js 笔试题 多次bind重写

Question

function foo() {

    return this.bar;

}

foo = foo.bind({bar:1});

foo = foo.bind({bar:2});

foo();

要让这里的foo()输入2，我的答案是：

Function.prototype.bind = function(obj,arg){

    var that = this;

    return function(){

     return that.call(obj,arg);

    }

};

function foo() {

    return this.bar;

}

foo = foo.bind({bar:1});

foo = foo.bind({bar:2});

foo();

然而输出1