How To Make Middleware In Node.js

Notice the call above to next(). Calling this function invokes the next middleware function in the app. The next() function is not a part of the Node.js or Express API but is the third argument that is passed to the middleware function. The next() function could be named anything, but by convention it is always named “next”. To avoid confusion, always use this convention.

To load the middleware function, call app.use(), specifying the middleware function. For example, the following code loads the myLogger middleware function before the route to the root path (/).

const express = require('express')
const app = express()
const myLogger = function(req, res, next) {
    console.log('LOGGED')
    next()
}
app.use(myLogger)
app.get('/', (req, res) => {
    res.send('Hello World!')
})
app.listen(3000)

Every time the app receives a request, it prints the message “LOGGED” to the terminal.

The order of middleware loading is important: middleware functions that are loaded first are also executed first.

If myLogger is loaded after the route to the root path, the request never reaches it and the app doesn’t print “LOGGED”, because the route handler of the root path terminates the request-response cycle.

The middleware function myLogger simply prints a message, then passes on the request to the next middleware function in the stack by calling the next() function.

Middleware function requestTime

Next, we’ll create a middleware function called “requestTime” and add a property called requestTime to the request object.

const requestTime = function(req, res, next) {
    req.requestTime = Date.now()
    next()
}

The app now uses the requestTime middleware function. Also, the callback function of the root path route uses the property that the middleware function adds to req (the request object).

const express = require('express')
const app = express()
const requestTime = function(req, res, next) {
    req.requestTime = Date.now()
    next()
}
app.use(requestTime)
app.get('/', (req, res) => {
    let responseText = 'Hello World!<br>'
    responseText += `<small>Requested at: ${req.requestTime}</small>`
    res.send(responseText)
})
app.listen(3000)

When you make a request to the root of the app, the app now displays the timestamp of your request in the browser.

Middleware function validateCookies

Finally, we’ll create a middleware function that validates incoming cookies and sends a 400 response if cookies are invalid.

Here’s an example function that validates cookies with an external async service.

async function cookieValidator(cookies) {
    try {
        await externallyValidateCookie(cookies.testCookie)
    } catch {
        throw new Error('Invalid cookies')
    }
}

Here we use the cookie-parser middleware to parse incoming cookies off the req object and pass them to our cookieValidator function. The validateCookies middleware returns a Promise that upon rejection will automatically trigger our error handler.

const express = require('express')
const cookieParser = require('cookie-parser')
const cookieValidator = require('./cookieValidator')
const app = express()
async function validateCookies(req, res, next) {
    await cookieValidator(req.cookies)
    next()
}
app.use(cookieParser())
app.use(validateCookies)
// error handler
app.use((err, req, res, next) => {
    res.status(400).send(err.message)
})
app.listen(3000)

Note how next() is called after await cookieValidator(req.cookies). This ensures that if cookieValidator resolves, the next middleware in the stack will get called. If you pass anything to the next() function (except the string 'route' or 'router'), Express regards the current request as being an error and will skip any remaining non-error handling routing and middleware functions.

Because you have access to the request object, the response object, the next middleware function in the stack, and the whole Node.js API, the possibilities with middleware functions are endless.

Configurable middleware

If you need your middleware to be configurable, export a function that accepts an options object or other parameters, which, then returns the middleware implementation based on the input parameters.

File: my-middleware.js

module.exports = function(options) {
    return function(req, res, next) {
        // Implement the middleware function based on the options object
        next()
    }
}
// The middleware can now be used as shown below.
const mw = require('./my-middleware.js')
app.use(mw({
    option1: '1',
    option2: '2'
}))

Summary

Depending on our short definition, middleware can be created for purposes such as database transactions, transactions between application servers, message controls, and authentication. Structures such as soap, rest, and JSON can be used in the communication of applications. The middleware acts according to all these purposes.