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javascriptecmascript-harmony

what is extends is equal to when desugarified, javascript harmony

Assuming I had an ecmascript 5 class

function vehicle(){
    this.hasWheels=true;
} 

vehicle.prototype.getWheels=function(){return this.haswheels;};

but it was defined using the new ecmascript class syntax, would creating a "car" class that extends vehicle use vehicle.prototype as a prototype or would it use a new instance of vehicle?

Solution

  • would creating a "car" class that extends vehicle use vehicle.prototype as a prototype or would it use a new instance of vehicle?

    Neither. It does not simply extend not the vehicle.prototype, and it does not create an instance (new vehicle).

    Instead, it properly creates a new object that inherits from the vehicle prototype, like

    Car.prototype = Object.create(Vehicle.prototype);

    However, it actually does more than that. In the maximally minimal classes proposal the extendsis defined in terms of the <| prototype operator (archived):

    class Car extends Vehicle { constructor(){} }

    is equivalent to

    const Car = Vehicle <| function Car(){};

    which in ES5 looks like

    function Car(){}
Car.__proto__ = Vehicle; // not so much ES5
Car.prototype = Object.create(Vehicle.prototype);