重写bind方法

var that = null;
Function.prototype.bind = function(obj,arg){
    if (that == null){
        that = this
    }
    return function(){
        return that.call(obj,arg);

    }
};
function foo() {
    return this.bar;
}

foo = foo.bind({bar:1});
foo = foo.bind({bar:2});
var result = foo();
console.log(result);//2