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Middleware

Order​

  • Middleware are functions that run before routes
  • Routes and middleware are executed in order they are specified

Third-Party Middleware​

  • Add third party middleware before routes
  • In this case using cors middleware allows Cross-Origin Resource Sharing
app.js
// Middleware
app.use(cors());

// Routes
app.get("/", (req, res) => {
    res.send("Hello World!");
});

Custom Middleware​

  • Use next() function to pass the control to middleware or route which is next in order
  • In this case testMiddlware() is called before /hello route is executed

Functions​

app.js
// Middleware
const testMiddleware = (req,res,next) => {
    console.log('testMiddleware activated');
    next();
}

// Routes
app.get('/hello', testMiddleware, (req, res) => {
    res.send('Hello World!');
});
  • Multiple functions can be added as middleware
  • They will be executed in order
app.js
// Routes
app.get('/hello', [testMiddlewareOne, testMiddlewareTwo], (req, res) => {
    res.send('Hello World!');
});

With app.use()​

app.js
// Middleware
app.use('/hello', (req,res,next) => {
    console.log('testMiddleware activated');
    next();
}

// Routes
app.get('/hello', (req, res) => {
    res.send('Hello World!');
});
  • Multiple middleware
app.js
// Middleware
app.use('/hello', (req,res,next) => {
    console.log('testMiddlewareOne activated');
    next();
}

// Routes
app.use('/hello', (req,res,next) => {
    console.log('testMiddlewareTwo activated');
    next();
}

Pass Data​

  • Data can be passed from middleware to route as reqBody variable
app.js
// Middleware
app.use('/hello', (req,res,next) => {
    console.log('testMiddleware activated');
    req.body.someData = 'someDataFromMiddleware';
    next();
}

// Routes
app.get('/hello', (req, res) => {
    console.log(req.body.someData);
    res.send('Hello World!');
});
  • Middleware CAN respond to requests using res. But it is not what they are designed to do