The TypeScript Promise error codes TS2304 and TS2529 are causing confusion among

I came across the code below:

function asyncTask(): Promise<string> {
    return new Promise<string>(resolve => resolve);
}

This code resulted in the following error:

TS2304: cannot find name 'Promise'

To address this issue, I decided to explicitly declare 'Promise' as follows:

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

import * as Promise from 'bluebird';

function asyncTask(): Promise<string> {
    return new Promise<string>(resolve =>resolve);
}

However, this led to a new error:

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

How can I resolve this conflicting situation?

Answer №1

Understanding Promises in TypeScript

In the realm of TypeScript, promises are a valuable tool for handling asynchronous operations. These promises were officially introduced as part of ES6, also known as ECMAScript 2015. However, there are ways to utilize promises in TypeScript without specifically targeting ES6.

Using Promises in TypeScript Without Targeting ES6

If you're looking to access constructors and methods defined in the es2015 specification within your TypeScript codebase, there is a workaround. By configuring TypeScript to target es5, excluding the default library, and manually referencing the necessary libraries, you can achieve the desired functionality.

To implement this approach, your tsconfig file may need to include specific references to the es6 type definitions while setting the target to ES5. This ensures that you have typings for features like Promise constructor and array.includes(), even if they are not natively supported by all browsers.

It's important to note that by taking this route, you may not receive type errors for unsupported browser features. However, if you are primarily targeting modern browsers or utilizing NodeJS where you have control over the runtime environment, this method can be highly effective.

Answer №2

It appears that TypeScript is safeguarding the Promise identifier. To workaround this issue, consider importing it under a different name or utilizing alternate typings. Several recommendations can be found in this TypeScript GitHub problem.

Async / await with Bluebird promises

The problem pertains to using a distinct promise library with Async Await, which explains why the Promise name is being protected. Specifically, when employing Async/Await in TypeScript, the compiler translates it into a Promise in the transpired code, hence defending the Promise constructor from potential overwrite. While there are ways to circumvent this protective measure, it's important to evaluate whether it's the appropriate course of action.

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