> For the complete documentation index, see [llms.txt](https://docs.mapbox.com/mapbox-gl-js/llms.txt)
# Change and inspect requests with transformRequest
Use the `Map` object's [`transformRequest`](https://docs.mapbox.com/mapbox-gl-js/api/map/#map-parameters) option to run a callback before the map makes any external request. The callback can rewrite the URL, attach headers, or set the `credentials` property for cross-origin requests — useful for self-hosted or third-party resources that need custom authentication or routing.
To change a request, return a [`RequestParameters`](https://docs.mapbox.com/mapbox-gl-js/api/properties/#requestparameters) object (or a `Promise` that resolves to one); return nothing to leave the request unchanged. This example adds a vector tile source hosted outside of Mapbox — the USGS National Hydrography Dataset — and signs every request to that host with an `auth_token` parameter, leaving requests to any other host unchanged.
> **Note**
>
> Mapbox-hosted resources are already signed with your access token, so `transformRequest` is only needed for resources you fetch from elsewhere. This dataset is public and ignores the `auth_token` parameter, which stands in for whatever your own server or third-party provider expects.
> Example code:
**JavaScript**
```html
Change and inspect requests with transformRequest
```