## Apply a promotion code

Use this method to apply a promotion code that your customer enters.

This method returns a `Promise` that resolves with an object containing the following fields:
* `type`: one of either `"success"` or `"error"`
* `session`: only populated when `type` is `"success"`. Contains a [Session](/js/custom_checkout/session_object) instance representing the updated state.
* `error`: only populated when `type` is `"error"`. Contains an object with a string `message` field which can be displayed to your customer.

- `promotionCode` (string) **required**
  The promotion code to apply to the Checkout Session.

### Apply a promotion code

```jsx
const checkoutState = useCheckout();
if (checkoutState.type === 'success') {
  const {applyPromotionCode} = checkoutState.checkout;
  applyPromotionCode('PROMOTION_CODE');
}
```