# Resolve API URLs

Use `baseUrl` for standard URL resolution; use `prefix` when a leading slash in a request path must stay under the configured API path.

## Choose how relative paths resolve

Set defaults on a [`ky`](https://bench-ky-6l.atloria.app/p/bench-ky-6l-QSDF7DxJ6h/developer/ky#default) instance with `ky.extend()`, then keep request paths relative:

```ts
import ky from 'ky';

const run = async (): Promise<void> => {
	const baseUrlApi = ky.extend({
		baseUrl: 'https://jsonplaceholder.typicode.com/users/',
	});

	const prefixApi = ky.extend({
		prefix: 'https://jsonplaceholder.typicode.com/users',
	});

	const baseUrlResponse = await baseUrlApi.get('1');
	const prefixResponse = await prefixApi.get('/1');

console.log(baseUrlResponse.url);
console.log(prefixResponse.url);
};

void run();
```

Both requests target `https://jsonplaceholder.typicode.com/users/1`. `baseUrl` resolves `'1'` relative to the trailing-slash path. `prefix` joins its path with `'/1'`, trimming the boundary slashes before URL resolution. Ky instances created with `extend()` inherit the parent instance's defaults.

## Pick the option that matches your paths

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `baseUrl` | `URL \| string \| undefined` | No value set | Resolves a relative input as a URL reference. An input beginning with `/` replaces the base URL's path, so a base of `https://example.com/api/` and input `/users` resolves to `https://example.com/users`. |
| `prefix` | `URL \| string \| undefined` | No value set | Joins the prefix and string input before resolving the resulting URL. Leading slashes on the input are trimmed at the join boundary, so `/users` stays under an `/api/` prefix. |

In most cases, choose `baseUrl`: it follows standard URL resolution. Include a trailing slash in a base URL that has a path, such as `https://example.com/api/`, so a page-relative input like `'users'` extends that path. Choose `prefix` when your callers use origin-relative paths such as `'/users'` but you want those paths appended under the API prefix.

An absolute input bypasses `baseUrl`. `prefix` applies only when the input is a string; when the input is a `Request`, both `baseUrl` and `prefix` are ignored.

## Related

- [Ky quick start](https://bench-ky-6l.atloria.app/p/bench-ky-6l-QSDF7DxJ6h/developer/ky-quick-start)
- [Instances and defaults](https://bench-ky-6l.atloria.app/p/bench-ky-6l-QSDF7DxJ6h/developer/instances-and-defaults)