TypeScript Promise TS2304 TS2529

Question

I had the following code:

function asyncFunc1(): Promise<string> {
    return new Promise<string>(x => x);
}

which produced the following error:

TS2304: cannot find name 'Promise'

So I changed it to explicitly declare 'Promise':

///<reference path="../typings/modules/bluebird/index.d.ts" />

import * as Promise from 'bluebird';

function asyncFunc1(): Promise<string> {
    return new Promise<string>(x => x);
}

and now I get the following error:

TS2529: Duplicate identifier 'Promise'. Compiler reserves name 'Promise' in top level scope of a module containing async functions

How can I solve this paradox?

Answer 1

It seems TypeScript is protecting the Promise identifier. Try importing as a name other than Promise, or use a different set of typings. There are a few suggestions in this TypeScript GitHub issue.

Async / await with Bluebird promises

The issue is regarding the use of a different promise library with Async Await, but this very situation is likely why they are protecting the Promise name. Specifically, when we use Async/Await in TypeScript the compiler converts this to a Promise in the emitted code, so the compiler is trying to protect the Promise constructor from being over-written. You can always trick the compiler with this sort of thing, but be sure to ask yourself if it's the right thing to do.