flatten

var testArray = [[1,2,[3]],4];

function flat(array) {
  var result = [];
  for(var i=0; i < array.length; i++) {
    if(array[i] instanceof Array) {
      result.push(...flat(array[i]));
    } else {
      result.push(array[i]);
    }
  }
  return result;
}
console.log(flat(testArray));