Question

What is the difference between calling a function call(), and calling implicit constructor of a inherited class?

Example1:

function DepthRectangle(w, h, d){
    Rectangle.call(this, w, h);
    this.depth = d;
}

Example2:

function DepthRectangle(w, h, d){
    this = new Rectangle(w, h);
    this.depth = d;
}

Constructor of Rectangle:

function Rectangle(w, h){
    this.width = w;
    this.height = h;
}

Answer 1

Well, your second example doesn’t work, because the this value is not assignable, it isn’t a valid left-hand side expression for assignments.

But, suppose that you have:

function DepthRectangle(w, h, d){
    var obj = new Rectangle(w, h);
    obj.depth = d;
    return obj;
}

In this case, the difference would be that the object returned by this function will inherit directly from Rectangle.prototype, and not from DepthRectangle.

For example, with the above function:

//...
DepthRectangle.prototype.inherited = "foo";
Rectangle.prototype.inherited = "bar";

var obj = new DepthRectangle(1,2,3); // note: new is not really needed
obj.inherited; // "bar", from Rectangle.prototype
obj instanceof DepthRectangle; // false

In your first example, using the call method will just execute the Rectangle function, but with the this value pointing to the object that was created on DepthRectangle, meaning that this object will in fact still inherit from DepthRectangle.prototype.