Improve Header's forEach method compliance with browser implementation. (#1074)

* Improve Header's forEach method compliance with browser implementation.

* Replaced callback.call with Reflect.apply on Headers' forEach.

* Set undefined as default value for Header' forEach second argument.

* Rewrote test case where 'this' is undefined.

* Ignored lint issues (no-eq-null and eqeqeq).

src/headers.js

@@ -167,9 +167,9 @@ export default class Headers extends URLSearchParams {
 		return value;
 	}
 
-	forEach(callback) {
+	forEach(callback, thisArg = undefined) {
 		for (const name of this.keys()) {
-			callback(this.get(name), name);
+			Reflect.apply(callback, thisArg, [this.get(name), name, this]);
 		}
 	}
 

src/request.js

@@ -82,6 +82,7 @@ export default class Request extends Body {
 			signal = init.signal;
 		}
 
+		// eslint-disable-next-line no-eq-null, eqeqeq
 		if (signal != null && !isAbortSignal(signal)) {
 			throw new TypeError('Expected signal to be an instanceof AbortSignal or EventTarget');
 		}

test/headers.js

@@ -53,6 +53,37 @@ describe('Headers', () => {
 		]);
 	});
 
+	it('should be iterable with forEach', () => {
+		const headers = new Headers();
+		headers.append('Accept', 'application/json');
+		headers.append('Accept', 'text/plain');
+		headers.append('Content-Type', 'text/html');
+
+		const results = [];
+		headers.forEach((value, key, object) => {
+			results.push({value, key, object});
+		});
+
+		expect(results.length).to.equal(2);
+		expect({key: 'accept', value: 'application/json, text/plain', object: headers}).to.deep.equal(results[0]);
+		expect({key: 'content-type', value: 'text/html', object: headers}).to.deep.equal(results[1]);
+	});
+
+	it('should set "this" to undefined by default on forEach', () => {
+		const headers = new Headers({Accept: 'application/json'});
+		headers.forEach(function () {
+			expect(this).to.be.undefined;
+		});
+	});
+
+	it('should accept thisArg as a second argument for forEach', () => {
+		const headers = new Headers({Accept: 'application/json'});
+		const thisArg = {};
+		headers.forEach(function () {
+			expect(this).to.equal(thisArg);
+		}, thisArg);
+	});
+
 	it('should allow iterating through all headers with for-of loop', () => {
 		const headers = new Headers([
 			['b', '2'],