## LoaderStart event

Triggered when the loader UI is mounted to the DOM and ready to be displayed.

See also: [Elements methods](/js/custom_checkout/custom_checkout_elements.md) | [Listen to Elements events](/js/custom_checkout/element_events.md)

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

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

- `handler` (function) **required**
  `handler(event) => void` is a **callback function** that you provide that will be called when the event is fired.

When called it will be passed an event object with the following properties:

  - `elementType` (string)
    The type of element that emitted this event.

### Handle a Checkout Element loaderstart event

```js
const paymentElement = checkout.createPaymentElement();
paymentElement.on('loaderstart', function(event) {
  // Handle loaderstart event
});
```

```es_next
const paymentElement = checkout.createPaymentElement();
paymentElement.on('loaderstart', (event) => {
  // Handle loaderstart event
});
```