In TypeScript, is there a similar concept to "sealed" or "final" modifiers?

I'm currently working on incorporating a method in a superclass that should be accessible for use, but not modifiable, in subclasses. Take a look at the following:

export abstract class BaseClass {
    universalBehavior(): void {
        doStuff(); // Perform some standardized actions consistently across all subclasses
        specializedBehavior(); // Delegate specialized actions to subclasses
    }

    protected abstract specializedBehavior(): void;
}

My aim is for any subclass of BaseClass to have the option to exclude the implementation of universalBehavior(), and furthermore, not even permitted to provide an implementation. Is this capability not yet feasible in TypeScript? Intellisense gives me errors when I omit the implementation in my subclasses. The most effective approach I've found so far is as follows:

export class SubClass extends BaseClass {
    universalBehavior(): void {
        super.universalBehavior();
    }

    specializedBehavior(): void {
        // Implement specific behavior for the subclass here
    }
}

Evidently, this poses an issue because I must ensure no subclass ever implements universalBehavior() with anything other than a call to super.universalBehavior().

inheritancetypescriptfinaltypescript2.0sealed

Answer №2

Here is an innovative way to utilize the 'sealed method' as a readonly property of type function, demonstrating how it can trigger a compiler error when attempting to override it in an 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'"
}

In TypeScript 2.0, there is support for creating "final" classes by using a private constructor:

class A {
    private constructor(){}
}

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

Answer №3

// Here is a clever trick I like to use:

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

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

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