# withBaseUrl

## withBaseUrl(baseUrl)

Wraps a fetch function to resolve relative URLs against a base URL. Useful for API clients with a consistent base URL.

```js
import {withBaseUrl} from 'fetch-extras';

const fetchWithBaseUrl = withBaseUrl('https://api.example.com')(fetch);
const response = await fetchWithBaseUrl('/users'); // Requests https://api.example.com/users
const data = await response.json();
```

Only string-based relative URLs are resolved against the base URL. Absolute URLs and URL objects are passed through unchanged. Protocol-relative inputs like `//cdn.example.com/file.js` are rejected to avoid ambiguity about escaping the configured origin. Relative paths are resolved against the base URL's pathname, while query-only and fragment-only inputs keep normal URL semantics:

```js
// Both of these work the same way
const fetch1 = withBaseUrl('https://api.example.com/v1')(fetch);
const fetch2 = withBaseUrl('https://api.example.com/v1/')(fetch);

await fetch1('users'); // https://api.example.com/v1/users
await fetch2('users'); // https://api.example.com/v1/users
await fetch1('?page=2'); // https://api.example.com/v1?page=2
```

Absolute inputs still bypass the base URL:

```js
const fetchWithBaseUrl = withBaseUrl('https://api.example.com')(fetch);

await fetchWithBaseUrl('https://other.example.com/users');
await fetchWithBaseUrl(new URL('https://other.example.com/users'));
await fetchWithBaseUrl(new Request('https://other.example.com/users'));
```

`withBaseUrl()` only resolves URLs. Absolute inputs still go through other wrappers.

Can be combined with other `with*` functions:

```js
import {pipeline, withBaseUrl, withHttpError, withTimeout} from 'fetch-extras';

const fetchWithAll = pipeline(
	fetch,
	withTimeout(5000),
	withBaseUrl('https://api.example.com'),
	withHttpError(),
);

const response = await fetchWithAll('/users');
```

Works with `paginate()`:

```js
import {paginate, withBaseUrl} from 'fetch-extras';

const fetchWithBaseUrl = withBaseUrl('https://api.github.com')(fetch);

for await (const commit of paginate('/repos/sindresorhus/ky/commits', {fetchFunction: fetchWithBaseUrl})) {
	console.log(commit.sha);
}
```

> [!NOTE]
> `withBaseUrl()` is intended for normal relative paths and explicit absolute URLs. Ambiguous `http:` / `https:` shorthand string inputs without `//` are intentionally unsupported.