Question

In the following code, why does instanceof return false for both Shape and Rectangle? Also why do the own properties of rec include both x and y from the superclass?

    function Shape(x, y) {
        this.x=x;
        this.y=y;
    }
    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 = Object.create(Shape.prototype);
    Rectangle.prototype.area = function() {
        return this.w * this.h;
    };
    var rec = new Rectangle(0,0,10,10);
    console.log("instanceof = " + rec instanceof Shape);
    console.log("instanceof = " + rec instanceof Rectangle);
    rec.move(2,3);
    console.log("area = " + rec.area());
    console.log(Object.getOwnPropertyNames(rec));

Answer 1

Because + is evaluated before instanceof. So you’re asking if:

"instanceof = " + rec

…a String, is an instanceof your constructors, which it won’t be.

Either add parenthesis to force the order:

console.log("instanceof = " + (rec instanceof Shape));

Or, since console.log accepts any number of arguments, pass it as its own:

console.log("instanceof = ", rec instanceof Shape);