# Form Validation
Form validation for custom elements using `ElementInternals`.
## Overview
When building form-associated custom elements, you want them to behave like native
`` elements - participating in form submission, validation, and lifecycle events.
With `static formAssociated = true` and a `value` property, your Component subclass
automatically gets:
- Form submission (value appears in FormData)
- Validation API (`checkValidity()`, `reportValidity()`, `setCustomValidity()`)
- Automatic validation for `required`, `minlength`, `maxlength`, `pattern` attributes
- Form lifecycle callbacks (reset, disabled state, browser restore)
- Focusability for validation UI
## Quick Start
Here's a minimal form-associated component:
```js
import { Component, elements } from 'tosijs'
class SimpleInput extends Component {
static preferredTagName = 'simple-input'
static formAssociated = true
value = ''
content = ({input}) => input({part: 'input', style: 'padding: 8px'})
connectedCallback() {
super.connectedCallback()
this.parts.input.addEventListener('input', (e) => {
this.value = e.target.value
})
}
render() {
super.render()
if (this.parts.input.value !== this.value) {
this.parts.input.value = this.value
}
}
}
const simpleInput = SimpleInput.elementCreator()
const { form, button, div } = elements
const output = div()
const myForm = form(
simpleInput({name: 'username', required: true}),
button({type: 'submit'}, 'Submit')
)
myForm.addEventListener('submit', (e) => {
e.preventDefault()
const fd = new FormData(e.target)
output.textContent = 'FormData: ' + [...fd.entries()].map(([k,v]) => `${k}=${v}`).join(', ')
})
preview.append(myForm, output)
```
```css
.preview form { display: flex; gap: 8px; align-items: center; }
.preview simple-input { display: inline-block; }
```
## Validation API
Form-associated components expose the standard validation API:
### Properties
- `validity: ValidityState` - Current validity state (readonly)
- `validationMessage: string` - Current validation message (readonly)
- `willValidate: boolean` - Whether element will be validated (readonly)
### Methods
- `checkValidity()` - Returns `true` if valid; fires `invalid` event if not
- `reportValidity()` - Like `checkValidity()` but shows browser validation UI
- `setCustomValidity(message)` - Set custom error (empty string clears)
- `setValidity(flags, message?, anchor?)` - Low-level validity control
- `setFormValue(value, state?)` - Explicitly set form value
- `validateValue()` - Run constraint validation (called automatically)
### ValidityStateFlags
When calling `setValidity()`, you can set these flags:
- `valueMissing` - required but empty
- `typeMismatch` - wrong type (email, url, etc.)
- `patternMismatch` - doesn't match pattern attribute
- `tooLong` - exceeds maxlength
- `tooShort` - below minlength
- `rangeUnderflow` / `rangeOverflow` - outside min/max range
- `stepMismatch` - doesn't match step
- `badInput` - browser can't parse input
- `customError` - custom error via setCustomValidity
## Automatic Validation
Component automatically validates against standard HTML constraint attributes
when `value` changes:
- `required` - value cannot be empty
- `minlength` - minimum string length
- `maxlength` - maximum string length
- `pattern` - regex pattern (anchored to full string)
```js
import { Component, elements } from 'tosijs'
class ValidatedInput extends Component {
static preferredTagName = 'validated-input'
static formAssociated = true
value = ''
content = ({input}) => input({part: 'input', style: 'padding: 8px'})
connectedCallback() {
super.connectedCallback()
this.parts.input.addEventListener('input', (e) => {
this.value = e.target.value
})
}
render() {
super.render()
if (this.parts.input.value !== this.value) {
this.parts.input.value = this.value
}
}
}
const validatedInput = ValidatedInput.elementCreator()
const { form, button, div } = elements
const output = div()
const myForm = form(
validatedInput({
name: 'code',
required: true,
minlength: '3',
maxlength: '10',
pattern: '[a-z]+'
}),
button({type: 'submit'}, 'Submit')
)
myForm.addEventListener('submit', (e) => {
e.preventDefault()
output.textContent = 'Valid! Value: ' + e.target.elements.code.value
})
preview.append(div('Enter 3-10 lowercase letters:'), myForm, output)
```
```css
.preview form { display: flex; gap: 8px; align-items: center; margin-top: 8px; }
.preview validated-input { display: inline-block; }
```
### Custom Validation
Override `validateValue()` for custom logic. Call `super.validateValue()` first
to include standard constraint validation:
validateValue() {
super.validateValue() // check required, minlength, etc.
// Add custom validation
if (this.value && !/^[a-z][a-z0-9_]*$/.test(this.value)) {
this.setValidity(
{ patternMismatch: true },
'Must start with letter, only lowercase letters/numbers/underscores',
this
)
}
}
## Form Lifecycle Callbacks
Component provides default implementations for form lifecycle callbacks:
### formResetCallback()
Called when the containing `