Is there an equivalent to “sealed” or “final” in TypeScript?

Question

I\'m trying to implement a method in a super class that should be available for use, but not changeable, in sub classes. Consider this:

export abstract clas         


        

Answer 1

No, at the time of this writing there is not. There is a proposal for such a keyword which is still being considered, but may or may not ever be implemented.

See:

• github.com/Microsoft/TypeScript/issues/9264, and
• github.com/Microsoft/TypeScript/issues/8306

Answer 2

// I use this workaround:

export default class CreateHandler extends BaseHandler {
    // final prop used as method
    public readonly create = (blueprint: Blueprint): Response<Record> => {
        return blueprint.create();
    };

    // normal method
    public store(blueprint: Blueprint): Response<Record> {
        return this.response(blueprint.create());
    }
}

Answer 3

Example of implementation hack of 'sealed method' as readonly property of type function which throws compiler error when attempting to override in extended class:

abstract class BaseClass {
    protected element: JQuery<HTMLElement>;
    constructor(element: JQuery<HTMLElement>) {
        this.element = element;
    }
    readonly public dispose = (): void => {
        this.element.remove();
    }
}

class MyClass extends BaseClass {
    constructor(element: JQuery<HTMLElement>) {
        super(element);
    }
    public dispose(): void { } // Compiler error: "Property 'dispose' in type 'MyClass' is not assignable to the same property in base type 'BaseClass'"
}

---

TypeScript 2.0 supports "final" classes through using of private constructor:

class A {
    private constructor(){}
}

class B extends A{} //Cannot extend a class 'A'. Class constructor is marked as private.ts(2675)