app.use(async function(req, res, next) {
try {
var myres = await new Promise((resolve, reject) => {
mysql_connection.query(\"select * from
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);
}
});
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
})
Using async function to be able to use await for DB queries is totally okay. But try to embed that code in try and catch block so you can catch the exceptions.