arrayFlatten.js

const input = [[1,2,[3]],4];

function arrFlatten() {
  let flat = [];
  for (let i = 0; i < arguments.length; i++) {
    if (arguments[i] instanceof Array) {
      flat.push.apply(flat, arrFlatten.apply(this, arguments[i]));
    } else {
      flat.push(arguments[i]);
    }
  }
  return flat;
}

console.log(arrFlatten(input));