Simple string padding for JS (like Ruby ljust and rjust)

string-padding.js

String.prototype.rJust = function(padLen, padStr) {
  return (new Array(padLen).join(padStr) + this.valueOf()).slice(-padLen);
};

String.prototype.lJust = function(padLen, padStr) {
  return (this.valueOf() + new Array(padLen).join(padStr)).substring(0, padLen);
};

/**
 * > "6969".lJust(10, "0")
 * < "6969000000"
 * > "6969".rJust(10, "0")
 * < "0000006969"
 */