Express JS express.json() Function - GeeksforGeeks (2024)

The express.json() function is a built-in middleware function in Express. It parses incoming requests with JSON payloads and is based on body-parser.It’s a critical component for handling JSON data in RESTful APIs and web applications, allowing the server to automatically parse JSON data from the request body.

Syntax:

express.json( [options] )

Parameters:

• The options parameter has various properties like inflate, limit, type, etc.

Return Value: It returns an Object.

Features

• Automatic Parsing: Automatically parses JSON data in the request body and makes it available on req.body.
• Error Handling: If the JSON is malformed, express.json() throws a 400 Bad Request error, preventing the request from proceeding.
• Lightweight: Built directly into Express, eliminating the need for additional packages to handle JSON parsing.
• Integration with Other Middleware: Can be used in conjunction with other middleware like express.urlencoded() for handling different content types.

Steps to Install the Express Module

Step 1: You can install this package by using this command.

npm install express

Step 2: After installing the express module, you can check your express version in the command prompt using the command.

npm version express

Step 3: After that, you can just create a folder and add a file, for example, index.js. To run this file you need to run the following command.

node index.js

Project Structure:

Example 1: Below is the code example of the express.json().

// Filename - index.jsconst 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);}); 

Step to run the program: Run the index.js file using the below command

node index.js

Output: Now make a POST request to http://localhost:3000/ with header set to ‘content-type: application/json’ and body {“name”:”GeeksforGeeks”}, then you will see the following output on your console:

Server listening on PORT 3000GeeksforGeeks

Example 2: Below is the code example of the express.json().

// Filename - index.jsconst 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);});

Steps to run the program: Run the index.js file using the below command:

node index.js

Output: Now make a POST request to http://localhost:3000/ with header set to ‘content-type: application/json’ and body {“name”:”GeeksforGeeks”}, then you will see the following output on your console:

Server listening on PORT 3000TypeError: Cannot read property 'name' of undefined

Summary

The express.json() middleware is an essential part of any Express.js application dealing with JSON data. It simplifies the process of handling JSON payloads, ensuring that the data is automatically parsed and available for use in your routes. By integrating express.json() into your application, you can streamline the handling of JSON requests and enhance the robustness of your API or web application.