This program converts nested arrays to flat array structure

flatArray.js

// JSBIN link - http://jsbin.com/wedoqeyezi/edit?js,output  (EDIT - http://jsbin.com/kuvepo/edit?js,console)
// The below works because the first iteration find this elements and checks whether they are array or not if not then it simply
// pushes the element to the flatArray else if they are array they are sent to recursive function for further processing till a
// value(not array) is reached and then finally pushed to flatArray.
var arr = [[1,2,[3]],4];
flatArray = [];
for(var i=0;i<arr.length;i++){
  if(arr[i].length === undefined){
    flatArray.push(arr[i]);
  }
  else{
    recursive(arr[i]);
  }
}
function recursive(arrRec){
 
  for(var j=0;j<arrRec.length;j++){
  if(arrRec[j].length === undefined){
    flatArray.push(arrRec[j]);
  }
  else{
    recursive(arrRec[j]);
  }
}
}
console.log(arr);
console.log(flatArray);