express.json() Middleware in Express.js

The express.json() middleware is a built-in middleware function in Express.js that parses incoming requests with JSON payloads and makes the data available on the req.body object. It is commonly used when your Express application needs to handle requests with JSON data, typically in API development.

Purpose of express.json()

• Parses JSON Request Bodies: It parses the incoming JSON request payload, converting it from a JSON string to a JavaScript object.
• Makes Data Available on req.body: Once the JSON is parsed, the resulting object is attached to req.body for easy access in route handlers.

Syntax

app.use(express.json());

This applies the express.json() middleware to every incoming request.

When to Use express.json()

• When you are building an API that handles POST, PUT, or PATCH requests containing JSON data.
• When a client (such as a web or mobile app) sends data to the server in JSON format (e.g., through a REST API or an AJAX request).

Example of express.json()

Here’s a simple Express.js example that demonstrates the use of the express.json() middleware to handle a POST request with a JSON payload.

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

// Apply the express.json() middleware to parse JSON request bodies
app.use(express.json());

// Define a route to handle a POST request
app.post('/submit', (req, res) => {
  const data = req.body; // Access the parsed JSON data
  res.send(`Received data: ${JSON.stringify(data)}`);
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

In this example:

• The express.json() middleware is used to parse the incoming JSON request.
• The route /submit expects a POST request with JSON data.
• The parsed data is available in req.body and is sent back in the response for demonstration purposes.

Sending a Request with JSON Data

You can use tools like Postman or cURL to test this. Here’s an example of a POST request using cURL:

curl -X POST http://localhost:3000/submit \
-H "Content-Type: application/json" \
-d '{"name": "John", "age": 30}'

• Content-Type: Specifies the request body is in JSON format.
• -d: Sends the actual JSON data {"name": "John", "age": 30}.

Handling Invalid JSON

If the request contains invalid JSON, the express.json() middleware will automatically return a 400 Bad Request error with a message like "Unexpected token". This ensures that only valid JSON requests are processed.

Example:

If a client sends a malformed JSON like:

curl -X POST http://localhost:3000/submit \
-H "Content-Type: application/json" \
-d '{"name": "John", "age": 30'  # Missing closing brace

The response will be a 400 Bad Request error due to the invalid JSON syntax.

Comparison with Other Body Parsers

Express also provides other middleware for parsing different types of request bodies:

• express.urlencoded(): Used to parse URL-encoded data (e.g., form submissions).
• express.text(): Parses text/plain bodies.
• express.raw(): Parses raw binary data.

Each middleware is tailored to handle specific content types, and express.json() specifically focuses on application/json content.

Summary

• express.json() is middleware in Express.js used to parse incoming requests with JSON payloads.
• It parses JSON-formatted request bodies and makes the data available on req.body.
• It is commonly used when handling API requests where JSON data is sent by the client.
• The middleware ensures proper error handling by returning a 400 Bad Request for invalid JSON.
• It should be used when you expect requests to contain JSON data, especially in APIs or web applications that interact with modern front-end frameworks or mobile apps.

This middleware simplifies handling JSON in Express applications and is essential for modern API development.