next.js/docs/01-app/03-api-reference/03-file-conventions/default.mdx

---
title: default.js
description: API Reference for the default.js file.
related:
  title: Learn more about Parallel Routes
  links:
    - app/api-reference/file-conventions/parallel-routes
---

The `default.js` file is used to render a fallback within [Parallel Routes](/docs/app/api-reference/file-conventions/parallel-routes) when Next.js cannot recover a [slot's](/docs/app/api-reference/file-conventions/parallel-routes#slots) active state after a full-page load.

During [soft navigation](/docs/app/getting-started/linking-and-navigating#client-side-transitions), Next.js keeps track of the active _state_ (subpage) for each slot. However, for hard navigations (full-page load), Next.js cannot recover the active state. In this case, a `default.js` file can be rendered for subpages that don't match the current URL.

Consider the following folder structure. The `@team` slot has a `settings` page, but `@analytics` does not.

<Image
  alt="Parallel Routes unmatched routes"
  srcLight="/docs/light/parallel-routes-unmatched-routes.png"
  srcDark="/docs/dark/parallel-routes-unmatched-routes.png"
  width="1600"
  height="930"
/>

When navigating to `/settings`, the `@team` slot will render the `settings` page while maintaining the currently active page for the `@analytics` slot.

On refresh, Next.js will render a `default.js` for `@analytics`. If `default.js` doesn't exist, a `404` is rendered instead.

Additionally, since `children` is an implicit slot, you also need to create a `default.js` file to render a fallback for `children` when Next.js cannot recover the active state of the parent page.

## Reference

### `params` (optional)

A promise that resolves to an object containing the [dynamic route parameters](/docs/app/api-reference/file-conventions/dynamic-routes) from the root segment down to the slot's subpages. For example:

```tsx filename="app/[artist]/@sidebar/default.js" switcher
export default async function Default({
  params,
}: {
  params: Promise<{ artist: string }>
}) {
  const { artist } = await params
}
```

```jsx filename="app/[artist]/@sidebar/default.js" switcher
export default async function Default({ params }) {
  const { artist } = await params
}
```

| Example                                    | URL          | `params`                                     |
| ------------------------------------------ | ------------ | -------------------------------------------- |
| `app/[artist]/@sidebar/default.js`         | `/zack`      | `Promise<{ artist: 'zack' }>`                |
| `app/[artist]/[album]/@sidebar/default.js` | `/zack/next` | `Promise<{ artist: 'zack', album: 'next' }>` |

- Since the `params` prop is a promise. You must use `async/await` or React's [`use`](https://react.dev/reference/react/use) function to access the values.
  - In version 14 and earlier, `params` was a synchronous prop. To help with backwards compatibility, you can still access it synchronously in Next.js 15, but this behavior will be deprecated in the future.