abstract-final-class.md

Abstract and Final Classes

Abstract Class

class AbstractClass {
    constructor() {
        if (new.target === AbstractClass) {
            throw new TypeError('Abstract class AbstractClass cannot be instantiated');
        }
    }
}

class ConcreteClass extends AbstractClass {}

const concreteInstance = new ConcreteClass();
// OK

const abstractInstance = new AbstractClass();
// TypeError: Abstract class AbstractClass cannot be instantiated

Final Class

class FinalClass {
    constructor() {
        if (new.target !== FinalClass) {
            throw new TypeError('Final class FinalClass cannot be extended');
        }
    }
}

class ConcreteClass extends FinalClass {}

const concreteInstance = new ConcreteClass();
// TypeError: Final class FinalClass cannot be extended

const finalInstance = new FinalClass();
// OK

Old Fashioned Way (Before ES6)

• Abstract Class

var AbstractClass = function () {
    if (this.constructor === AbstractClass) {
        throw new TypeError('Abstract class AbstractClass cannot be instantiated');
    }
    
    this.method = function () {};
};

• Final Class

var FinalClass = function () {
    if (this.constructor !== FinalClass) {
        throw new TypeError('Final class FinalClass cannot be extended');
    }
    
    this.method = function () {};
};