# Systems >> Form >> Validate ||30
```js script
import { html } from '@mdjs/mdjs-preview';
import { LionInput } from '@lion/ui/input.js';
import '@lion/ui/define/lion-checkbox-group.js';
import '@lion/ui/define/lion-checkbox.js';
import '@lion/ui/define/lion-combobox.js';
import '@lion/ui/define/lion-fieldset.js';
import '@lion/ui/define/lion-form.js';
import '@lion/ui/define/lion-input-amount.js';
import '@lion/ui/define/lion-input-date.js';
import '@lion/ui/define/lion-input-datepicker.js';
import '@lion/ui/define/lion-input-email.js';
import '@lion/ui/define/lion-input-iban.js';
import '@lion/ui/define/lion-input-range.js';
import '@lion/ui/define/lion-input-stepper.js';
import '@lion/ui/define/lion-input.js';
import '@lion/ui/define/lion-listbox.js';
import '@lion/ui/define/lion-listbox.js';
import '@lion/ui/define/lion-option.js';
import '@lion/ui/define/lion-options.js';
import '@lion/ui/define/lion-radio-group.js';
import '@lion/ui/define/lion-radio.js';
import '@lion/ui/define/lion-select.js';
import '@lion/ui/define/lion-select-rich.js';
import '@lion/ui/define/lion-textarea.js';
import {
DefaultSuccess,
EqualsLength,
IsDate,
IsEmail,
IsNumber,
IsString,
MaxDate,
MaxLength,
MaxNumber,
MinDate,
MinLength,
MinMaxDate,
MinMaxLength,
MinMaxNumber,
MinNumber,
Required,
Validator,
Pattern,
Unparseable,
} from '@lion/ui/form-core.js';
import { localize } from '@lion/ui/localize.js';
import { loadDefaultFeedbackMessages } from '@lion/ui/validate-messages.js';
import { renderLitAsNode } from '@lion/ui/helpers.js';
```
## When validation happens
Validation happens on every change of `modelValue`. A change in modelValue can be originated from either user interaction or an imperative action from an Application Developer. We could therefore say validation happens 'realtime'. This is aligned with the platform.
## When validation feedback will be shown
Although validation happens realtime under the hood, displaying validity feedback in realtime may not always lead to the desired User Experience. Together with [interaction states](../../../fundamentals/systems/form/interaction-states.md),
validity states can determine whether a validation message should be shown along the input field.
## Assessing field validation state programmatically
When a field has validation feedback it can be accessed using the `hasFeedbackFor` and `showsFeedbackFor` properties. This can be used if the validity of the field is needed in code.
- `hasFeedbackFor` is the state of the field regardless of whether the feedback is shown or not.
- `showsFeedbackFor` represents whether or not validation feedback is being shown. This takes into account interaction state such as a field being `touched` and `dirty`, `prefilled` or `submitted`
```js
// Field is invalid and has not interacted with
const field = document.querySelector('#my-field');
field.hasFeedbackFor.includes('error'); // returns true
field.showsFeedbackFor.includes('error'); // returns false
// Field invalid, dirty and touched
field.hasFeedbackFor.includes('error'); // returns true
field.showsFeedbackFor.includes('error'); // returns true
```
## Features
- Allow for advanced UX scenarios by updating validation state on every value change
- Provide a powerful way of writing validation via pure functions
- Default validators
- Custom validators
- Multiple validation types(error, warning, info, success)
Our validation system is designed to:
- Allow for advanced UX scenarios by updating validation state on every value change
- Provide a powerful way of writing validations via classes.
## Validators
All validators are extensions of the `Validator` class. They should be applied to the element implementing `ValidateMixin` as follows:
```html
```
As you can see the 'validators' property expects a map (an array of arrays). So, every Validator is a class consisting of:
- validator function
- validator parameters (optional)
- validator config (optional)
### Validator classes
All validators extend from the default `Validator` class. Below is an example of what a validator could look like:
```js
class EqualsText extends Validator {
static validatorName = 'EqualsText';
/**
* Returns true when 'activated' (c.q. in error/warning/info state)
* @param {string} modelValue
*/
execute(modelValue, param) {
return modelValue !== param;
}
static getMessage({ fieldName, modelValue, formControl }) {
return `Please make sure values are equal`;
}
}
```
The `execute()` method returns a boolean by default, but it can also be customized to return enums instead. In this example below, we customize the method to return one among some possible enums as strings.
```js
class EnumOutComeValidator extends Validator {
static validatorName = 'EnumOutCome';
/**
* @param {string} modelValue
* @param {number} maxLength
* @returns {false|'overload'|'error'}
*/
execute(modelValue, maxLength) {
if (modelValue.length > maxLength) {
return 'overload';
} else if (modelValue.length < 0) {
return 'error';
}
return false;
}
/**
* @param {{outcome: 'overload'|'error'|false}} opts
*/
static async getMessage({ outcome }) {
const results = {
overload: 'Too many characters!',
error: 'Something went wrong..',
false: 'All good!',
};
return results[outcome];
}
}
```
```html
```
## Installation
```bash
npm i --save @lion/ui
```
```js
import '@lion/ui/define/lion-input.js';
import { %ValidatorName% } from '@lion/ui/form-core.js';
```
## Default Validators
By default, the validate system ships with the following validators:
- **Required**
- **String Validators**, IsString, EqualsLength, MinLength, MaxLength, MinMaxLength, Pattern
- **Number Validators**, IsNumber, MinNumber, MaxNumber, MinMaxNumber
- **Date Validators**, IsDate, MinDate, MaxDate, MinMaxDate, IsDateDisabled
- **Email Validator**, IsEmail
All validators return `false` if the required validity state is met.
All validators are considered self explanatory due to their explicit namings.
```bash
npm i --save @lion/ui
```
```js
import { loadDefaultFeedbackMessages } from '@lion/ui/validate-messages.js';
```
```js preview-story
export const defaultValidationMessages = () => {
loadDefaultFeedbackMessages();
return html`
`;
};
```
Once the method `loadDefaultFeedbackMessages` is called, it will make sure that all validators provided in `@lion/form-core` will have a default error message. It uses the `@lion/localize` system to provide these translations and has support for more than 15 locales.
### Required
The required validator can be put onto every form field element and will make sure that element is not empty. For an input that may mean that it is not an empty string, while for a `checkbox-group` it means at least one `checkbox` needs to be checked.
```js preview-story
export const requiredValidator = () => html`
`;
```
The default `Required` validation message is "Please, enter a(n) {fieldName}". In which the "fieldName" can be provided separately via the `fieldName` attribute, with a fallback to the label. See [override fieldname](#override-fieldname) for more information.
Specific required messages for a select and file-upload are created.
### String Validators
Useful on input elements it allows to define how many characters can be entered.
```js preview-story
export const stringValidators = () => html`
Rocks" is in this input #LionRocks'}
label="Pattern"
>
`;
```
### Number Validators
Number validations assume that it's modelValue is actually a number.
Therefore it may only be used on input that have an appropriate parser/formatter like the `input-amount`.
```js preview-story
export const numberValidators = () => html`
`;
```
### Date Validators
Date validators work with real javascript dates. Use them on `input-date` or `input-datepicker`.
```js preview-story
export const dateValidators = () => {
const today = new Date();
const year = today.getFullYear();
const month = today.getMonth();
const day = today.getDate();
const yesterday = new Date(year, month, day - 1);
const tomorrow = new Date(year, month, day + 1);
return html`
`;
};
```
### Email Validator
```js preview-story
export const emailValidator = () => html`
`;
```
### Override default messages with custom text
To get default validation messages you need to import and call the `loadDefaultFeedbackMessages` function once in your application.
Sometimes the default messages don't make sense for your input field. In that case you want to override it by adding a `getMessage` function to your validator.
```js preview-story
export const defaultMessages = () => html`
`;
```
### Override default messages with custom HTML
Similarly to overriding the default messages with a custom text it is possible to override the messages with a custom HTML.
```js preview-story
export const defaultMessagesWithCustomHtml = () => html`
`;
```
### Override fieldName
In the scenario that the default messages are correct, but you only want to change the `fieldName`, this can both be done for a single specific validator or for all at once.
```js preview-story
export const overrideFieldName = () => html`
`;
```
## Custom Validators
Here is an example how you can make your own validator and providing the error messages directly within. You can even hard code localization in there if needed or you can use a localization system.
> Note that we import an `lion-input` here as an example of a form control implementing `ValidateMixin`.
> We could equally well use `lion-textarea`, `lion-select`, `lion-fieldset` etc. to illustrate our example.
```js preview-story
export const customValidators = () => {
const isInitialsRegex = /^([A-Z]\.)+$/;
class IsInitialsExample extends Validator {
static get validatorName() {
return 'IsExampleInitials';
}
execute(value) {
let hasFeedback = false;
const isStringValidator = new IsString();
if (isStringValidator.execute(value) || !isInitialsRegex.test(value)) {
hasFeedback = true;
}
return hasFeedback;
}
static getMessage({ fieldName }) {
return `Please enter a valid ${fieldName} in the format "L.I.".`;
}
}
return html`
`;
};
```
The custom `IsInitialsExample` checks if the value is fitting our regex, but does not prevent the user from submitting other values.
Retrieving validity states is as easy as checking for:
```js
myInitialsInput.hasFeedbackFor.include('error');
```
## Validation Examples
### Checkbox Validator
You can apply validation to the ``, similar to how you would do so in any fieldset. The interaction states of the `` are evaluated in order to hide or show feedback messages.
```js preview-story
export const checkboxValidation = () => {
const validate = e => {
const checkboxGroup = e.target.parentElement.querySelector('#scientists');
checkboxGroup.submitted = !checkboxGroup.submitted;
};
return html`
`;
};
```
Below is a more advanced validator on the group that evaluates the children checkboxes' checked states.
```js preview-story
export const checkboxValidationAdvanced = () => {
class HasMinTwoChecked extends Validator {
execute(value) {
return value.length < 2;
}
static get validatorName() {
return 'HasMinTwoChecked';
}
static async getMessage() {
return 'You need to select at least 2 values.';
}
}
const validate = e => {
const checkboxGroup = e.target.parentElement.querySelector('#scientists2');
checkboxGroup.submitted = !checkboxGroup.submitted;
};
return html`
`;
};
```
### Radio Validator
```js preview-story
export const radioValidation = () => {
const validate = e => {
const radioGroup = e.target.parentElement.querySelector('#dinos1');
radioGroup.submitted = !radioGroup.submitted;
};
return html`
`;
};
```
You can also create a validator that validates whether a certain option is checked.
```js preview-story
export const radioValidationAdvanced = () => {
class IsBrontosaurus extends Validator {
static get validatorName() {
return 'IsBrontosaurus';
}
execute(value) {
let showFeedback = false;
if (value !== 'brontosaurus') {
showFeedback = true;
}
return showFeedback;
}
static async getMessage() {
return 'You need to select "brontosaurus"';
}
}
const validate = e => {
const radioGroup = e.target.parentElement.querySelector('#dinos2');
radioGroup.submitted = !radioGroup.submitted;
};
return html`
`;
};
```
### Combobox
Validation can be used as normal, below is an example of a combobox with a `Required` validator.
```js preview-story
export const validationCombobox = () => html`
RockyRocky IIRocky IIIRocky IVRocky VRocky Balboa
`;
```
## Validation Types
The most common use case for validation is the assessment of whether the input is in error state. An error state would be considered blocking: it prevents a form from being submitted to the server.
However, the validation system also supports three non blocking validation feedback types. Summed up, we have the following four types:
- **error**: blocking the field from being submitted to the server. For example:
"Please enter an amount higher than 1000,00 euro."
- **warning**: something looks wrong, but it is not blocking. For example an optional email input:
"Please enter a valid e-mail address in the format `name@example.com`."
- **info**: shows extra information. For example a message of a scheduled payment planner:
"Ends on 15/05/2020 after 5 payments."
- **success**: will only be triggered if there was a Message from one of the above validation types and is now correct. For example: "Ok, correct."
The api for warning validators and info validators are as follows:
```js preview-story
export const validationTypes = () => {
try {
class MyTypesInput extends LionInput {
static get validationTypes() {
return ['error', 'warning', 'info', 'success'];
}
}
customElements.define('my-types-input', MyTypesInput);
} catch (err) {
// expected as it is a demo
}
return html`
`;
};
```
Success validators work a bit differently. Their success state is defined by the lack of a previously existing erroneous state (which can be an error or warning state).
So, an error validator going from invalid (true) state to invalid(false) state, will trigger the success validator.
## Asynchronous validation
By default, all validations are run synchronously. However, for instance when validation can only take place on server level, asynchronous validation will be needed.
You can make your async validators as follows:
```js preview-story
export const asynchronousValidation = () => {
function pause(ms = 0) {
return new Promise(resolve => {
setTimeout(() => {
resolve();
}, ms);
});
}
class AsyncValidator extends Validator {
constructor(...args) {
super(...args);
}
static get validatorName() {
return 'asyncValidator';
}
static get async() {
return true;
}
async execute() {
console.log('async pending...');
await pause(2000);
console.log('async done...');
return true;
}
static getMessage({ modelValue }) {
return `validated for modelValue: ${modelValue}...`;
}
}
return html`
`;
};
```
## Dynamic parameter change
```js preview-story
export const dynamicParameterChange = () => {
const beginDate = new Date('09/09/1990');
const minDateValidatorRef = new MinDate(beginDate, {
message: 'Fill in a date after your birth date',
});
return html`
`;
};
```
## Disabled inputs validation
According to the W3C specs, Disabled fields should not be validated. Therefore if the attribute disabled is present on a `lion-input` it will not be validated.
```js preview-story
export const disabledInputsValidation = () => html`
`;
```
## Backend validation
Backend validation is needed in some cases. For example, you have a sign up form, the username could be already taken. You can add an `AsyncValidator` to check the availability when the `username` field is changed but you may have a conflict when you submit the form (another user take the username in parallel).
```js preview-story
export const backendValidation = () => {
// Mock
function fakeFetch(ms = 0, withError = false) {
return new Promise(resolve => {
setTimeout(() => {
resolve(
new Response(
JSON.stringify({
message: withError ? 'Username is already taken' : '',
}),
{ status: withError ? 400 : 200 },
),
);
}, ms);
});
}
let backendValidationResolver;
let backendErrorMessage = 'Unknown Error';
let isBackendCallPending = false;
const submitHandler = ev => {
const lionForm = ev.target;
if (lionForm.hasFeedbackFor.includes('error')) {
const firstFormElWithError = lionForm.formElements.find(el =>
el.hasFeedbackFor.includes('error'),
);
firstFormElWithError.focus();
return;
}
isBackendCallPending = true;
lionForm.formElements.username.validate(); // => backendValidationResolver will be assigned
const formData = lionForm.serializedValue;
fakeFetch(2000, formData.simulateError.length).then(async response => {
if (response.status !== 200) {
backendErrorMessage = (await response.json())?.message;
backendValidationResolver(true);
}
backendValidationResolver(false);
isBackendCallPending = false;
});
};
class BackendValidator extends Validator {
static get validatorName() {
return 'backendValidator';
}
static get async() {
return true;
}
async execute() {
if (isBackendCallPending) {
return await new Promise(resolve => (backendValidationResolver = resolve));
}
return false;
}
static getMessage({ fieldName, modelValue, params: param }) {
return backendErrorMessage;
}
}
return html`
`;
};
```
## Multiple field validation
When validation is dependent on muliple fields, two approaches can be considered:
- Fieldset validation
- Validators with knowledge about context
### Fieldset validation
Assume we have a field `startDate` and `endDate` field, with condition `startDate` < `endDate`.
The easiest way to achieve this, is by adding a Validator to a fieldset wrapping those fields.
```js preview-story
const isInterpretable = mv => mv && !(mv instanceof Unparseable);
class Interval extends Validator {
static get validatorName() {
return 'Interval';
}
execute({ startDate, endDate }) {
if (isInterpretable(startDate) && isInterpretable(endDate)) {
return !(startDate < endDate);
}
return false;
}
static async getMessage() {
return `The start date should be before the end date`;
}
}
export const fieldsetValidation = () => html`
`;
```
### Validators with knowledge about context
Assume we want to create password validation with a confirmation field.
We don't want to show feedback on group level, but right beneath the fields.
```js preview-story
const isInterpretableValue = mv => mv && !(mv instanceof Unparseable);
class PasswordMatch extends Validator {
static get validatorName() {
return 'PasswordsMatch';
}
execute(modelValue, { first, second }) {
if (isInterpretableValue(first.modelValue) && isInterpretableValue(second.modelValue)) {
return first.modelValue !== second.modelValue;
}
return false;
}
}
// TODO: use ref directive once on Lit-element 3
const first = renderLitAsNode(html`
`);
const second = renderLitAsNode(html`
`);
first.validators = [
new PasswordMatch(
{ first, second },
{ getMessage: () => 'Please match with confirmation field' },
),
];
second.validators = [
new PasswordMatch({ first, second }, { getMessage: () => 'Please match with initial field' }),
];
export const contextValidation = () => html` ${first}${second} `;
```