Flattens a deeply nested array

flatten.js

function flatten(xs, output=[]) {
	xs.map(x => {
		if (Array.isArray(x)) {
			flatten(x, output);
		}
		else {
			output.push(x);
		}
	});

	return output;
}

const a = [[1,2,[3]],4];
const f = flatten(a);