feat: add cookies.setSerialized to create a cookie from a string

.changeset/red-bikes-beg.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: add new method to create cookie from a string

packages/kit/src/exports/public.d.ts

@@ -239,6 +239,17 @@ export interface Cookies {
 		opts: import('cookie').CookieSerializeOptions & { path: string }
 	) => void;
 
+	/**
+	 * Sets a cookie from a string. This will add a `set-cookie` header to the response, but also make the cookie available via `cookies.get` or `cookies.getAll` during the current request.
+	 *
+	 * No default values. It will set only properties you specified in a cookie.
+	 *
+	 * If you do not specify name, value and path, it will throw an error.
+	 * @param cookie the cookie represented as string
+	 * @since 2.21.0
+	 */
+	setFromString: (cookie: string) => void;
+
 	/**
 	 * Deletes a cookie by setting its value to an empty string and setting the expiry date in the past.
 	 *

packages/kit/src/runtime/server/cookie.js

@@ -1,6 +1,7 @@
 import { parse, serialize } from 'cookie';
 import { normalize_path, resolve } from '../../utils/url.js';
 import { add_data_suffix } from '../pathname.js';
+import * as set_cookie_parser from 'set-cookie-parser';
 
 // eslint-disable-next-line no-control-regex -- control characters are invalid in cookie names
 const INVALID_COOKIE_CHARACTER_REGEX = /[\x00-\x1F\x7F()<>@,;:"/[\]?={} \t]/;
@@ -129,6 +130,44 @@ export function get_cookies(request, url) {
 			set_internal(name, value, { ...defaults, ...options });
 		},
 
+		/**
+		 * @param {string} cookie
+		 */
+		setFromString(cookie) {
+			if (cookie === '') {
+				throw new Error('Cannot pass empty string');
+			}
+
+			const parsed = set_cookie_parser.parseString(cookie);
+			const { name, value, path, sameSite, secure, httpOnly, ...opts } = parsed;
+
+			if (name === undefined || value === undefined || path === undefined) {
+				throw new Error('Name, Value and Path are mandatory for cookie to be created.');
+			}
+
+			/**
+			 * @type {true|false|"lax"|"strict"|"none"|undefined}
+			 */
+			const normalized_same_site = (() => {
+				if (sameSite === undefined || typeof sameSite === 'boolean') {
+					return sameSite;
+				}
+				const lower = sameSite.toLowerCase();
+				if (lower === 'lax' || lower === 'strict' || lower === 'none') {
+					return /** @type {"lax"|"strict"|"none"} */ (lower);
+				}
+				return undefined;
+			})();
+
+			this.set(name, value, {
+				...opts,
+				path,
+				sameSite: normalized_same_site,
+				secure: secure ?? false,
+				httpOnly: httpOnly ?? false
+			});
+		},
+
 		/**
 		 * @param {string} name
 		 *  @param {import('./page/types.js').Cookie['options']} options

packages/kit/src/runtime/server/cookie.spec.js

@@ -213,3 +213,43 @@ test("set_internal isn't affected by defaults", () => {
 	expect(cookies.get('test')).toEqual('foo');
 	expect(new_cookies['test']?.options).toEqual(options);
 });
+
+test('no default values when setFromString is called', () => {
+	const { cookies, new_cookies } = cookies_setup();
+	const cookie_string = 'a=b; Path=/;';
+	cookies.setFromString(cookie_string);
+	const opts = new_cookies['a']?.options;
+	assert.equal(opts?.path, '/');
+	assert.equal(opts?.secure, false);
+	assert.equal(opts?.httpOnly, false);
+	assert.equal(opts?.sameSite, undefined);
+});
+
+test('set all parameters when setFromString is called', () => {
+	const { cookies, new_cookies } = cookies_setup();
+	const cookie_string =
+		'a=b; Path=/; Max-Age=3600; Expires=Thu, 03 Apr 2025 00:41:07 GMT; Secure; HttpOnly; SameSite=Strict; domain=example.com';
+	cookies.setFromString(cookie_string);
+	const opts = new_cookies['a']?.options;
+	assert.equal(opts?.path, '/');
+	assert.equal(opts?.secure, true);
+	assert.equal(opts?.httpOnly, true);
+	assert.equal(opts?.sameSite, 'strict');
+	assert.equal(opts?.domain, 'example.com');
+	assert.equal(opts?.maxAge, 3600);
+	assert.isNotNull(opts.expires);
+});
+
+test('throw error when setFromString is called with empty string', () => {
+	const { cookies } = cookies_setup();
+	assert.throws(() => cookies.setFromString(''), 'Cannot pass empty string');
+});
+
+test('throw error when setFromString is called without name, value and path', () => {
+	const { cookies } = cookies_setup();
+	const cookie_string = 'Max-Age=3600; Expires=Thu, 03 Apr 2025 00:41:07 GMT; Secure; HttpOnly;';
+	assert.throws(
+		() => cookies.setFromString(cookie_string),
+		'Name, Value and Path are mandatory for cookie to be created.'
+	);
+});

packages/kit/types/index.d.ts

@@ -221,6 +221,16 @@ declare module '@sveltejs/kit' {
 			opts: import('cookie').CookieSerializeOptions & { path: string }
 		) => void;
 
+		/**
+		 * Sets a cookie from a string. This will add a `set-cookie` header to the response, but also make the cookie available via `cookies.get` or `cookies.getAll` during the current request.
+		 *
+		 * No default values. It will set only properties you specified in a cookie.
+		 *
+		 * If you do not specify name, value and path, it will throw an error.
+		 * @param cookie the cookie represented as string
+		 */
+		setFromString: (cookie: string) => void;
+
 		/**
 		 * Deletes a cookie by setting its value to an empty string and setting the expiry date in the past.
 		 *