Add support for passing a function to ky.extend()

readme.md

@@ -510,6 +510,22 @@ console.log('unicorn' in response);
 //=> true
 ```
 
+You can also refer to parent defaults by providing a function to `.extend()`.
+
+```js
+import ky from 'ky';
+
+const api = ky.create({prefixUrl: 'https://example.com/api'});
+
+const usersApi = api.extend((options) => ({prefixUrl: `${options.prefixUrl}/users`}));
+
+const response = await usersApi.get('123');
+//=> 'https://example.com/api/users/123'
+
+const response = await api.get('version');
+//=> 'https://example.com/api/version'
+```
+
 ### ky.create(defaultOptions)
 
 Create a new Ky instance with complete new defaults.

source/index.ts

@@ -17,7 +17,14 @@ const createInstance = (defaults?: Partial<Options>): KyInstance => {
 	}
 
 	ky.create = (newDefaults?: Partial<Options>) => createInstance(validateAndMerge(newDefaults));
-	ky.extend = (newDefaults?: Partial<Options>) => createInstance(validateAndMerge(defaults, newDefaults));
+	ky.extend = (newDefaults?: Partial<Options> | ((parentDefaults: Partial<Options>) => Partial<Options>)) => {
+		if (typeof newDefaults === 'function') {
+			newDefaults = newDefaults(defaults ?? {});
+		}
+
+		return createInstance(validateAndMerge(defaults, newDefaults));
+	};
+
 	ky.stop = stop;
 
 	return ky as KyInstance;

source/types/ky.ts

@@ -83,7 +83,7 @@ export type KyInstance = {
 
 	@returns A new Ky instance.
 	*/
-	extend: (defaultOptions: Options) => KyInstance;
+	extend: (defaultOptions: Options | ((parentOptions: Options) => Options)) => KyInstance;
 
 	/**
 	A `Symbol` that can be returned by a `beforeRetry` hook to stop the retry. This will also short circuit the remaining `beforeRetry` hooks.

test/main.ts

@@ -542,7 +542,7 @@ test('ky.create() does not mangle search params', async t => {
 	await server.close();
 });
 
-test('ky.extend()', async t => {
+const extendHooksMacro = test.macro<[{useFunction: boolean}]>(async (t, {useFunction}) => {
 	const server = await createHttpTestServer();
 	server.get('/', (_request, response) => {
 		response.end();
@@ -551,25 +551,28 @@ test('ky.extend()', async t => {
 	let isOriginBeforeRequestTrigged = false;
 	let isExtendBeforeRequestTrigged = false;
 
+	const intermediateOptions = {
+		hooks: {
+			beforeRequest: [
+				() => {
+					isOriginBeforeRequestTrigged = true;
+				},
+			],
+		},
+	};
+	const extendedOptions = {
+		hooks: {
+			beforeRequest: [
+				() => {
+					isExtendBeforeRequestTrigged = true;
+				},
+			],
+		},
+	};
+
 	const extended = ky
-		.extend({
-			hooks: {
-				beforeRequest: [
-					() => {
-						isOriginBeforeRequestTrigged = true;
-					},
-				],
-			},
-		})
-		.extend({
-			hooks: {
-				beforeRequest: [
-					() => {
-						isExtendBeforeRequestTrigged = true;
-					},
-				],
-			},
-		});
+		.extend(useFunction ? () => intermediateOptions : intermediateOptions)
+		.extend(useFunction ? () => extendedOptions : extendedOptions);
 
 	await extended(server.url);
 
@@ -582,6 +585,60 @@ test('ky.extend()', async t => {
 	await server.close();
 });
 
+test('ky.extend() appends hooks', extendHooksMacro, {useFunction: false});
+
+test('ky.extend() with function appends hooks', extendHooksMacro, {useFunction: false});
+
+test('ky.extend() with function overrides primitives in parent defaults', async t => {
+	const server = await createHttpTestServer();
+	server.get('*', (request, response) => {
+		response.end(request.url);
+	});
+
+	const api = ky.create({prefixUrl: `${server.url}/api`});
+	const usersApi = api.extend(options => ({prefixUrl: `${options.prefixUrl!.toString()}/users`}));
+
+	t.is(await usersApi.get('123').text(), '/api/users/123');
+	t.is(await api.get('version').text(), '/api/version');
+
+	{
+		const {ok} = await api.head(server.url);
+		t.true(ok);
+	}
+
+	{
+		const {ok} = await usersApi.head(server.url);
+		t.true(ok);
+	}
+
+	await server.close();
+});
+
+test('ky.extend() with function retains parent defaults when not specified', async t => {
+	const server = await createHttpTestServer();
+	server.get('*', (request, response) => {
+		response.end(request.url);
+	});
+
+	const api = ky.create({prefixUrl: `${server.url}/api`});
+	const extendedApi = api.extend(() => ({}));
+
+	t.is(await api.get('version').text(), '/api/version');
+	t.is(await extendedApi.get('something').text(), '/api/something');
+
+	{
+		const {ok} = await api.head(server.url);
+		t.true(ok);
+	}
+
+	{
+		const {ok} = await extendedApi.head(server.url);
+		t.true(ok);
+	}
+
+	await server.close();
+});
+
 test('throws DOMException/Error with name AbortError when aborted by user', async t => {
 	const server = await createHttpTestServer();
 	// eslint-disable-next-line @typescript-eslint/no-empty-function