CODEWITHSUNDEEP
javascript
Edward Tanguay
Edward Tanguay

Reputation: 193312

Why doesn't .__proto__ = .prototype in this example?

I understand that "__proto__ is an internal property of an object, pointing to its prototype" so in the following example I would think that c2.prototype would equal c2.__proto__. Why do they not have the same value?

<!DOCTYPE html>
<html>
    <head>
        <script type="text/javascript">
            window.onload = function() {
                var Circle = function(radius) {
                    this.radius = radius;
                    this.doubleRadius = function() {
                        return this.radius * 2;
                    }
                }

                var c1 = new Circle(4);

                Circle.prototype.area = function() {
                    return Math.PI*this.radius*this.radius;
                }

                var c2 = new Circle(5);

                console.log('--- first circle object, created before adding "area" method');
                console.log(c1.radius);
                console.log(c1.doubleRadius());
                console.log(c1.area());

                console.log('--- second circle object, created after adding "area" method');
                console.log(c2.radius);
                console.log(c2.doubleRadius());
                console.log(c2.area());

                console.log(c2.prototype); // undefined
                console.log(c2.__proto__); // Object { area=function() }

            }
        </script>
    </head>
<body>
</body>
</html>

Upvotes: 3

Views: 180

Answers (3)

xdazz
xdazz

Reputation: 160853

Try the below.

console.log(c2.constructor.prototype);
console.log(c2.__proto__);

Acturly, .__proto__ == .constructor.prototype when c2 is a object.

Upvotes: 1

Ruan Mendes
Ruan Mendes

Reputation: 92274

The simple answer is that c2.constructor.prototype == c2.__proto__

Constructors have a .prototype property. Instances don't, but they do have .__proto__ and .constructor properties

Upvotes: 3

Engineer
Engineer

Reputation: 48793

obj.__proto__ is short version of obj.constructor.prototype, not of obj.prototype:

console.log(c2.constructor.prototype === c2.__proto__);   //true
console.log(c2.prototype === c2.__proto__);   //false

Upvotes: 2

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