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