Is there an equivalent to "sealed" or "final" in TypeScript?

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

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)

// 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());    }}