# Extendable Promise A promise that can be delayed (extended) via repeated calls to `waitUntil`. "Extendable" here refers to the ability to extend the duration of the promise, not inheritance/subtypes/etc. The name and API are modelled after the [`ExtendableEvent`](https://developer.mozilla.org/en-US/docs/Web/API/ExtendableEvent) interface that was introduced to the web part of the Service Worker API. As such, it provides a `waitUntil` method that takes a promise as argument that extends settlement of the parent promise until all promises that were added in this way have settled. You can think about it like `Promise.allSettled`, except you can add promises to the list after the fact. Example: ```js const p = new ExtendablePromise(timeout(200).then(() => 1)); await timeout(100) p.waitUntil(timeout(300).then(() => Promise.reject(Error('2')))); const res = await p; // total duration ~400ms (100 + 300) assertEquals(res, [ { status: "fulfilled", value: 1 }, { status: "rejected", reason: Error('2') }, ]); ``` After all promises have settled, further calls to `waitUntil` become no-ops. The constructor takes a promise as argument, which sets the minimum duration for the extendable promise. If an extendable promise is created without supplying an initial promise, it is "idle" (`await`ing it pauses execution) until the first call to `waitUntil` "primes" it. Initializing it with a synchronous value or a settled promise immediately settles the extendable promise with that value and all calls to `waitUntil` are no-ops. -------- <p align="center"><a href="https://workers.tools"><img src="https://workers.tools/assets/img/logo.svg" width="100" height="100" /></a> <p align="center">This module is part of the Worker Tools collection<br/>⁕ [Worker Tools](https://workers.tools) are a collection of TypeScript libraries for writing web servers in [Worker Runtimes](https://workers.js.org) such as Cloudflare Workers, Deno Deploy and Service Workers in the browser. If you liked this module, you might also like: - 🧭 [__Worker Router__][router] --- Complete routing solution that works across CF Workers, Deno and Service Workers - 🔋 [__Worker Middleware__][middleware] --- A suite of standalone HTTP server-side middleware with TypeScript support - 📄 [__Worker HTML__][html] --- HTML templating and streaming response library - 📦 [__Storage Area__][kv-storage] --- Key-value store abstraction across [Cloudflare KV][cloudflare-kv-storage], [Deno][deno-kv-storage] and browsers. - 🆗 [__Response Creators__][response-creators] --- Factory functions for responses with pre-filled status and status text - 🎏 [__Stream Response__][stream-response] --- Use async generators to build streaming responses for SSE, etc... - 🥏 [__JSON Fetch__][json-fetch] --- Drop-in replacements for Fetch API classes with first class support for JSON. - 🦑 [__JSON Stream__][json-stream] --- Streaming JSON parser/stingifier with first class support for web streams. Worker Tools also includes a number of polyfills that help bridge the gap between Worker Runtimes: - ✏️ [__HTML Rewriter__][html-rewriter] --- Cloudflare's HTML Rewriter for use in Deno, browsers, etc... - 📍 [__Location Polyfill__][location-polyfill] --- A `Location` polyfill for Cloudflare Workers. - 🦕 [__Deno Fetch Event Adapter__][deno-fetch-event-adapter] --- Dispatches global `fetch` events using Deno’s native HTTP server. [router]: https://workers.tools/router [middleware]: https://workers.tools/middleware [html]: https://workers.tools/html [kv-storage]: https://workers.tools/kv-storage [cloudflare-kv-storage]: https://workers.tools/cloudflare-kv-storage [deno-kv-storage]: https://workers.tools/deno-kv-storage [kv-storage-polyfill]: https://workers.tools/kv-storage-polyfill [response-creators]: https://workers.tools/response-creators [stream-response]: https://workers.tools/stream-response [json-fetch]: https://workers.tools/json-fetch [json-stream]: https://workers.tools/json-stream [request-cookie-store]: https://workers.tools/request-cookie-store [extendable-promise]: https://workers.tools/extendable-promise [html-rewriter]: https://workers.tools/html-rewriter [location-polyfill]: https://workers.tools/location-polyfill [deno-fetch-event-adapter]: https://workers.tools/deno-fetch-event-adapter Fore more visit [workers.tools](https://workers.tools).