[promise-function-async] False positives with functions that return any

[sindresorhus]

Repro

{
  "rules": {
    "typescript/promise-function-async": "error"
  }
}

function trim(query) {
	return query.trim();
}

Expected Result

I expected the rule to only report on functions that return a promise.

Actual Result

It reports on functions returning any.

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	1.5.0
@typescript-eslint/parser	1.5.0
TypeScript	3.3.3333
ESLint	5.15.3
node	10.15.1
npm	6.9.0

[j-f1]

This is technically correct since an object of type any could be a Promise. Perhaps there should be an option allowAny, on by default, that allows any to be used here?

[sindresorhus]

While it might be technically correct, I don't see how it's ever wanted behavior for this rule. It would mean you would have to add async to anything that returns any, even originally synchronous functions.

[ThomasdenH]

Does the lint trigger on function a(): Promise<string> | string? I think the rule should only trigger when it definitely only returns promises, not when it could return a promise.

Edit: Never mind. This rule intends to solve functions 'either returning a Promise or something else' as well as 'returning a rejected Promise and throwing an error'.

If a function returns 'any', the user disables type checking in Typescript and relies on manually checked logic. I think the expected behaviour is thus that it doesn't lint in the specific case of any.

[j-f1]

That’s why i suggested adding an option that would allow any by default but let the user disallow it. However, if it’s very unlikely to be a useful option, there’s no need to add it.

[G-Rath]

I've got the same problem with axios & class getters:

class Mine {
  public get t() {
    return Axios;
  }
}

This gets flagged as should be marked with async (which is actually not allowed with get).

[scottohara]

Also seeing this in a webpack.config.js that has functions that return various configurations of webpack plugins, e.g.

(note: this is from a project that has a mixed codebase of JS and TS files, and the webpack config is Javascript)

function extractCss(hashFilename) {
	return new MiniCssExtractPlugin({filename: hashFilename ? "[name]-[chunkhash:6].css" : "[name].css"});
}

function defineAppConfig(maxDataAgeDays) {
	return new webpack.DefinePlugin({MAX_DATA_AGE_DAYS: maxDataAgeDays});
}

Both of the above functions trigger @typescript-eslint/promise-function-async, but as far as I can tell neither of these plugin constructors return any or Promise types?

[burtek]

I'm getting similar error on v1.13.0

import { Reducer } from 'redux';
const reducer: Reducer<Record<string, string>> = (state = {}, _action) => state;

Versions

package	version
@typescript-eslint/eslint-plugin	1.13.0
@typescript-eslint/parser	1.13.0
TypeScript	3.5.3
ESLint	6.1.0
node	10.14.2
npm	6.4.1

[bradzacher]

have you tried the allowAny: false option?
Also if you try the 2.0 canary version, it should have the above option as the default value.

[bradzacher]

(closing as this didn't get closed when #553 was merged)