jwmerrill / ts-promise-repro

Example of an async/await type checking inconsistency in typescript.

This project is designed to show an inconsistency in Typescript's type checking of async functions as of version 3.4 when targeting ES5.

Down-leveling async functions requires the Promise constructor to be available in scope. As of version 3.4, Typescript may or may not enforce this depending on the return value of an async function.

Install

npm ci

Repro

npx tsc

Summary

The following code fails to type check as expected

// src/fails.ts
async function helper() {
  await fetch('https://www.example.com');
}

export async function main() {
  await helper();
}

with

error TS2468: Cannot find global value 'Promise'.

src/fails.ts:1:16 - error TS2705: An async function or method in ES5/ES3 requires the 'Promise' constructor.  Make sure you have a declaration for the 'Promise' constructor or include 'ES2015' in your `--lib` option.

1 async function helper() {
                 ~~~~~~

but very similar code that adds a return value to the definition of helper() passes type checking.

// works-1.ts
async function helper() {
  await fetch('https://www.example.com');
  return true;
}

export async function main() {
  await helper();
}

Typescript also fails to reject async functions without a local call-site:

// works-2.ts
export async function main() {
  await fetch('https://www.example.com');
}
```