Java Script - Adding null value to array, if the element is not present at particular index

You can use the splice() method on the array

var arr=[2,4,6];
var l = arr[arr.length-1];
for(var i=0; i<=l; i++){
    if(arr[i] !== i){
        arr.splice(i, 0, null);
    }
}

Output : [null, null, 2, null, 4, null, 6]

This modifies the original array.

Given you have the only input arr which you want to fill null inside. Try this:

var arr = [2, 4, 6];

var output = [];
while (arr.length>0){
    var first = arr.splice(0,1);
    while (output.length<first[0])
        output.push(null);
    output.push(first[0]);
}

// output should be [null,null,2,null,4,null,6];

You just need to find the max value in the array and then iterate from 0 to that max, checking each value to see if it was present in the source or not:

var arr = [2, 4, 6];

var max = Math.max.apply(Math, arr);
var result = [];
for (var i = 0; i <= max; i++) {
    if (arr.indexOf(i) !== -1) {
        result[i] = i;
    } else {
        result[i] = null;
    }
}

Working demo: http://jsfiddle.net/jfriend00/c7p8mkqy/

As I asked in my comments, I'd like to know what problem you're actually trying to solve because it seems like both the original and the newly created data structures are inefficient structures that could probably use different form of data and work more efficiently. But, we can only help you make a wiser choice if you explain the actual problem, rather just your attempted solution.

Try:

var arr = [2, 4, 6];
var new_arr = [];
var i = 0;

while(i < 7){
	var pos = arr.indexOf(i++);
	new_arr.push(pos !== -1 ? arr[pos] : null)
}

 document.write(JSON.stringify(new_arr, null, 4))

var arr = [2, 4, 6];
var result = new Array(7);
arr.forEach(function(a) { result[a] = a;});

Interesting quiz:

var arr = [2, 4, 6]
var n = 0

while(arr.length > n) {
  if(arr[n] !== n) {
    arr = arr.slice(0,n).concat(null, arr.slice(n))
  }
  n++
}

console.log(arr) // [null, null, 2, null, 4, null, 6]

This approach applies to array consists of random number of sorted integers.

var arr = [2, 4, 6];
var narr = (new Array(arr.sort()[arr.length-1]))
arr.map(function(v){
    narr[v] = v;
}); 
for (var i = 0; i<narr.length; i++) narr[i]||(narr[i]=null);
console.log(narr);

Try splice():

var arr = [2, 4, 6];
var i = 0,
  l = arr[arr.length - 1];
while (i < l) {
  if(i !== arr[i])
    arr.splice(i, 0, null);
  i++;
}
console.log(arr); //[ null, null, 2, null, 4, null, 6 ]

I will write a permanence case for all answers soon.

function createArrayFromArray(array, length) {

var new_array = new Array(length);

   for (var i = 0; i < new_array.length; i++) {
      new_array[i] = null;
   }

  for (var i = 0; i < array.length; i++) {
      new_array[array[i]] = array[i];
   }

   return new_array;
}

console.log(createArrayFromArray(arr, 7)); //[null, null, 2, null, 4, null, 6]

check it...

var arr = new Array(7);

arr[0] = "null";
arr[1] = "null";
arr[2] = 2;
arr[3] = "null";
arr[4] = 4;
arr[5] = "null";
arr[6] = 6;

document.write(arr.toString());