Express js Middleware

Middlewares are different types of functions that have access to the req object, res object and the next middleware function in the application’s request-response cycle.

Basically, middlewares are used to handle and modify the req and res object or pass control to the next middleware function according to the conditions.

Example:

In the below example, we are checking through middleware that if the password is equal to 123 then a user can access the home page otherwise not.

index.js

const express = require('express');
const app = express();
// app.use() is used to apply a middleware
app.use(
// A MIDDLEWARE
(req, res, next) => {
    let password = req.query.password;
    // if password is equal to 123 then call next();
    if(password == 123) next();
    res.send('Invalid Password. You can't access the home page.');
}
);

app.get('/', (req, res) => {
    res.send('Welcome to the Home Page.');
});

app.listen(3000, () => console.log('Your app listening on port 3000'));

Now run the index.js file by running node index command.

Result:

http://localhost:3000/?password=521

http://localhost:3000/?password=123

In the Express JS, there are some types of middleware –

• Application-level middleware
• Router-level middleware
• Error-handling middleware
• Built-in middleware
• Third-party middleware

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