为什么在JavaScript中将object和Constructor.prototype设置为Base？

Question

Here is my little program. When I check the value of rec in the debug mode, the object is Base { x=0, y=0, w=10, more...}. Should it be Rectangle? Also the constructor.prototype is Base. Why not Shape?

    function Base() {
    }

    function Shape(x, y) {
        this.x = x;
        this.y = y;
    }

    Shape.prototype = new Base();
    Shape.prototype.move = function(x, y) {
        this.x += x;
        this.y += y;
        console.log("x = " + this.x + " y = " + this.y);
    };

    function Rectangle(x, y, w, h) {
        Shape.call(this, x, y);
        this.w = w;
        this.h = h;
    }

    Rectangle.prototype = new Shape();
    Rectangle.prototype.area = function() {
        return this.w * this.h;
    };
    var rec = new Rectangle(0, 0, 10, 10);
    console.log(instanceOf(rec, Rectangle));

    function instanceOf(object, constructor) { 
        while (object != null) {
            if (object == constructor.prototype)
                return true;
            if ( typeof object == 'xml') {
                return constructor.prototype == XML.prototype;
            }
            object = object.__proto__;
        }
        return false;
    }

Answer 1

Have a look at Why [not] to use the new keyword here? . You might not use it and create a new instance of it, but rather just inherit from Base.prototype .

Also the constructor.prototype is Base. Why not Shape?

I'm not sure which constructor you are referring to here:

• The constructor property of all your objects is Base , as all of them inherit this prototype from the Base.prototype object. You did not overwrite it after setting up the inheritance chains. It is not really necessary, but good style: Shape.prototype.constructor = Shape and Rectangle.prototype.constructor = Rectangle - where those prototype objects are the overwritten ones which inherit from Base .

• The constructor parameter of your instanceOf function. You pass in Rectangle there, so constructor.prototype is the prototype object of Rectangle , which inherits from Base but is different.

When I check the value of rec in the debug mode, the object is Base { x=0, y=0, w=10, more...}

Usually not. Is Base something special, eg a host object? Your rec object is an instance of Base , so it might be displayed differently because of that.

rec is just an object which inherits from Rectangle.prototype which inherits from Shape.prototype which inherits from Base.prototype which inherits from​… Assuming Base is the function you defined, from Object.prototype which inherits from null .