模拟 extjs 底层继承

 // 混合继承
    function Person(name,age) {
        this.name = name;
        this.age = age;
    }
    Person.prototype = {
        constructor : Person,
        sayHello : function () {
            alert("hello");
        }
    }

    function Boy(name,age,sex) {
        //Person.call(this,name,age);
        this.sex = sex;
    }
    Boy.prototype = new Person();           //  虽然 在 new Person 的时候，并没有传入参数，但是 还是实例化了 Person 模板 ， 只是参数的值默认为 undefined

    var b = new Boy("z3",25,"男");
    alert(b.name);          // undefined
    alert(b.sex);           // 男

 // 改进方法 ， 模拟 extjs 底层继承实现方式
    function Person(name,age) {
        this.name = name;
        this.age = age;
    }
    Person.prototype = {
        constructor : Person,
        sayHello : function () {
            alert("hello");
        }
    }

    function Boy(name,age,sex) {
        Boy.superClass.constructor.call(this,name,age);
        this.sex = sex;
    }

    function extend(sub,sup) {
        var F = new Function();
        F.prototype = sup.prototype;
        sub.prototype = new F();
        sub.prototype.constructor = sub;
        sub.superClass = sup.prototype;
        if(sup.prototype.constructor = Object.prototype.constructor){
            sup.prototype.constructor = sup;
        }
    }
    extend(Boy,Person);
    var b = new Boy("z3",25,"男");
    alert(b.name);              // z3