面向对象的JS(4) 继承机制

在JavaScript中有好几种方法都可以实现继承。原型继承——使用prototype作为一种继承机制有许多优点,下面举例:

function Parent() {
    var parentPrivate = "parent private data";
    var that = this;
    this.parentMethodForPrivate = function () {
        return parentPrivate;
    };
    console.log("parent");
}
Parent.prototype = {
    parentData: "parent data",
    parentMethod: function (arg) {
        return "parent method";
    },
    overrideMethod: function (arg) {
        return arg + " overriden parent method";
    }
}

function Child() {
    // super constructor is not called, we have to invoke it
    Parent.call(this);
    console.log(this.parentData);
    var that = this;
    this.parentPrivate = function () {
        return that.parentMethodForPrivate();
    };
    console.log("child");
}
//inheritance
Child.prototype = new Parent(); // parent
Child.prototype.constructor = Child;
//lets add extented functions
Child.prototype.extensionMethod = function () {
    return " child’ s" + this.parentData;
};
//override inherited functions
Child.prototype.overrideMethod = function () {
    //parent’s method is called
    return" Invoking from child " + Parent.prototype.overrideMethod.call(this, "test");
};
var child = new Child(); // parent
// parent data
// child
console.log(child.extensionMethod()); //child’s parent data
console.log(child.parentData); //parent data
console.log(child.parentMethod()); //parent method
console.log(child.overrideMethod()); //Invoking from child test overriden parent method
console.log(child.parentPrivate()); // parent private data
console.log(child instanceof Parent); //true
console.log(child instanceof Child); //true

当访问子对象的成员时，从子对象开始找起,如果找不到则在其原型对象中搜索。如果还找不到，则在其父类对象和原型中搜索。以此类推一直搜索到Object的原型。这种层次被称之为"原型链"。下图描述了这种关系：