flat.js

export default function flat () {
	var depth = isNaN(arguments[0]) ? 1 : Number(arguments[0]);

	return depth ? Array.prototype.reduce.call(this, function (acc, cur) {
		if (Array.isArray(cur)) {
			acc.push.apply(acc, flat.call(cur, depth - 1));
		} else {
			acc.push(cur);
		}

		return acc;
	}, []) : Array.prototype.slice.call(this);
}

flat.test.js

import flat from '../src/flat';

test('flat() examples', () => {
	// basic nesting
	expect(flat.call([1, 2, [3, 4]], 0)).toEqual([1, 2, [3, 4]]);
	expect(flat.call([1, 2, [3, 4]], 1)).toEqual([1, 2, 3, 4]);
	expect(flat.call([1, 2, [3, 4]])).toEqual([1, 2, 3, 4]);

	// double nesting
	expect(flat.call([[1], [2], [3], [4]])).toEqual([1, 2, 3, 4]);

	// triple nesting
	expect(flat.call([1, 2, [3, 4, [5, 6]]])).toEqual([1, 2, 3, 4, [5, 6]]);
	expect(flat.call([1, 2, [3, 4, [5, 6]]], 0)).toEqual([1, 2, [3, 4, [5, 6]]]);
	expect(flat.call([1, 2, [3, 4, [5, 6]]], 1)).toEqual([1, 2, 3, 4, [5, 6]]);
	expect(flat.call([1, 2, [3, 4, [5, 6]]], 2)).toEqual([1, 2, 3, 4, 5, 6]);

	// complex nesting
	expect(flat.call([1, 2, 3, [4, [5, 6], 7, [8, 9, 10]]], 2)).toEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
	expect(flat.call([[[1]], [2], [3], [4]])).toEqual([[1], 2, 3, 4]);
	expect(flat.call([[[1]], [2], [3], [4]], 2)).toEqual([1, 2, 3, 4]);
});