flatten an array of arbitrarily nested arrays

flatten.js

let data = [[1,2,[3]],4];

function flatten(array) {
	let res = [];
	for (let item of array) {
		if (Array.isArray(item)) {
			res = res.concat(flatten(item));
		} else {
			res.push(item);
		}
	}
	return res;
}
console.log(flatten(data));