output.txt

zfill1
it took 198
zfill2
it took 1160
zfill3
it took 39

test.js

function zfill1(number, size) {
  number = number.toString();
  while (number.length < size) number = "0" + number;
  return number;
}

function zfill2(number, places) {
  number = number.toString();
  var zero = places - number.length + 1;
  return new Array(+(zero > 0 && zero)).join("0") + number;
}

function zfill3(number, places) {
  return ('000000'+number).slice(-6);
}

function time(cb) {
  var begin = new Date();
  cb();
  var end = new Date();
  console.info("it took", end - begin);
}

function benchit(fn) {
  time(function() {
    for (var i = 0; i < 100000; ++i) {
      var size = Math.floor(Math.random() * 100);
      fn(i, size);
    }
  });
}

console.info("zfill1");
benchit(zfill1);
console.info("zfill2");
benchit(zfill2);
console.info("zfill3");
benchit(zfill3);