proposal-class-modifiers

An ES proposal to add class-level inheritance modifiers.

Motivation

The latest proposals for ES include data properties for class. If this encourages an increase in developers from class-based languages adopting ES for various uses, they will also want to be able to control the inheritability of the classes they write.

Goal

The idea is to add abstract and final as class modifiers. These prefixes will prevent instantiation and inheritance respectively. They are mutually exclusive as the only type of class that is usefully neither instantiable nor inheritable is one with only static data and methods.

Behavior

• abstract - causes Reflect.construct and new to throw an exception if target.prototype === newTarget.prototype.
• final - causes a derived class definition to throw an exception if it extends the target class.

Examples

abstract class Base {
    constructor() {
        console.log("abstract class 'Base' was instantiated!");
    }
}

new Base(); //throws exception

final class Derived extends Base {
    constructor() {
        constructor.log("final class 'Derived' was instantiated!");
    }
}

new Derived(); //Prints both console.log messages

class C extends Derived {}; //throws exception

abstract final class D {}; //throws exception