> ## Documentation Index
> Fetch the complete documentation index at: https://docs.qu.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# FetchPreflightFunc

```ts theme={null}
type FetchPreflightFunc: (req) => Promise<FetchRequest>;
```

Called before any network request, allowing updated headers (e.g. Bearer tokens), etc.

## Parameters

| Parameter | Type                                                |
| :-------- | :-------------------------------------------------- |
| `req`     | [`FetchRequest`](/sdk/content/classes/FetchRequest) |

## Returns

`Promise`\<[`FetchRequest`](/sdk/content/classes/FetchRequest)>

## Source

[utils/fetch.ts:52](https://github.com/dominant-strategies/quais.js/blob/c1c12d43f9d34c6baad2b0542bd6d0acd6fefcbf/src/utils/fetch.ts#L52)