js中的delete

delete删除的是构造函数中的属性，不能删除原型对象中的属性

function Foo(){}
    Foo.prototype.bar = 42;
    var foo = new Foo();
    delete foo.bar;
    alert(foo.bar);           // 42
    delete Foo.prototype.bar; // delete it from the prototype Object
    alert(foo.bar);           // undefined