Ruby-like Question: Make this function shorter (ActionScript 3)

Question

I just wrote this incredibly verbose code to turn numbers like 2 into 02. Can you make this function shorter, please (maintaning the functionality)?

    public static function format(n:int, minimumLength:int):String {
        var retVal:String = n.toString();
        var stillNeed:int = minimumLength - retVal.length;
        for (var i:int = 0; i < stillNeed; i++) {
            retVal = "0" + retVal;
        }
        return retVal;
    }

Please use types for variables. Extra points (good-vibe points, not SO points) if there's already a built-in function that I don't know about.

If anybody wants to post some extremely short equivalent in some other language, that would be fun too.

Answer 1

This wouldn't be the fastest implementation (it does some unnecessary copying and has a loop), but it is nice and readable:

public static function pad(num:int, minLength:uint):String {
    var str:String = num.toString();
    while (str.length < minLength) str = "0" + str;
    return str;
}

Answer 2

I don't think there is a built-in way, but this might be cleaner (if not necessarily better performing):

//20 zeroes, could be more if needed
public static var Zeroes:String = "00000000000000000000"   

public static function format(n:Number, minimumLength:int):String {
var retVal:String = (n.toFixed(0)); // cut off the decimals
var stillNeed:int = minimumLength - retVal.length;
retVal = Zeroes.substring(0, stillNeed) + retVal; 
return retVal;
}

The "zeroes" var eliminates the need for looping, just prepend however many zeroes you need from a prebuilt string.