Creating Custom Middleware
Discover how to create and chain your own middleware to tailor request processing in Express applications.
Middleware functions are the backbone of an Express.js application. They allow us to intercept requests, modify data, and control the flow between incoming and outgoing responses.
While built-in and third-party middleware provide useful functionality, we often need to create custom middleware to address specific application needs, including:
Logging
Authentication
Validation
Error handling
Data transformation
Response modification
Security enforcement
In this lesson, we’ll focus on writing custom middleware, organizing middleware for reusability, and implementing effective error handling.
Designing and writing custom middleware
Custom middleware makes writing reusable code easier, simplifying requests and adding features tailored to your app. For instance, this middleware function converts all response object keys to uppercase before sending them back to the client.
{
"name": "node-server",
"version": "1.0.0",
"main": "server.js",
"scripts": {
"start": "node server.js",
"test": "mocha"
},
"dependencies": {
"async": "^3.2.4",
"cors": "^2.8.5",
"express": "^4.18.2",
"lodash": "^4.17.21"
},
"devDependencies": {
"mocha": "^11.1.0",
"supertest": "^7.0.0"
}
}Explanation:
Line 4: This defines the
transformResponsemiddleware function, which takesreq,res, andnextas parameters.Line 5: This stores the original
res.jsonmethod.Lines 7–11: This replaces the original
res.jsonmethod to transform response data by converting all keys to uppercase before sending it to the client. The transformation uses ...