shiftZeros.js

function shiftZeros(arr) {
  var len = arr.length-1,
      lastidx = len,
      ZERO = 0;

  for (var i = 0; i <= len; i++) {    
    if (lastidx <= i) break;

    while (arr[lastidx] === ZERO) {
      lastidx--;
    }
    
    var el = arr[i];

    if (el === ZERO) {
      arr[i] = arr[lastidx];
      arr[lastidx] = el;
    }   
  }
  
  return arr
}

shiftZeros([1, 2, 3, 0, 4, 0, 0, 1, 0, 0]); // [1, 2, 3, 1, 4, 0, 0, 0, 0, 0]