Why Doesn't JavaScript's Inheritance Mechanism Automatically Update the Constructor Property?

Understanding Inheritance and the Constructor Property in JavaScript

The provided code snippet demonstrates inheritance in JavaScript. However, it also raises questions about the constructor property and instanceof operator.

Why is the constructor property not updated for b and c?

When setting b.prototype to new a(), the prototype object of a becomes the prototype of b. This doesn't update the constructor property of b, which remains the function b. Similarly, c inherits from b and thus also inherits b's constructor, which ultimately points to a.

Is the inheritance mechanism faulty?

No, the inheritance mechanism is correct. In JavaScript, the constructor property is a property of the prototype object, not the instance itself. When creating an instance, the internal [[proto]] property points to the prototype of the constructor, not the constructor function itself.

Updating the constructor property

To update the constructor property, one can use the following pattern:

function Square(side) {
    if (this instanceof Square) {
        this.side = side;
    } else {
        return new Square(side);
    }
}

Understanding instanceof

The instanceof operator does not rely on the constructor property of the instance. Instead, it checks whether the prototype chain of the instance object contains the prototype of the constructor function. In the given code:

c.prototype = new b();
console.log(new c() instanceof c); // true

Even though the constructor property of (new c()) points to a(), instanceof checks the prototype chain and correctly identifies it as an instance of c because the prototype chain includes c.prototype.

Conclusion

Inheritance in JavaScript involves setting the prototype of the derived constructor to an instance of the base constructor. The constructor property remains a property of the prototype object, and the instanceof operator checks the prototype chain to determine if an instance belongs to a specific constructor.

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