Find unique range numbers
If what you want is to call the same function with the same array and obtain random numbers from that array every time, but without them repeating, well, that's almost literally the definition of "shuffle".

Have a look at this answer on stackoverflow. The idea in your case is that you shuffle the array and pick the first (or last) n elements.

That means something like (almost entirely copied from linked answer):

function shuffle(array) {
  var currentIndex = array.length, temporaryValue, randomIndex;

  while (0 !== currentIndex) {
    randomIndex = Math.floor(Math.random() * currentIndex);
    currentIndex -= 1;
    temporaryValue = array[currentIndex];
    array[currentIndex] = array[randomIndex];
    array[randomIndex] = temporaryValue;
  }

  return array;
}

var arr = [];
for (var i = 10; i &lt;= 20; i++) {
   arr.push(i);
}
shuffle(arr);
console.log(arr.slice(0,3));
console.log(arr.slice(3,6));


The advantage is that you need to randomize only once, no need to loop through the array and generate random numbers every time.

The disadvantage is that if you want different numbers, you'll have to keep track of the index by yourself, because requesting arr.slice(0,3) will always return the same numbers. 
But considering that you'd have to keep track of which numbers have already been used anyway, this should not be a problem.

Also, this may actually not be a disadvantage depending on what you use the function for.
(Of course you could shuffle every time, but that would lose any speed benefit of shuffling).

Another approach would be to shuffle once and then having a function that returns the first n numbers but also removes them from the list.
The obvious disadvantage in that case would be that you have to keep a copy of your original array, and when the shuffled array is empty, copy it and shuffle it again.

If you still want to use your approach (which works anyway), at least make sure to use meaningful variable names. In my opinion not is probably the worst possible name for a variable.