
       });
@@ -74,6 +86,7 @@ describe('formatNumberToParts', () => {
 
   describe("style: 'decimal'", () => {
     describe('no minimumFractionDigits', () => {
+      /** @type {Array.<Array.<?>>} */
       const specs = [
         ['en-GB', 3500, [i('3'), g(','), i('500')]],
         ['en-GB', -3500, [m, i('3'), g(','), i('500')]],
@@ -88,10 +101,12 @@ describe('formatNumberToParts', () => {
       ];
 
       specs.forEach(([locale, amount, expectedResult]) => {
-        it(`formats ${locale} ${amount} as "${stringifyParts(expectedResult)}"`, () => {
+        it(`formats ${locale} ${amount} as "${stringifyParts(
+          /** @type {FormatNumberPart[]} */ (expectedResult),
+        )}"`, () => {
           localize.locale = locale;
           expect(
-            formatNumberToParts(amount, {
+            formatNumberToParts(Number(amount), {
               style: 'decimal',
             }),
           ).to.deep.equal(expectedResult);
@@ -100,6 +115,7 @@ describe('formatNumberToParts', () => {
     });
 
     describe('minimumFractionDigits: 2', () => {
+      /** @type {Array.<Array.<?>>} */
       const specs = [
         ['en-GB', 3500, [i('3'), g(','), i('500'), d('.'), f('00')]],
         ['en-GB', -3500, [m, i('3'), g(','), i('500'), d('.'), f('00')]],
@@ -114,10 +130,12 @@ describe('formatNumberToParts', () => {
       ];
 
       specs.forEach(([locale, amount, expectedResult]) => {
-        it(`formats ${locale} ${amount} as "${stringifyParts(expectedResult)}"`, () => {
+        it(`formats ${locale} ${amount} as "${stringifyParts(
+          /** @type {FormatNumberPart[]} */ (expectedResult),
+        )}"`, () => {
           localize.locale = locale;
           expect(
-            formatNumberToParts(amount, {
+            formatNumberToParts(Number(amount), {
               style: 'decimal',
               minimumFractionDigits: 2,
             }),
@@ -142,12 +160,14 @@ describe('formatNumberToParts', () => {
     ];
 
     specs.forEach(([locale, amount, expectedResult]) => {
-      it(`formats ${locale} ${amount} as "${stringifyParts(expectedResult)}"`, () => {
+      it(`formats ${locale} ${amount} as "${stringifyParts(
+        /** @type {FormatNumberPart[]} */ (expectedResult),
+      )}"`, () => {
         expect(
-          formatNumberToParts(amount / 100, {
-            locale,
+          formatNumberToParts(Number(amount) / 100, {
             style: 'percent',
             minimumFractionDigits: 2,
+            locale: String(locale),
           }),
         ).to.deep.equal(expectedResult);
       });
diff --git a/packages/localize/types/LocalizeMixinTypes.d.ts b/packages/localize/types/LocalizeMixinTypes.d.ts
new file mode 100644
index 000000000..a25e65d24
--- /dev/null
+++ b/packages/localize/types/LocalizeMixinTypes.d.ts
@@ -0,0 +1,52 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+
+export interface FormatNumberPart {
+  type: string;
+  value: string;
+}
+
+interface StringToFunctionMap {
+  [key: string]: Function;
+}
+
+export type NamespaceObject = StringToFunctionMap | string;
+
+interface msgVariables {
+  [key: string]: unknown;
+}
+
+interface msgOptions {
+  locale?: string;
+}
+
+declare class LocalizeMixinHost {
+  // FIXME: return value type check doesn't seem to be `working!
+  static get localizeNamespaces(): StringToFunctionMap[];
+
+  static get waitForLocalizeNamespaces(): boolean;
+
+  public localizeNamespacesLoaded(): Promise<Object>;
+
+  /**
+   * Hook into LitElement to only render once all translations are loaded
+   */
+  public performUpdate(): Promise<void>;
+
+  public onLocaleReady(): void;
+  public onLocaleChanged(): void;
+  public onLocaleUpdated(): void;
+  public connectedCallback(): void;
+  public disconnectedCallback(): void;
+  public msgLit(keys: string | string[], variables?: msgVariables, options?: msgOptions): void;
+
+  private __getUniqueNamespaces(): void;
+  private __localizeAddLocaleChangedListener(): void;
+  private __localizeRemoveLocaleChangedListener(): void;
+  private __localizeOnLocaleChanged(event: CustomEvent): void;
+}
+
+declare function LocalizeMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<LocalizeMixinHost> & typeof LocalizeMixinHost;
+
+export type LocalizeMixin = typeof LocalizeMixinImplementation;
diff --git a/packages/localize/types/index.d.ts b/packages/localize/types/index.d.ts
new file mode 100644
index 000000000..6ae7b1561
--- /dev/null
+++ b/packages/localize/types/index.d.ts
@@ -0,0 +1,8 @@
+declare module '@bundled-es-modules/message-format/MessageFormat.js' {
+  let main: any;
+  export = main;
+}
+
+declare module '@bundled-es-modules/fetch-mock' {
+  export const fetchMock: any;
+}
diff --git a/packages/singleton-manager/src/SingletonManagerClass.js b/packages/singleton-manager/src/SingletonManagerClass.js
index c35c6c225..49e9b7659 100644
--- a/packages/singleton-manager/src/SingletonManagerClass.js
+++ b/packages/singleton-manager/src/SingletonManagerClass.js
@@ -3,6 +3,11 @@ export class SingletonManagerClass {
     this._map = new Map();
   }
 
+  /**
+   * @param {string} key
+   * @param {any} value
+   * @throws {Error} Will throw if the key is already defined
+   */
   set(key, value) {
     if (this.has(key)) {
       throw new Error(`The key "${key}" is already defined and can not be overridden.`);
@@ -10,10 +15,17 @@ export class SingletonManagerClass {
     this._map.set(key, value);
   }
 
+  /**
+   * @param {string} key
+   * @returns
+   */
   get(key) {
     return this._map.get(key);
   }
 
+  /**
+   * @param {string} key
+   */
   has(key) {
     return this._map.has(key);
   }
diff --git a/tsconfig.json b/tsconfig.json
index 02514af21..34468d335 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,15 +10,24 @@
     "strict": true,
     "noImplicitThis": true,
     "alwaysStrict": true,
-    "types": ["node", "mocha"],
+    "types": ["node", "mocha", "sinon"],
     "esModuleInterop": true
   },
-  "include": ["packages/core/**/*.js", "packages/tabs/**/*.js"],
+  "include": [
+    "packages/core/**/*.js",
+    "packages/tabs/**/*.js",
+    "packages/singleton-manager/**/*.js",
+    "packages/localize/**/*.js",
+    "packages/localize/**/*.ts"
+  ],
   "exclude": [
     "node_modules",
     "**/node_modules/*",
     "**/coverage/*",
     "**/dist/**/*",
-    "packages/**/test-helpers"
+    "packages/**/test-helpers",
+    // ignore test/demos for singleton manager until overlays are typed as it's used in there
+    "packages/singleton-manager/demo/",
+    "packages/singleton-manager/test/"
   ]
 }
diff --git a/web-test-runner-browserstack.config.js b/web-test-runner-browserstack.config.js
index 104921967..dfb502dc6 100644
--- a/web-test-runner-browserstack.config.js
+++ b/web-test-runner-browserstack.config.js
@@ -20,11 +20,13 @@ module.exports = {
   sessionStartTimeout: 60000,
   concurrency: 1,
   coverageConfig: {
+    report: true,
+    reportDir: 'coverage',
     threshold: {
-      statements: 80,
+      statements: 90,
       branches: 70,
-      functions: 70,
-      lines: 80,
+      functions: 80,
+      lines: 90,
     },
   },
   browsers: [
diff --git a/web-test-runner.config.js b/web-test-runner.config.js
index 366c9fa76..04bdee22f 100644
--- a/web-test-runner.config.js
+++ b/web-test-runner.config.js
@@ -6,11 +6,13 @@ module.exports = {
   sessionStartTimeout: 30000,
   concurrency: 5,
   coverageConfig: {
+    report: true,
+    reportDir: 'coverage',
     threshold: {
-      statements: 80,
-      branches: 70,
-      functions: 70,
-      lines: 80,
+      statements: 90,
+      branches: 65,
+      functions: 80,
+      lines: 90,
     },
   },
 };