Require url to be an instance of URL when paginating

Fixes #1818

Author: szmarczak

documentation/4-pagination.md

@@ -129,6 +129,10 @@ It should return an object representing Got options pointing to the next page. I
 The options are merged automatically with the previous request.\
 Therefore the options returned by `pagination.paginate(…)` must reflect changes only.
 
+**Note:**
+> - The `url` option (if set) accepts **only** a [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL) instance.\
+>   This prevents `prefixUrl` ambiguity. In order to use a relative URL string, merge it via `new URL(relativeUrl, response.url)`.
+
 #### `filter`
 
 **Type: `Function`**\

source/core/options.ts

@@ -692,7 +692,7 @@ const defaultInternals: Options['_internals'] = {
 
 			if (next) {
 				return {
-					url: new URL(next.reference, response.requestUrl),
+					url: new URL(next.reference, response.url),
 				};
 			}
 

source/create.ts

@@ -201,6 +201,8 @@ const create = (defaults: InstanceDefaults): Got => {
 			} else {
 				normalizedOptions.merge(optionsToMerge);
 
+				assert.any([is.urlInstance, is.undefined], optionsToMerge.url);
+
 				if (optionsToMerge.url !== undefined) {
 					normalizedOptions.prefixUrl = '';
 					normalizedOptions.url = optionsToMerge.url;

test/pagination.ts

@@ -575,7 +575,7 @@ test('next url in json response', withServer, async (t, server, got) => {
 				}
 
 				return {
-					url: next,
+					url: new URL(next),
 					prefixUrl: '',
 					searchParams: undefined,
 				};
@@ -728,6 +728,20 @@ test('retrieves all elements - relative url', withServer, async (t, server, got)
 	t.deepEqual(result, [1, 2]);
 });
 
+test('throws if url is not an instance of URL', withServer, async (t, server, got) => {
+	attachHandler(server, 2);
+
+	await t.throwsAsync(got.paginate.all<number>('', {
+		pagination: {
+			paginate: () => ({
+				url: 'not an instance of URL',
+			}),
+		},
+	}), {
+		instanceOf: TypeError,
+	});
+});
+
 test('throws when transform does not return an array', withServer, async (t, server) => {
 	server.get('/', (_request, response) => {
 		response.end(JSON.stringify(''));