Javascript Array flatten(prevous)

Array.prototype.flatten = function(prevous) {
 var flattenedArr = prevous || [];

 for (var i = 0; i < this.length; i++) {
  if(Array.isArray(this[i])) {
   this[i].flatten(flattenedArr);/*  w  ww .  ja v  a2  s  .  c  o m*/
  } else if(this[i] === null) {
  } else {
   flattenedArr.push(this[i]);
  }
 }

 return flattenedArr;
}