leftpad.js

/**
 * url:http://blog.h5jun.com/post/left-pad.html
 */
if (!String.prototype.repeat) {
  String.prototype.repeat = function(count) {
    "use strict";
    if (this == null) {
      throw new TypeError("can\"t convert " + this + " to object");
    }
    var str = "" + this;
    count = +count;
    if (count != count) {
      count = 0;
    }
    if (count < 0) {
      throw new RangeError("repeat count must be non-negative");
    }
    if (count == Infinity) {
      throw new RangeError("repeat count must be less than infinity");
    }
    count = Math.floor(count);
    if (str.length == 0 || count == 0) {
      return "";
    }
    // Ensuring count is a 31-bit integer allows us to heavily optimize the
    // main part. But anyway, most current (August 2014) browsers can"t handle
    // strings 1 << 28 chars or longer, so:
    if (str.length * count >= 1 << 28) {
      throw new RangeError("repeat count must not overflow maximum string size");
    }
    var rpt = "";
    for (;;) {
      if ((count & 1) == 1) {
        rpt += str;
      }
      count >>>= 1;
      if (count == 0) {
        break;
      }
      str += str;
    }
    // Could we try:
    // return Array(count + 1).join(this);
    return rpt;
  }
}

function leftpad(str, len, ch){
  str = "" + str;
  ch = ("" + ch) || " ";
  var padlen = len - str.length;
  if(padlen <= 0){ 
    return str;
  }else{
    return ch.repeat(padlen) + str;
  }
}