A promise which execution is deferred until awaited

deferred_promise.ts

/** Defers an execution until a promise is awaited */
class DeferredPromise<T = void> extends Promise<T> {
    private executor: () => Promise<T>;
    private resultPromise: Promise<T> | undefined; // Memoize the promise, to avoid executing the executor twice if awaited twice

    constructor(executor: () => Promise<T>) {
        super(() => {
            // Dummy callback to make Promise happy
        });
        this.executor = executor;
    }

    // Overrides Promise's then, so it executes the executor when the promise is awaited.
    public then<TResult1 = T, TResult2 = never>(
        onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | null | undefined,
        onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined
    ): Promise<TResult1 | TResult2> {
        if (!this.resultPromise) {
            this.resultPromise = this.executor();
        }

        return this.resultPromise.then(onfulfilled, onrejected);
    }
}