> For the complete documentation index, see [llms.txt](https://redd.gitbook.io/react-advanced-form/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://redd.gitbook.io/react-advanced-form/components/field/callbacks/on-change.md).

# onChange

## Specification

Event handler called on each field value change.

## Definition

```typescript
type OnChange = (params) => void
```

## Parameters

| Parameter name | Type     | Description                        |
| -------------- | -------- | ---------------------------------- |
| `event`        | `Event`  | Native event reference.            |
| `prevValue`    | `any`    | The previous value of a field.     |
| `nextValue`    | `any`    | The next value of a field.         |
| `fieldProps`   | `Object` | Props of the current field.        |
| `fields`       | `Object` | Reference to all fields of a form. |
| `form`         | `Object` | Form component reference.          |

## Default behavior

By default, `onChange` behaves as a callback method, allowing you to base your external logic based on the field updates. This doesn't change the controlled flow of the field.

The same onChange method can be used as the handler when working with controlled fields. In that case it is responsible for updating the data source used for setting a value on the field.

{% content-ref url="/pages/-LMpxPdDjjqDJohvphz-" %}
[Controlled fields](/react-advanced-form/architecture/controlled-fields.md)
{% endcontent-ref %}

## 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 {
  handleUsernameChange = ({
    event,
    nextValue,
    prevValue,
    fieldProps,
    fields,
    form
  }) => {
    // ...
  }

  render() {
    return (
      <Form>
        <Input
          name="username"
          onChange={this.handleUsernameChange}
          required />
      </Form>
    )
  }
}
```