Pad a number with leading zeroes in Javascript

As far as I have been able to tell there isn’t a built-in way with Javascript to pad a number with leading zeroes to make it a fixed width like with e.g. sprintf in other programming langages. This post looks at a way to pad a number with zeroes with Javascript.

Please note: this doesn’t work for negative numbers; it was designed specifically for padding with leading a zeros a number which will be used in date formats where negatives are not possible (there will be a post later this week about this). I’ll write another post to revise what’s here and will work with negative numbers.

The following function will pad a number with leading zeroes so the resulting string is "length" length. For example if length is 2 and the passed in number is 5 the value returned is 05.

function pad(number, length) {
   
    var str = '' + number;
    while (str.length < length) {
        str = '0' + str;
    }
   
    return str;

}

Some code to test the above:

document.write(pad(1, 1) + '<br />');
document.write(pad(1, 2) + '<br />');
document.write(pad(15, 2) + '<br />');
document.write(pad(1, 3) + '<br />');
document.write(pad(15, 3) + '<br />');
document.write(pad(155, 3) + '<br />');
document.write(pad(1, 4) + '<br />');
document.write(pad(1, 5) + '<br />');

And the resulting output:

1
01
15
001
015
155
0001
00001

On Tuesday I’ll look at a second solution if you only ever need to pad a number to two numbers, like you would for days and months, and hours, minutes and seconds.