if(num<10) num="000"+num;
else if(num<100) num="00"+num;
else if(num<1000) num="0"+num;

function FormatNumberLength(num, length) {
    var r = "" + num;
    while (r.length < length) {
        r = "0" + r;
    }
    return r;
}


FormatNumberLength(10000, 5) outputs '10000'
FormatNumberLength(1000, 5)  outputs '01000'
FormatNumberLength(100, 5)   outputs '00100'
FormatNumberLength(10, 5)    outputs '00010'

String.prototype.padLeft = function (length, character) {
     return new Array(length - this.length + 1).join(character || '0') + this;
}

var num = '12';

alert(num.padLeft(4, '0'));

function FormatInteger(num, length) {

    return (num / Math.pow(10, length)).toFixed(length).substr(2);
}

function PadDigits(input, totalDigits)
{
    var result = input;
    if (totalDigits > input.length)
    {
        for (i=0; i < (totalDigits - input.length); i++)
        {
            result = '0' + result;
        }
    }
    return result;
}

function prefixZeros(number, maxDigits)
{
    var length = maxDigits - number.toString().length;
    if(length <= 0)
    	return number;

    var leadingZeros = new Array(length + 1);
    return leadingZeros.join('0') + number.toString();
}
//Call it like prefixZeros(100, 5); //Alerts 00100