How to output numbers with leading zeros in JavaScript?

Is there a way to prepend leading zeros to numbers so that it results in a string of fixed length? For example, 5 becomes "05" if I specify 2 places.

> NOTE: Potentially outdated. ECMAScript 2017 includes String.prototype.padStart.

You'll have to convert the number to a string since numbers don't make sense with leading zeros. Something like this:

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

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 read it.

UPDATE: Small one-liner function using the ES2017 String.prototype.padStart method:

const zeroPad = (num, places) => String(num).padStart(places, '0')

console.log(zeroPad(5, 2)); // "05"
console.log(zeroPad(5, 4)); // "0005"
console.log(zeroPad(5, 6)); // "000005"
console.log(zeroPad(1234, 2)); // "1234"

Another ES5 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" :)

You could extend the Number object:

Number.prototype.pad = function(size) {
    var s = String(this);
    while (s.length < (size || 2)) {s = "0" + s;}
    return s;
}

Examples:

(9).pad();  //returns "09"

(7).pad(3);  //returns "007"

From https://gist.github.com/1180489

function pad(a, b){
  return(1e15 + a + '').slice(-b);
}

With comments:

function pad(
  a, // the number to convert 
  b // number of resulting characters
){
  return (
    1e15 + a + // combine with large number
    "" // convert to string
  ).slice(-b) // cut leading "1"
}

function zfill(num, len) {return (Array(len).join("0") + num).slice(-len);}

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...