array flatten

const flatten = (array) => {
	let flattened = [];
	for (const item of array) {
  	flattened = flattened.concat(Array.isArray(item) ? flatten(item) : item);
  }
  return flattened;
};

console.log(flatten([[1,2,[3]],4])); // [1, 2, 3, 4]