Someone with more JS experience can chime in, but isn't this really inefficient in JavaScript? Wouldn't appending rather than pre-pending be better due to the way strings and memory are handled? Or at the bare minimum create the left padding in the loop and tack on str after? Can you use ch.repeat(len) + str; yet in node or if not just do the same idea of doubling in size ch until len is satisfied?

    while (++i < len) {
      str = ch + str;
    }

And isn't this a bug?

leftpad("x", 2, '00') will return "00x"