ES6 Promise / Typescript and the Bluebird Promise

Question

I have a nodejs / typescript 2 project and use the es6-promise package. Now i would like to get rid of the extra package because i can target ES6 in typescript directly.

So i removed the es6-promise package and changed the tsconfig.json to target es6.

{
  "compilerOptions": {
    "target": "es6",
    // ...
  }
}

Many 3rd party packages uses the Bluebird promise but the promise definition is incompatible to the default es6 promise as stated on different posts on github

• bluebird 3.0 definifion is not assignable to ES6 Promises
• Provide a way to load Bluebird globally in es6 compilation target.
• Add Symbol.toStringTag to promise instance

So i am getting the following error.

TS2322: Type 'Bluebird' is not assignable to type 'Promise'. Property '[Symbol.toStringTag]' is missing in type 'Bluebird'.

There is a other types package on npm @types/bluebird-global. In one of the blog post a user suggests to use it instead of @types/bluebird but some 3rd party packages (e.g. sequelize typings) reference bluebird and not bluebird-global so i get another error for the missing typings of bluebird.

What is a good solution to get this to work?

Answer 1

I was dealing with

TS2322: Type 'Bluebird' is not assignable to type 'Promise'. Property '[Symbol.toStringTag]' is missing in type 'Bluebird'.

and found this thread: https://github.com/DefinitelyTyped/DefinitelyTyped/issues/10801

The TL;DR; version of it is to do one of the following:

1. In each of your .ts entry files add the following to overwrite the global promise:

   import * as Bluebird from 'bluebird';

   declare global { export interface Promise<T> extends Bluebird<T> {} }

Or

2. wrap all promises in a Bluebird promise constructor. There is some runtime overhead here and it is listed as an anti pattern on Bluebird's site.

As an aside, I could not get the second option to work, but the first worked fine for me.

Answer 2

Since there is no [Symbol.toStringTag] in Bluebird, it's incompatible indeed. There are other things that differ Bluebird implementation from native Promise - scheduler, error handling... The correct way to handle this is:

const promise: Promise<type> = Promise.resolve<type>(bluebirdPromise);

If runtime compatibility is not an issue for sure, this can be addressed with type casting only in relatively type-safe manner:

const promise: Promise<type> = <Promise<type>><any><Bluebird<type>>bluebirdPromise;

or

const promise: Promise<type> = <Promise<type>><PromiseLike<type>>bluebirdPromise;