## Checkout events

Listen to Checkout events to respond to changes caused by customer actions on your checkout page.

## Change event

The change event is triggered when [Checkout Session](/js/custom_checkout/session_object.md)
data changes, such as when the customer changes their shipping
address.  The event payload is always a [Checkout Session](/js/custom_checkout/session_object.md)
object.

**Syntax:** `checkout.on(...)`

- `event` ('change') **required**
  The name of the event. In this case, `change`.

- `handler` (function) **required**
  `handler(session) => void` is a **callback function** that
you provide that will be called when the event is fired. When called
it will be passed a [Checkout Session](/js/custom_checkout/session_object.md) object.

### Listen to change events

```js
checkout.on('change', function(session) {
  // Read new data
});
```

```es_next
checkout.on('change', (session) => {
  // Read new data
});
```