fix(headers): clone getSetCookie list & add getSetCookie type (#1917)

lib/fetch/headers.js

@@ -392,7 +392,13 @@ class Headers {
     // 2. Return the values of all headers in this’s header list whose name is
     //    a byte-case-insensitive match for `Set-Cookie`, in order.
 
-    return this[kHeadersList].cookies ?? []
+    const list = this[kHeadersList].cookies
+
+    if (list) {
+      return [...list]
+    }
+
+    return []
   }
 
   // https://fetch.spec.whatwg.org/#concept-header-list-sort-and-combine

test/fetch/headers.js

@@ -665,3 +665,21 @@ tap.test('invalid headers', (t) => {
 
   t.end()
 })
+
+tap.test('Headers.prototype.getSetCookie', (t) => {
+  t.test('Mutating the returned list does not affect the set-cookie list', (t) => {
+    const h = new Headers([
+      ['set-cookie', 'a=b'],
+      ['set-cookie', 'c=d']
+    ])
+
+    const old = h.getSetCookie()
+    h.getSetCookie().push('oh=no')
+    const now = h.getSetCookie()
+
+    t.same(old, now)
+    t.end()
+  })
+
+  t.end()
+})

test/types/fetch.test-d.ts

@@ -168,4 +168,6 @@ expectType<Response>(response.clone())
 
 expectType<Request>(new Request('https://example.com', { body: 'Hello, world', duplex: 'half' }))
 expectAssignable<RequestInit>({ duplex: 'half' })
-expectNotAssignable<RequestInit>({ duplex: 'not valid' })
+expectNotAssignable<RequestInit>({ duplex: 'not valid' })
+
+expectType<string[]>(headers.getSetCookie())

types/fetch.d.ts

@@ -59,6 +59,7 @@ export declare class Headers implements SpecIterable<[string, string]> {
   readonly get: (name: string) => string | null
   readonly has: (name: string) => boolean
   readonly set: (name: string, value: string) => void
+  readonly getSetCookie: () => string[]
   readonly forEach: (
     callbackfn: (value: string, key: string, iterable: Headers) => void,
     thisArg?: unknown