Suggestion: add flag to notify when awaiting a non-promise

Search Terms

async promise force error

Suggestion

Add a tsconfig.json flag to display info/warning/error when awaiting non-PromiseLike code. This will catch developer errors.

Use Cases

Catch developer errors when writing async code.

Example

The following code is valid:

await Promise.all(urls.map(url => {
  fetch(url);
});
console.log('fetching complete');

The developer intends to do something once all the fetches complete, but since map is not returning any value, it ends up awaiting [undefined, undefined, ...], which means fetching complete is logged before the fetches complete. It would be nice if TS gave an info/warning/error in this case.

The correct code is:

await Promise.all(urls.map(url => {
  return fetch(url);
});
console.log('fetching complete');

This example is paired down to the bare minimum for simplicity, but IRL code is more complex.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

No source file or test is identified in the issue, so start by locating TypeScript's handling of await expressions and tsconfig diagnostics. Done means defining and implementing a configurable info, warning, or error for awaiting non-PromiseLike code, with coverage for the provided Promise.all example.