var number = 1310;

var number = 120;

var number = 10;

var number = 7;

function pad_with_zeroes(number, length) {

    var my_string = '' + number;
    while (my_string.length < length) {
        my_string = '0' + my_string;
    }

    return my_string;

}

var number = 1310;
var number = 120;
var number = 10;
var number = 7;

// only needs to happen once
var zeroString = "00000";

// one assignment gets the padded number
var paddedNum = zeroString.substring((number + "").length, 4) + bareNum;

//output
alert("The padded number string is: " + paddedNum);

var bareNum = 42 + '';
var zeroString = "000000000000000";
var paddedNum = zeroString.substring(bareNumber.length, 14) + bareNum

//to: 0 - to left, 1 - to right
String.prototype.pad = function(_char, len, to) {
    if (!this || !_char || this.length >= len) {
        return this;
    }
    to = to || 0;

    var ret = this;

    var max = (len - this.length)/_char.length + 1;
    while (--max) {
        ret = (to) ? ret + _char : _char + ret;
    }

    return ret;
};

someString.pad(neededChars, neededLength)

'332'.pad('0', 6); //'000332'
'332'.pad('0', 6, 1); //'332000'

function pad(n, len) {
  return (new Array(len + 1).join('0') + n).slice(-len);
}

// You want to pad four places:
>>> var N = Math.pow(10, 4)
>>> var number = 1310
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"1310"
>>> var number = 120
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"0120"
>>> var number = 10
>>> number < N ? ("" + (N + number)).slice(1) : "" + number
"0010"
…

/**
 * Pad a number with leading zeros to "pad" places:
 *
 * @param number: The number to pad
 * @param pad: The maximum number of leading zeros
 */
function padNumber(number, pad) {
    var N = Math.pow(10, pad);
    return number < N ? ("" + (N + number)).slice(1) : "" + number
}

Array(5).join("0").slice(0,4-number.length)+ number

S('123').padLeft(5, '0').s   --> 00123