Get value from function then make it global - node.js

Question

How can I get the value from inside the function to use it when posting data to database? Here I want to make the value of imageName inside the multer.diskStorage to be global so I can use it when posting the path to database
var imageName;
var storage = multer.diskStorage({
destination: function (req, file, cb) {
cb(null, './public/assets/images');
},
filename: function (req, file, cb) {
if (!file.originalname.match(/\.(png|jpg|jpeg)$/)) {
var err = new Error();
err.code = 'filetype';
return cb(err);
} else {
imageName = Date.now() + '_' + file.originalname;
cb(null, imageName);
}
}
});
router.post('/newsfeeds', parser.single("myfile"), function (req, res) {
newsfeed.path = imageName;
newsfeed.save(function (err) {
});
});

Answer 1

This is a way to implement multer. You can use upload as a function or as a middleware, I use it as a function because it allows me a more detailed handling of errors.
It was not necessary a global variable, in this case at the moment that multer processes the image fills the req.file property with the information of the image, as we overwrite the function filename now in req.file.filename this is the custom value that we need.
const multer = require('multer');
const multerStorage = multer.diskStorage({
destination: function (req, file, cb) {
cb(null, './public/assets/images');
},
filename: function (req, file, cb) {
if (!file.originalname.match(/\.(png|jpg|jpeg)$/)) {
var err = new Error();
err.code = 'filetype';
return cb(err);
} else {
const imageName = Date.now() + '_' + file.originalname;
cb(null, imageName);
}
}
});
const upload = multer({ storage: multerStorage }).single('myfile');
router.post('/newsfeeds',(req, res) => {
upload(req, res, function (err) {
if (err || !req.file) {
// An error occurred while loading the image.
return res.status(400);
}
newsfeed.path = req.file.filename;
newsfeed.save(function (err) {
});
});
});
Here you can find a table with the rest of the properties of the image
that you have after multer processes the image. Documentation

Answer 2

Global variables tend to be frowned upon in the programming community, so you should first ask yourself if it's really what you want to do.
If you do decide it's the best approach, though, in Node you can attach a property to the global variable. In the browser, you can attach it to window.
As for your issue with the order of execution (from the OP's comments), you can add a Promise instead of a value to the global scope. Then, anytime you want that value, call .then() on your value, or run it in an async function with await.