steamrollerV1.js

function steamrollArray(arr) {
  while (hasArrElement(arr)){
    arr = expandArrElement(arr);
  }
  return arr;
}

function hasArrElement(arr) {
  for (var i = 0; i < arr.length; i++ ) {
    if (Array.isArray(arr[i])){
      return true;
    }
  }
  return false;
}

function expandArrElement(arr) {
  var returnArr = [];
  while (arr.length > 0) {
    if (Array.isArray(arr[0])){
      var arrElement = arr.shift();
      while (arrElement.length > 0) {
        returnArr.push(arrElement.shift());
      }
    } else {
      returnArr.push(arr.shift());
    }
  }
  return returnArr;
}

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