LibJS: Implement the "Change Array by copy" stage 3 proposal

[linusg]

Part 1: Array.prototype functions

[davidot]

Nice work, couldn't really find anything but tiny nitpicks. And one commit order problem

Also while you're at it you could rename groupBy to group which was another new thing in the last meeting

[Lubrsi]

Looks good from having a quick glance over. I noticed that the tests don't check the contents of the original array are unmodified.

[Lubrsi]

I just noticed while watching the video that this is missing tests for unscopable for all of the functions except with. Here's an example one:

serenity/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.groupBy.js

Lines 73 to 81 in c10d48b

	test("is unscopable", () => {
	expect(Array.prototype[Symbol.unscopables].groupBy).toBeTrue();
	const array = [];
	with (array) {
	expect(() => {
	groupBy;
	}).toThrowWithMessage(ReferenceError, "'groupBy' is not defined");
	}
	});

[Lubrsi]

Same for null and undefined this values, which ToObject throws on:

serenity/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.groupBy.js

Lines 12 to 24 in c10d48b

	test("null or undefined this value", () => {
	expect(() => {
	Array.prototype.groupBy.call();
	}).toThrowWithMessage(TypeError, "ToObject on null or undefined");
	expect(() => {
	Array.prototype.groupBy.call(undefined);
	}).toThrowWithMessage(TypeError, "ToObject on null or undefined");
	expect(() => {
	Array.prototype.groupBy.call(null);
	}).toThrowWithMessage(TypeError, "ToObject on null or undefined");
	});

[linusg]

Thanks for the review both! I think I have all suggestions implemented 👍