Add prefixUrl option (#11)

Fixes #1

index.js

@@ -87,7 +87,6 @@ const timeout = (promise, ms) => Promise.race([
 
 class Ky {
 	constructor(input, {timeout = 10000, hooks = {beforeRequest: []}, throwHttpErrors = true, json, ...otherOptions}) {
-		this._input = input;
 		this._retryCount = 0;
 
 		this._options = {
@@ -96,7 +95,17 @@ class Ky {
 			retry: 2,
 			...otherOptions
 		};
+		this._options.prefixUrl = String(this._options.prefixUrl || '');
+		this._input = String(input || '');
 
+		if (this._options.prefixUrl && this._input.startsWith('/')) {
+			throw new Error('`input` must not begin with a slash when using `prefixUrl`');
+		}
+		if (this._options.prefixUrl && !this._options.prefixUrl.endsWith('/')) {
+			this._options.prefixUrl += '/';
+		}
+
+		this._input = this._options.prefixUrl + this._input;
 		this._timeout = timeout;
 		this._hooks = hooks;
 		this._throwHttpErrors = throwHttpErrors;

readme.md

@@ -25,6 +25,7 @@ Ky targets [modern browsers](#browser-support). For older browsers, you will nee
 - Retries failed requests
 - JSON option
 - Timeout support
+- URL prefix option
 - Instances with custom defaults
 - Hooks
 
@@ -109,6 +110,26 @@ Shortcut for sending JSON. Use this instead of the `body` option. Accepts a plai
 
 Sets `options.method` to the method name and makes a request.
 
+#### prefixUrl
+
+Type: `string` [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL)
+
+When specified, `prefixUrl` will be prepended to `input`. The prefix can be any valid URL, either relative or absolute. A trailing slash `/` is optional, one will be added automatically, if needed, when joining `prefixUrl` and `input`. The `input` argument cannot start with a `/` when using this option.
+
+Useful when used with [`ky.extend()`](#kyextenddefaultoptions) to create niche-specific Ky-instances.
+
+```js
+// On https://example.com
+
+(async () => {
+	await ky('unicorn', {prefixUrl: '/api'});
+	//=> 'https://example.com/api/unicorn'
+
+	await ky('unicorn', {prefixUrl: 'https://cats.com'});
+	//=> 'https://cats.com/unicorn'
+})();
+```
+
 #### retry
 
 Type: `number`<br>
@@ -153,6 +174,20 @@ Setting this to `false` may be useful if you are checking for resource availabil
 
 Create a new `ky` instance with some defaults overridden with your own.
 
+```js
+// On https://my-site.com
+
+const api = ky.extend({prefixUrl: 'https://example.com/api'});
+
+(async () => {
+	await api.get('/users/123');
+	//=> 'https://example.com/api/users/123'
+
+	await api.get('/status', {prefixUrl: ''});
+	//=> 'https://my-site.com/status'
+})();
+```
+
 #### defaultOptions
 
 Type: `Object`

test/_require.js

@@ -1,6 +1,8 @@
+import {URL} from 'url';
 import fetch, {Headers} from 'node-fetch';
 
 global.window = {
 	fetch,
 	Headers
 };
+global.URL = URL;

test/prefix-url.js

@@ -0,0 +1,34 @@
+import test from 'ava';
+import createTestServer from 'create-test-server';
+import ky from '..';
+
+test('prefixUrl option', async t => {
+	const server = await createTestServer();
+	server.get('/', (request, response) => {
+		if (request.query.page === '/https://cat.com/') {
+			response.end('meow');
+			return;
+		}
+		response.end('zebra');
+	});
+	server.get('/api/unicorn', (request, response) => {
+		response.end('rainbow');
+	});
+
+	t.is(await ky(`${server.url}/api/unicorn`, {prefixUrl: false}).text(), 'rainbow');
+	t.is(await ky(`${server.url}/api/unicorn`, {prefixUrl: ''}).text(), 'rainbow');
+	t.is(await ky('api/unicorn', {prefixUrl: server.url}).text(), 'rainbow');
+	t.is(await ky('unicorn', {prefixUrl: `${server.url}/api`}).text(), 'rainbow');
+	t.is(await ky('unicorn', {prefixUrl: `${server.url}/api/`}).text(), 'rainbow');
+	t.is(await ky('', {prefixUrl: server.url}).text(), 'zebra');
+	t.is(await ky('', {prefixUrl: `${server.url}/`}).text(), 'zebra');
+	t.is(await ky('https://cat.com/', {prefixUrl: new URL(`${server.url}/?page=`)}).text(), 'meow');
+	t.is(await ky(new URL('https://cat.com'), {prefixUrl: `${server.url}/?page=`}).text(), 'meow');
+	t.is(await ky(new URL('https://cat.com'), {prefixUrl: new URL(`${server.url}/?page=`)}).text(), 'meow');
+
+	t.throws(() => {
+		ky('/unicorn', {prefixUrl: `${server.url}/api`});
+	}, '`input` must not begin with a slash when using `prefixUrl`');
+
+	await server.close();
+});