Error Handling Middleware and next

next

Middleware functions are functions that have access to the request object (req), the response object (res) in the application’s request-response cycle.

One example of application-level middleware is express.json(), which is a function call before the routing and controller logic to deal with any possible incoming req.bodys.

Express middleware function also have access to a third parameter: the next middleware function, which is commonly denoted by a variable named 'next'.

The next middleware function can be used to move onto the 'next' middleware function in the chain. For example the following would log the request method and time before passing control onto the following middleware function:

const express = require('express');
const app = express();
 
app.use((req, res, next) => {
  console.log('Request method: ', req.method);
  console.log('Request url: ', req.url);
  console.log('Time:', Date.now());
  next(); // this passes the request and response onto the following middleware function
});
 
app.use((req, res, next) => {
  console.log('I am logged next');
  next(); // without invoking next, the request and response would not be passed onto the routes
});
 
app.get('/api/cats', getCats);

express.json()

Express comes with some built-in middleware methods to perform common tasks. A particularly useful one for building api's is express.json() (opens in a new tab) which can be used to parse requests with JSON bodies. nb It must be invoked when passing it to app.use(express.json()).

The middleware will JSON.parse the request body and attach it to the request on a key of req.body.

When implementing methods such as POST or PATCH that commonly require bodies this is very useful.

// POST /api/users with a body of {"name": "new user", "likes": ["coding"] }
 
const express = require('express');
const app = express();
 
app.use(express.json());
 
app.post('/api/users', (req, res, next) => {
  console.log(req.body) // { name: 'new user', likes: ['coding'] }
  // logic for adding a new user
})