iterative implementation of array flattening in javascript

flatten_iterative.js

var flatten_v1 = function(array){
  var indices = [array.length - 1];
  var arrays = [array];

  var output = [];

  while(indices.length){
    if(indices[0] === -1){
      indices.shift();
      arrays.shift();
    } else if(arrays[0][indices[0]] instanceof Array){
      indices[0]--;
      arrays.unshift(arrays[0][indices[0] + 1]);
      indices.unshift(arrays[1][indices[0] + 1].length - 1);
    } else {
      output.unshift(arrays[0][indices[0]]);
      indices[0]--;
    }
  }

  return output;
}

var flatten_v2 = function(array){
  var indices = [];
  var arrays = [];

  var currentArray = array;
  var currentIndex = currentArray.length - 1;

  var output = [];

  while(currentIndex !== -1 || indices.length > 0){
    if(currentIndex === -1){
      currentIndex = indices.shift();
      currentArray = arrays.shift();
    } else if(currentArray[currentIndex] instanceof Array){
      arrays.unshift(currentArray);
      currentArray = currentArray[currentIndex];

      indices.unshift(currentIndex - 1);
      currentIndex = currentArray.length - 1;
    } else {
      output.unshift(currentArray[currentIndex]);
      currentIndex--;
    }
  }

  return output;
};

var flatten_v3 = function(array){
  var currentArray = array;
  var currentIndex = 0;

  // these are the indices of the unflattened arrays, 
  //    indices[n] = index state for unflattened array in output[n]
  var indices = [];

  // output will have the unflattened arrays at the beginning, everything
  //    afterwards is a result of the flattening process
  var output = [];

  while(currentIndex !== currentArray.length || indices.length > 0){
    if(currentIndex === currentArray.length){
      currentIndex = indices.shift();
      currentArray = output.shift();
    } else if(currentArray[currentIndex] instanceof Array){
      output.unshift(currentArray);
      currentArray = currentArray[currentIndex];

      indices.unshift(currentIndex + 1);
      currentIndex = 0;
    } else {
      output.push(currentArray[currentIndex]);
      currentIndex++;
    }
  }

  return output;
};

console.log(flatten_v1([ 1, 2, [ 3, 4, [ 5, 6 ], 7 ], 8, 9, [ 10, 11, [ 12, [ [ 13 ] ]]]]));
console.log(flatten_v2([ 1, 2, [ 3, 4, [ 5, 6 ], 7 ], 8, 9, [ 10, 11, [ 12, [ [ 13 ] ]]]]));
console.log(flatten_v3([ 1, 2, [ 3, 4, [ 5, 6 ], 7 ], 8, 9, [ 10, 11, [ 12, [ [ 13 ] ]]]]));