ExpressJS express.json() Function

The express.json() function is a built-in middleware in Express that is used for parsing incoming requests with JSON payload. The express.json middleware is important for parsing incoming JSON payloads and making that data available in the req.body or further processing within the routes. Without using express.json, Express will not automatically parse the JSON data in the request body.

By using the express.json middleware, you can handle POST, PUT, or PATCH requests that send JSON data from the client to the server.

Syntax

ExpressJSon( [options] )

• Parameters: The options parameter has various properties like inflate, limit, type, etc.
• Return Value: It returns an Object. 

How express.json() Works?

The primary function of express.json() is to parse requests with a Content-Type header of application/json. Once parsed, the resulting data is stored in the req.body, allowing easy access to the JSON content sent from the client.

Steps To Use ExpressJSon() Function

Step 1: Create a Node.js application using the following command.

mkdir nodejs
cd nodejs
npm init -y

Step 2: Install the required dependencies.

npm install express

Step 3: Create the required files and start the server.

node index.js

Handling JSON Data in ExpressJS

// Filename - index.js

const express = require('express');
const app = express();
const PORT = 3000;

app.use(express.json());

app.post('/', function (req, res) {
    console.log(req.body.name);
    res.end();
});

app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Output

When a POST request is made to http://localhost:3000/ with the header Content-Type: application/json and the body {"name":"GeeksforGeeks"}, the following output is displayed on the console:

In this example

• The ExpressJSon() middleware is applied to parse incoming JSON request bodies, making the data accessible via req.body.
• A POST route is defined at the root (/) to handle requests, logging the name property from the JSON payload to the console.

Without Using ExpressJSON()

// Filename - index.js
const express = require('express');
const app = express();
const PORT = 3000;
// Without this middleware 
// app.use(express.json()); 
app.post('/', function (req, res) {
    console.log(req.body.name);
    res.end();
});
app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Output:

When a POST request is made to http://localhost:3000/ with the header Content-Type: application/json and the body {"name":"GeeksforGeeks"}, the following output is displayed on the console:

In this example, the ExpressJSon() middleware is commented out. Without it, the req.body will be undefined, and attempting to access req.body.name will result in a TypeError.

Why is ExpressJSon() Important?

When you send a JSON request to the server, the data must be parsed before it can be used. The ExpressJSon() middleware is used to parse incoming JSON formatted data in the request body.

Without it, the JSON data in the request body would not be available in req.body as a usable object. This middleware automatically converts the raw JSON data into JavaScript objects that can be manipulated in your code.

Common Use Cases:

• APIs: Sending and receiving JSON data in RESTful APIs.
• POST Requests: When the client submits form data or other data via the POST method, it is often in JSON format, and ExpressJSon() helps parse that data.
• Handling Complex Data: JSON can be used to send complex data structures, and ExpressJSon() makes it easy to handle.

Best Practices with ExpressJSon()

• Validating JSON Data: While ExpressJSon() parses JSON data, it's important to validate that the incoming data matches your expected format.
• Error Handling: Wrap your logic in try-catch blocks when processing JSON data to catch any potential errors in the parsing or subsequent operations.
• Limiting Payload Size: You can limit the size of JSON payloads to protect your server from large and potentially malicious requests.