Rule proposal: no-unnecessary-async

[cjoecker]

Description

The rule would be beneficial in avoiding async functions that are, in reality, synchronous, and no async is necessary.
Unnecessary async function breaks unit tests that use fake timers.

Fail

async function getLast(arr: number[]) {
	return arr.at(-1);
}

Pass

function getLast(arr: number[]) {
	return arr.at(-1);
}

async function getLast() {
	return await getLastFromDb();
}

Proposed rule name

no-unnecessary-async

Additional Info

No response

[sindresorhus]

I think it's a good idea in general, but without types, it's really infeasible, and we don't have types available here. It may more appropriate for https://github.com/typescript-eslint/typescript-eslint

[cjoecker]

@sindresorhus isn't it possible for the rule to check if there is an await or new Promise inside the function? If there is none, we can know there is no promise used inside the async function and show the error.

[sindresorhus]

That would miss functions that return a promise:

async function getLast(arr: number[]) {
	return thisReturnsAPromise();
}

[fregante]

Pretty sure this exists and it's disabled in XO.

[fregante]

https://eslint.org/docs/latest/rules/require-await
https://typescript-eslint.io/rules/require-await/

https://github.com/xojs/eslint-config-xo-typescript/blob/9791a067d6a119a21a4db72c02f1da95e25ffbb6/index.js#L590-L592

[cjoecker]

@fregante you are right! Those rules do that, thanks for the hint!