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# asyncRule

## Specification

Asynchronous rule of the field applied as the top priority validation *after* its sibling [`Field.props.rule`](/react-advanced-form/components/field/props/rule.md) resolves. Executed on field blur.

* `asyncRule` is designed for on blur asynchronous validation of the fields.
* `asyncRule` is designed for precise unique field validation.
* `asyncRule` must always return the specified `AsyncRulePayload` Object.
* `asyncRule` will not be called if it has been previously resolved and the value of the field has not changed.

## Definition

```typescript
type AsyncRule = (params) => AsyncRulePayload
```

```typescript
type AsyncRulePayload = {
  valid: boolean,
  extra?: {
    [exraKey: string]: any
  }
}
```

## Parameters

| Parameter name | Type     | Description                       |
| -------------- | -------- | --------------------------------- |
| `value`        | `any`    | The current value of the field.   |
| `fieldProps`   | `Object` | Props of the current field.       |
| `fields`       | `Object` | Map of all fields.                |
| `form`         | `Object` | A reference to the current `Form` |

## Example

```jsx
import React from 'react'
import { Form } from 'react-advanced-form'
import { Input } from 'react-advanced-form-addons'

export default class Example extends React.Component {
  validateUsername = ({ value, fieldProps, fields, form }) => {
    return fetch('https://check.if/user/exists', {
      method: 'POST',
      body: JSON.stringify(value),
    })
    .then(res => res.json())
    .then((res) => {
      const { statusCode } = res

      return {
        valid: (statusCode === 'SUCCESS')
      }
    })
  }

  render() {
    return (
      <Form>
        <Input
          name="username"
          asyncRule={this.validateUsername} />
      </Form>
    );
  }
}
```


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