Express middleware with async operation that happens only once - node.js

Question

I am trying to append some data to my request object using a middleware, but I want to do it only once the server is up.
So I tried doing it with a middleware, while trying to use a function's context, but it's a bit problematic to perform such an action on a middleware, because I cannot pass a promise as a middleware.
This is what I'm trying to do:
const setupData = async () => {
const data = await getSomeData();
return (req, res, next) => {
req.data = data;
next();
}
}
app.use(setupData());
I tried using the solution suggested here, but it won't work as this will happen on every request.
Any idea how can I go around this? I can always put the info on a global var, but I would like to avoid it.
I also saw some in-memory packages to help with it (such as node-cache), but I would like to do it with a middleware.
Thanks in advance

Answer 1

Just cache the result using a normal variable:
let data = null;
function setupData (req, res, next) {
if (data !== null) {
req.data = data;
next();
}
else {
getSomeData().then(result => {
data = result
req.data = data;
next();
});
}
}
app.use(setupData);
This is the minimal, least complicated implementation. You can of course refactor it to be much DRYer and less prone to mistakes by taking out the caching logic:
Cleaner Implementation
let cache = null;
async function getCachedData() {
if (cache === null) {
cache = await getSomeData();
}
return cache;
}
Which makes setupData much cleaner:
function setupData (req, res, next) {
getCachedData().then(data => {
req.data = data;
next();
});
}
Either way, the cache is triggered on the first request. This means that there is a possibility that a second request may arrive before the data is possibly cached. So at startup the getSomeData() function may run more than once.
Really call getSomeData() ONLY ONCE
If you really want to call getSomeData only once you must call it before setting up Express:
async function main () {
const data = await getSomeData();
const app = express();
//
// set up express middlewares...
//
app.use((req,res,next) => {
req.data = data;
next();
});
//
// set up routes...
//
app.listen(config.port);
}
main(); // start everything
The key here is to realize that we have been trying to do everything backwards: to set up a constant value asynchronously AFTER starting to set up Express. The natural flow of the program wants the constant value to exist BEFORE we begin setting up Express so we only perform everything else inside the async function (here called main). Not try to run the async function while setting up Express.

Answer 2

You can do it without async:-
const setupData = (req, res, next) => {
// You can put a condition here so that it runs only once
getSomeData().then((data) => {
req.app.locals.data = data //The data can be accessed in the next middleware using req.app.locals
next();
}).catch((error) => {
console.log("Error Occured");
res.status(400).end("Error Occurred");
})
}
app.use(setupData);
You should see the documentation of getSomeData and see how it works