How to output integers with leading zeros in JavaScript

Question

I can round to x amount of decimal places with math.round but is there a way to round left of the decimal? for example 5 becomes 05 if I specify 2 places

Answer 1

You're asking for zero padding? Not really rounding. You'll have to convert it to a string since numbers don't make sense with leading zeros. Something like this...

function pad(num, size) {
    var s = num+"";
    while (s.length < size) s = "0" + s;
    return s;
}

Or if you know you'd never be using more than X number of zeros this might be better. This assumes you'd never want more than 10 digits.

function pad(num, size) {
    var s = "000000000" + num;
    return s.substr(s.length-size);
}

If you care about negative numbers you'll have to strip the "-" and readd it.

Answer 2

Another approach:

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

zeroPad(5, 2); // "05"
zeroPad(5, 4); // "0005"
zeroPad(5, 6); // "000005"
zeroPad(1234, 2); // "1234" :)

Answer 3

Just for fun (I had some time to kill), a more sophisticated implementation which caches the zero-string:

pad.zeros = new Array(5).join('0');
function pad(num, len) {
    var str = String(num),
        diff = len - str.length;
    if(diff <= 0) return str;
    if(diff > pad.zeros.length)
        pad.zeros = new Array(diff + 1).join('0');
    return pad.zeros.substr(0, diff) + str;
}

If the padding count is large and the function is called often enough, it actually outperforms the other methods...