[chvid]

You need to go something like:

  module.exports = function leftpad (str, len, ch) { return Array(Math.max(0, len - String(str).length)).join(ch || ' ') + String(str); };

Unfortunately we need to wrap str twice so maybe a one-liner is not quite in place.

[vespakoen]

The array + join is slower

http://jsperf.com/leftpadtesting

Repeat + slice is too

http://jsperf.com/leftpad

[vespakoen]

So this I guess

module.exports = function leftpad (str, len, ch) { str = String(str); if (ch === 0) { ch = '0'; } return Array(Math.max(0, len - str.length)).join(ch || ' ') + str; };

[vespakoen]

Cool, I like the Math.max. Two liner then str = String(str);\n...

[chvid]

And I did not notice the 0 check in the original code either :-D

[gsb]

Also, this doesn't support zero padding with ch=0.