polyfill of bind in javascript

bindPolyfill.js

Function.prototype.myBind = function() {
    var arg1 = [].slice.call(arguments); 
    var fn = this; 
    var that = arg1[0]; 
    var param = arg1.slice(1); 
    return function() {
      return fn.apply(that, param.concat(arguments));
    }
  }
  
  
// Example of working correctly   
x = 9;
var module = {
    x: 81,
    getX: function () {
        return this.x;
    }
};

module.getX(); // 81

var getX = module.getX;
getX(); // 9, because in this case, "this" refers to the global object

// create a new function with 'this' bound to module
var boundGetX = getX.myBind(module);
var boundGetXBind = getX.bind(module);
boundGetX(); // 81
boundGetXBind(); // 81