function flatten(input) {
  var output = [];
  for(var i=0; i<input.length; i++) {
    if(Array.isArray(input[i])) {
      output = output.concat(flatten(input[i]));
    } else {
      output = output.push(input[i]);
  }
  return output;
}

flatten([[1,2,[3]],4]);