Async/await in Express routing

Issue

The good practice is to use the latest standards of language. In JavaScript, standard ES7 saw the light of day in 2016. Using express, we may want to use async/await feature, but how?

Express route

Let’s create some express route f.ex user.

const getUser = async (req, res, next) => {
    const { id } = req.params;
    const user = await User.findById(id); // f.ex. mongoose findById method
};

router.get("/user/:id", getUser);

Above, we’ve defined simple route for /user/:id". Our async/await is not completed yet, because we are not catching any errors in getUser. Let’s add a try statement to our example.

const getUser = async (req, res, next) => {
    const { id } = req.params;
    try {
        const user = await User.findById(id); // f.ex. mongoose findById method
    } catch(e) {
        // here goes out error handler
    }
}

router.get("/user/:id", getUser);

Now our getUser function will catch errors in the catch block, but it’s not a perfect solution. I can’t imagine using async/await with try/catch in whole application… We can create some middleware to remove the try/catch statement.

const asyncMiddleware = fn => (req, res, next) => {
    Promise.resolve(fn(req, res, next))
        .catch(next);
};

const getUser = async (req, res, next) => {
    const { id } = req.params;
    const user = await User.findById(id); // f.ex. mongoose findById method
}

router.get("/user/:id", asyncMiddleware(getUser));

Above, we’ve declared our asyncMiddleware. Inside this function, we are just resolving getUser. If any of await statements returns us a rejected promise, then it will go into catch function.

What are the advantages?

1. Of course, this is syntactic sugar. We can write asynchronous code which looks like synchronous.
2. In our asyncMiddleware, we can handle all errors from our routes callbacks async functions and move them to some error middleware with next.