Comment by venomsnake

  module.exports = leftpad;
   function leftpad (str, len, ch) {
     str = String(str);
     var i = -1;
     if (!ch && ch !== 0) ch = ' ';
     len = len - str.length;
     while (++i < len) {
      str = ch + str;
     }
   return str;
  }

Isn't that the least efficient way to do that function? Prepending a string has always been very expensive operation.

Calculating needed length. Using repeat and just concatenating 2 strings would be faster.