Intellij Idea warning - "Promise returned is ignored" with aysnc/await

Question

I'm using Express.js in my code with Node.js v7.3. In this I've created a User Router which forwards the requests to my User Controller.

I'm using async/await inside the User Controller to do asynchronous calls. The problem is that IntelliJ gives me a warning saying that

Promise returned from login() is ignored.

The thing is I'm not even returning anything from the login() method.

Here's the code -

UserRouter.js

router.post('/login', function (req, res, next) {
    userController.login(req, res); // I get the warning here
});

UserController.js

exports.login = async function (req, res) {
    try {
        const verifiedUser = await someFunction(req.body.access_code);
        let user = await User.findOrCreateUser(verifiedUser);
        res.status(200).send(user);
    }
    catch (err) {
        res.status(400).send({success: false, error: err});
    }
};

If I write the same login method using native promises only then I don't get this warning. Am I understanding something wrong here or is IntelliJ at fault?

EDIT -

Thanks to @Stephen, I understand that an async function returns a promise but wouldn't it be better if Intellij identifies that nothing is being returned from the async function and doesn't show that warning because when I chain a .then() after the login() function, it provides an undefined object into the then result. It means if we don't return something from the async function explicitly then undefined is returned?

Answer 1

The userController.login() function returns a promise, but you're not doing anything with the result from the promise by utilizing its then() function.

For example:

userController.login(req, res).then(() => {
    // Do something after login is successful.
});

or in the ES2017 syntax:

await userController.login(req, res);

If you don't actually want to do anything there, I guess you can just ignore the warning. The warning is mostly there because not using the then() function on a promise is usually a code smell.

Answer 2

The thing is I'm not even returning anything from the login() method.

A function declared "async" returns a Promise by definition. See for example https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

However the IDEA warning is only an inspection. You can press "alt-enter, right" on the warning and change the inspection level to make the warning go away. The inspection is in the "JavaScript -> Probable bugs" category and is named "Result of method call returning a promise is ignored".

Answer 3

You should use the "void" operator.

From MDN: void is for "evaluating expressions that produce a value into places where an expression that evaluates to undefined is desired."

router.post('/login', function (req, res, next) {
    void userController.login(req, res); // Warning will not be shown now
});

Answer 4

if you are really manic as me and the then() is not required but you need the warning to go away, a possible solution is:

functionWithAsync.error(console.error);