Duplicate identifier 'Promise' (TS2529) too restrictive.

[electricessence]

Error:(8, 9) TS2529:Duplicate identifier 'Promise'. Compiler reserves name 'Promise' in top level scope of a module containing async functions.

TypeScript Version: 2.1.1 (targeting ES5)
Code

import {Promise} from "myOwnPromiseLib";

What I'm confused about is when I look at the __awaiter code it expects Promise (P) to be available.
In a modular environment, how else am I supposed to inject my own Promise lib so it can use it?

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
 return new (P || (P = Promise))(function (resolve, reject) {
 function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
 function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
 function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
 step((generator = generator.apply(thisArg, _arguments)).next());
 });
};

[electricessence]

For my current case, I'm able to get this working by:

export interface Awaiter
{
 (thisArg:any, _arguments:any[], P:PromiseConstructorLike, generator:Function):void;
}

export function awaiter(
 thisArg:any,
 _arguments:any[],
 P:PromiseConstructorLike,
 generator:Function)
{
 if(!P) throw "Must provide Promise constructor. Try injecting Promise using awaiter.factory(PromiseConstructorLike).";
 return new P((resolve, reject) =>
 {
 const g = generator = generator.apply(thisArg, _arguments);
 step(g.next());

 function fulfilled(value:any)
 {
 try
 { step(g.next(value)); }
 catch(e)
 { reject(e); }
 }

 function rejected(value:any)
 {
 try
 { step(g["throw"](value)); }
 catch(e)
 { reject(e); }
 }

 function step(result:any)
 {
 result.done
 ? resolve(result.value)
 : new P(resolve => { resolve(result.value); }).then(fulfilled, rejected);
 }
 });
}

export module awaiter
{
 export function factory(Promise:PromiseConstructorLike):Awaiter
 {
 return (thisArg:any, _arguments:any[], P:PromiseConstructorLike, generator:Function) =>
 {
 awaiter(thisArg, _arguments, P || Promise, generator);
 };
 }
}

[yortus]

For background on why this was done, see #6068 and #6631. I personally agree it's too restrictive but those links explain the justification for the current behaviour.

[electricessence]

Troy Gerwien (@yortus) Ok that makes sense. And apparently this may change once generators are supported in es5?
But what I'm really confused about for es5, is (because I've been living in module land for so long) how to I specify Promise as a global so it can be consumed by the default __awaiter function?

Ok, why not simply DI it like what I pasted above? I know what I have here isn't perfect, but it's working for me so it's possible that each module can decide it's own promise lib.

[yortus]

how to I specify Promise as a global so it can be consumed by the default __awaiter function?

There are two ways:

1. Polyfill Promise

1. use a Promise polyfill so Promise is globally defined at runtime (e.g. es6-promise or similar)
2. Ensure the TypeScript project includes ambient Promise typings, (e.g. use option "lib": ["es2015.promise"])

2. Annotate all your async functions

e.g.,

// Only works with target <= ES5
import BluebirdPromise = require('bluebird');
async function myFunc(): BluebirdPromise {
 await someOp();
}

The type annotation BluebirdPromise will be emitted into a value position in the .js file and passed to __awaiter, so your async function will use and return a bluebird promise in this case.

I'm not a fan of the second approach for reasons listed here.

[electricessence]

Ok, now I'm more interested how I can simply keep using my own helpers and not require a global reference.

I've got this working the way I want at the moment, but now I'm getting:

Error:TS2318:Cannot find global type 'Promise'.

[yortus]

Did you try "lib": ["es2015.promise"] in your tsconfig?

[electricessence]

I could do that but I'm not using es2015.promise. Does it end up just being a hack so I can ignore?

[yortus]

Are you polyfilling Promise at runtime so it's globally available as Promise? If so, the lib option will just tell the compiler that a global Promise exists and prevent the TS2318 error, and it should both compile and run fine.

If you are not using a global polyfill but importing some promise implementation locally, then I think you must either:

• a) use the annotation approach to avoid compiler errors, even if you provide your own __awaiter. -or-
• b) use the lib option to prevent TS2318 but then ensure your custom __awaiter ignores it's third argument (the P argument). This is a bit hacky because you are telling the compiler a global Promise exists when it doesn't. It might cause runtime errors elsewhere in your code if it references this phantom global Promise.

[electricessence]

Well now that I've updated to 2.2 dev, it's gotten even weirder:
#12374