eolas/Programming_Languages/NodeJS/Middlewear.md

tags
Programming_Languages backend node-js middlewear

What is middlewear?

• Anything that terminates the req, res cycle counts as middleware. It is basically anything that acts as an intermediary once the request is received but before the resource is sent. A good example would be the app.use(express.json() or app.use(bodyParser.json) functions we call in order to be able to parse JSON that is sent from the client.
• You will most likely have multiple middlewear functions running at once. We call this intermediary part of the cycle the request processing pipeline.
• Generally all middlewear will be added as a property on the Express app instance with the app.use(...) syntax.

Creating custom middlewear functions

Basic schema

app.use((req, res, next) => {
 	// do some middlewear
	next()
})

next

The next parameter is key, it allows Express to move onto the next middlewear function once the custom middlewear executes. Without it, the request processing pipeline will get blocked. Middlewear functions are basically asynchronous requests and as such they use a similar syntax as Promises (e.g then) for sequencing processes.

Example of sequence

app.use((req, res, next) => {
    console.log('Do process A...')	
	next()
})

app.use((req, res, next) => {
    console.log('Do process B...')	
	next()
})

Would return the following once the server starts:

Do process A...
Do process B...

It makes more sense of course to define our middlewear within a function and then pass it as an argument to app.use()

Useful built-in middlewear

express.static()

app.use(express.static())

Allows you to serve static files. Let's say we have a file called something.txt that resides at public/something.txt We can expose this to express with app.use(static('public')). Then if we navigate to localhost:3000/readme.txt the file will be served in the browser. (Not the public subdirectory is not included in the URL, it will be served from root).

express.urlencoded()

app.use(express.urlencoded())

Generally we handle the data of API requests via a JSON body and the express.json() middlewear. However, in cases where the data is sent from the client in the form of key=value&key=value appendages to the request URL, urlencoded allows us to parse them.