Express JS use async function on requests

Question

app.use(async function(req, res, next) {
    try {
        var myres = await new Promise((resolve, reject) => {
            mysql_connection.query("select * from Users;", (err, rows) => {
                if (err) {
                    reject(err);
                } else {
                    resolve(rows);
                }
            });
        });
    } catch (error) {
        console.log(error);
    }
});

Question is. using async function to be able to use await for DB queries is ok? i am afraid that it can cause some problems on expressjs side.

Answer 1

async..await is syntactic sugar for promises, and a promise is just a pattern that relies on callbacks. The use of async functions is acceptable wherever they are supported by the environment. async..await is supported since Node.js 7.6.0.

async function always returns a promise. As long as this implicit return value doesn't cause problems, it's perfectly fine everywhere, including Express. It doesn't matter whether it's used for database queries or anything else.

Unless API supports promises, errors should be entirely handled in async function. Function body should be wrapped with try..catch to rule out unhandled rejections which may result in exceptions in future Node versions.

The original code contains no next calls and just suppresses an error. As a rule of thumb, async middleware should be structured like that:

app.use(async function(req, res, next) {
    try {
        ...
        next();
    } catch (error) {
        next(error);
    }
});

Answer 2

Async await can be used with no problem for DB queries. You could use try catch however there is a more elegant solution which allows you to use the error handling middleware which express offers:

You wrap your middleware with this function:

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

Then you can use it in the following manner:

const asyncMiddleware = require('./utils/asyncMiddleware');

router.get('/', asyncMiddleware(async (req, res, next) => {
    /* 
      if there is an error thrown in getUserFromDb, asyncMiddleware
      will pass it to next() and express will handle the error;
    */
    const user = await getUserFromDb({ id: req.params.id })
    res.json(user);
}));

If an error is thrown the control will be handed over to the error handling middleware which is middlware which has four arguments like this:

app.use(function (err, req, res, next) {
   // your error code
})