TypeScript type definition for promise.prototype.finally

6
votes

I was using this ES6 Promise compatible finally implementation called promise.prototype.finally in a Node application that I want to convert to TypeScript however there are no typing available for this package that I can find on DefinitelyTyped. In these situations I've written up my own impromptu type definitions for just the subset of functionality that I need but in this case it is a library that modifies the prototype of the Promise object and I haven't encountered any conventional way to represent this in TypeScript. Any ideas?

Possibly related:

3
typescriptes6-promisedefinitelytyped

3 Answers

15
votes

Whilst the answer from Slava is correct, it only deals with the typing of the finally block. To actually incorporate the shim into your code, so you can write p.finally(() => { ... }), you need to invoke the shim.

Unfortunately the typings on DefinitelyTyped do not currently support the shim function so until this is supported, I'd advise to add the typings yourself.

declare interface Promise<T> {
  finally<U>(onFinally?: () => U | Promise<U>): Promise<U>;
}

declare module 'promise.prototype.finally' {
  export function shim(): void;
}

The types are now available. Install with

npm install --save-dev @types/promise.prototype.finally

In your TypeScript code, during application bootstrap, you can then call

import { shim } from 'promise.prototype.finally';
shim();

This will add the finally block to the Promise prototype, allowing you to use finally as required.

11
votes

For anyone wondering how to get this working natively without any shims: as of TS 2.7 it's possible.

Note that TS 2.7 is not yet fully (26. Feb. 2018) ES2018 compatible. While there are still a few things missing, Promise.finally made it into the 2.7 release. Also tsconfig-schema should already accept ES2018 as target but yet TS 2.7 has no knowledge of ES2018. For now to use the new features, such as Promise.finally, which already are in 2.7, you'll have to use "target": "ESNEXT" in your tsconfig.json.

Then you'll be able to write code like this:

myAsyncFunction().then
(
  (var) => console.log("executing then. " + var)
)
.catch
(
  (var) => console.log("executing catch. " + var)
)
.finally
(
  () => console.log("executing finally.")
)

Notice how finally will not take any arguments due to it's nature.

IMPORTANT: while TS will transpile correctly and understand what you're doing you still have to check if your JS-Engine supports Promise.finally. In my case I was using NodeJs 8.x and of course the produced JS was not executable because Promise.Finally is supported starting with in the latest NodeJs 10.x nightly builds Node 10.x (stable), see this link.

1
votes

You can write your own d.ts file and reference it in the tsconfig.json file. Of you do it you can contribute to the DefinitelyTyped git for others like yourself

Update:

If I understood correctly what you mean you can extend the existing Promise class in your own d.ts file. Make the method to be optional so it won't tell you the actual Promise class is not implementing the interface correctly. You need to extend it as an interface.

Your d.ts file should look like this 

declare module 'es6-promise/dist/es6-promise' {
    export interface Promise <R> {
      finally?<U>(onFinally?: () => U | Promise<U>): Promise<U>;
    }
}

And it should work properly...

I created a project for you as an example: promise-extension-typescript-example

I created a pull request to the DefinitelyTyped git repository, hope it will be accepted and you can download it from there...