Question

Is there a smart way to convert numbers to string and automatically add zeroes so the length is the same as the maximum?

like this:

for (var i=0; i<30; i++) {
    var c = i.toString();
    if (c.length == 1) {
        c = '0'+=c;
    }
}

But smarter

Answer 1

There’s a million ways, this is one of them, and its actually quite efficient too..

UPDATE 2
Here’s code for the ‘third interpretation of the OP’s question

This will output [“00”, “01”, “02” …. ,”30″]

var max = 30, i = max, maxLength = max.toString().length, num,  zeroes=[];
while (i--) zeroes.push("0");
zeroes = zeroes.join();

for (i=0; i < max ; i++) {
    num = i.toString();
    console.log(zeroes.substring(0, maxLength - num.length) + num);
}

UPDATE 1 adapted the code for both ‘possible’ interpretations of the OP’s question.

If what you want is code that based on n=5 , max = 30 produces 29 “0”s followed by “5” then this is the code you want

var n = 5, max = 30;

var a = n.toString().split(), zeroesToAdd = max - a.length;
while(zeroesToAdd--) a.unshift("0");
var num = a.join("");

alert(num);​

If what you want is code that based on n=5 , max = 30 produces 2 (the length of 30.toString()) “0”s followed by “5” then this is the code you want

var n = 5, maxLength = (30).toString().length;

var a = n.toString().split(), zeroesToAdd = maxLength - a.length;
while(zeroesToAdd--) a.unshift("0");
var num = a.join("");

alert(num);​

The only difference here is in the first line.

These do not use string concatenation (extremely inefficient), instead they use Array.unshift and Array.join