Allow input start with slash

by replacing slashes at the start of the input

readme.md

@@ -167,7 +167,7 @@ Accepts any value supported by [`URLSearchParams()`](https://developer.mozilla.o
 
 Type: `string | URL`
 
-A prefix to prepend to the `input` URL when making the request. It can be any valid URL, either relative or absolute. A trailing slash `/` is optional and will be added automatically, if needed, when it is joined with `input`. Only takes effect when `input` is a string. The `input` argument cannot start with a slash `/` when using this option.
+A prefix to prepend to the `input` URL when making the request. It can be any valid URL, either relative or absolute. A trailing slash `/` is optional and will be added automatically, if needed, when it is joined with `input`. Only takes effect when `input` is a string.
 
 Useful when used with [`ky.extend()`](#kyextenddefaultoptions) to create niche-specific Ky-instances.
 

source/core/Ky.ts

@@ -146,9 +146,7 @@ export class Ky {
 		}
 
 		if (this._options.prefixUrl && typeof this._input === 'string') {
-			if (this._input.startsWith('/')) {
-				throw new Error('`input` must not begin with a slash when using `prefixUrl`');
-			}
+			this._input = this._input.replace(/^\/+/, '');
 
 			if (!this._options.prefixUrl.endsWith('/')) {
 				this._options.prefixUrl += '/';

source/types/options.ts

@@ -66,7 +66,7 @@ export type KyOptions = {
 	searchParams?: SearchParamsOption;
 
 	/**
-	A prefix to prepend to the `input` URL when making the request. It can be any valid URL, either relative or absolute. A trailing slash `/` is optional and will be added automatically, if needed, when it is joined with `input`. Only takes effect when `input` is a string. The `input` argument cannot start with a slash `/` when using this option.
+	A prefix to prepend to the `input` URL when making the request. It can be any valid URL, either relative or absolute. A trailing slash `/` is optional and will be added automatically, if needed, when it is joined with `input`. Only takes effect when `input` is a string.
 
 	Useful when used with [`ky.extend()`](#kyextenddefaultoptions) to create niche-specific Ky-instances.
 

test/prefix-url.ts

@@ -27,14 +27,5 @@ test('prefixUrl option', async t => {
 	t.is(await ky('', {prefixUrl: `${server.url}/`}).text(), 'zebra');
 	t.is(await ky('', {prefixUrl: new URL(server.url)}).text(), 'zebra');
 
-	t.throws(
-		() => {
-			void ky('/unicorn', {prefixUrl: `${server.url}/api`});
-		},
-		{
-			message: '`input` must not begin with a slash when using `prefixUrl`',
-		},
-	);
-
 	await server.close();
 });