this理解

Function.prototype.bind = function(scope){
  var fn = this;
  return function(){
    return fn.apply(scope);
  }
}

this.x = 9;    // this refers to global "window" object here in the browser
var module = {
  x: 81,
  getX: function() { return this.x; }
};

module.getX(); // 81

var retrieveX = module.getX;
retrieveX();   
// 9

var boundGetX = retrieveX.bind(module);
boundGetX(); // 81

var foo = {
    x: 3
}

var bar = function(){
    console.log(this.x);
}

bar(); // undefined

var boundFunc = bar.bind(foo);

boundFunc(); // 3