JSON request and response patterns in Express

Introduction

JSON is a simple way to send and receive data between a client and a server. Express helps handle JSON easily in web apps.

When building APIs that send data to web or mobile apps

When a client sends data like form inputs to the server

When you want to respond with structured data instead of HTML pages

When you need to exchange data between different systems in a simple format

Syntax

Express

app.use(express.json())

app.post('/path', (req, res) => {
  const data = req.body
  res.json({ message: 'Received', yourData: data })
})

Use express.json() middleware to parse incoming JSON requests automatically.

Use res.json() to send JSON responses easily.

Examples

This example receives user data in JSON and sends it back with a success status.

Express

app.use(express.json())

app.post('/user', (req, res) => {
  const user = req.body
  res.json({ status: 'success', user })
})

This example sends a JSON response with app info when a GET request is made.

Express

app.get('/info', (req, res) => {
  res.json({ app: 'MyApp', version: '1.0' })
})

Sample Program

This program creates a simple Express server that listens on port 3000. It uses JSON middleware to parse incoming JSON data. When a POST request is sent to /echo with JSON data, it responds by sending back a JSON object confirming receipt and showing the data.

Express

import express from 'express'

const app = express()
const port = 3000

app.use(express.json())

app.post('/echo', (req, res) => {
  const receivedData = req.body
  res.json({ message: 'Data received', data: receivedData })
})

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`)
})

Important Notes

Always use express.json() before routes that expect JSON data.

Clients must set the Content-Type header to application/json when sending JSON.

Use res.json() instead of res.send() for automatic JSON formatting and headers.

Summary

Express makes handling JSON requests and responses simple with built-in middleware.

Use express.json() to parse incoming JSON data automatically.

Use res.json() to send JSON responses with correct headers.

Practice

1. What does the express.json() middleware do in an Express app?

easy

A. It sends JSON responses to the client.

B. It parses incoming JSON request bodies automatically.

C. It logs JSON data to the console.

D. It validates JSON schema in requests.

Solution

1. Step 1: Understand middleware purpose

   The express.json() middleware is designed to parse JSON data sent in the request body.

2. Step 2: Differentiate from response methods

   Sending JSON responses is done by res.json(), not express.json().

3. Final Answer:

   It parses incoming JSON request bodies automatically. -> Option B

4. Quick Check:

   express.json() parses JSON requests [OK]

Hint: express.json() parses JSON request bodies [OK]

Common Mistakes:

• Confusing express.json() with res.json()
• Thinking it sends JSON responses
• Assuming it validates JSON schema

2. Which of the following is the correct way to send a JSON response with Express?

easy

A. res.sendJson({ message: 'Hello' })

B. res.send({ json: 'Hello' })

C. res.json({ message: 'Hello' })

D. res.jsonify({ message: 'Hello' })

Solution

1. Step 1: Identify Express response methods

   Express provides res.json() to send JSON responses with proper headers.

2. Step 2: Check method names

   Methods like sendJson or jsonify do not exist in Express.

3. Final Answer:

   res.json({ message: 'Hello' }) -> Option C

4. Quick Check:

   Use res.json() to send JSON responses [OK]

Hint: Use res.json() to send JSON responses [OK]

Common Mistakes:

• Using non-existent methods like sendJson or jsonify
• Using res.send() without JSON formatting
• Confusing method names

3. Given this Express route, what will be the JSON response when a POST request with body { "name": "Alice" } is sent?

app.use(express.json());
app.post('/greet', (req, res) => {
  const user = req.body.name;
  res.json({ greeting: `Hello, ${user}!` });
});

medium

A. Empty response

B. { "greeting": "Hello, undefined!" }

C. SyntaxError

D. { "greeting": "Hello, Alice!" }

Solution

1. Step 1: Parse JSON body with express.json()

   The middleware parses the JSON body, so req.body.name is 'Alice'.

2. Step 2: Construct JSON response

   The response sends { greeting: `Hello, Alice!` } as JSON.

3. Final Answer:

   { "greeting": "Hello, Alice!" } -> Option D

4. Quick Check:

   Parsed JSON body used in res.json() response [OK]

Hint: express.json() parses body; res.json() sends response [OK]

Common Mistakes:

• Forgetting to use express.json() middleware
• Accessing req.body before parsing
• Expecting undefined instead of 'Alice'

4. What is wrong with this Express code snippet for handling JSON requests?

app.post('/data', (req, res) => {
  const data = req.body;
  res.json({ received: data });
});

medium

A. Missing express.json() middleware to parse JSON body.

B. Using res.json() instead of res.send().

C. Incorrect route method; should be app.get.

D. No error; code works fine.

Solution

1. Step 1: Check JSON parsing middleware

   The code accesses req.body but does not show express.json() middleware usage.

2. Step 2: Understand middleware necessity

   Without express.json(), req.body will be undefined for JSON requests.

3. Final Answer:

   Missing express.json() middleware to parse JSON body. -> Option A

4. Quick Check:

   express.json() needed to parse JSON requests [OK]

Hint: Always add express.json() to parse JSON bodies [OK]

Common Mistakes:

• Assuming req.body is parsed automatically
• Confusing res.json() with res.send()
• Using wrong HTTP method for JSON POST

5. You want to create an Express route that accepts a JSON array of numbers in the request body and responds with a JSON object containing the sum of those numbers. Which code snippet correctly implements this?

hard

A. app.post('/sum', express.json(), (req, res) => {
const numbers = req.body;
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

B. app.post('/sum', (req, res) => {
const numbers = JSON.parse(req.body);
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

C. app.post('/sum', express.urlencoded(), (req, res) => {
const numbers = req.body;
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

D. app.post('/sum', (req, res) => {
const numbers = req.body.numbers;
const sum = numbers.reduce((a, b) => a + b, 0);
res.send({ sum });
});

Solution

1. Step 1: Use express.json() middleware to parse JSON array

   app.post('/sum', express.json(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); correctly uses express.json() middleware inline to parse the JSON array in the request body.

2. Step 2: Sum array and send JSON response

   It sums the numbers with reduce and sends the result with res.json(), which sets correct headers.

3. Step 3: Identify errors in other options

   app.post('/sum', (req, res) => { const numbers = JSON.parse(req.body); const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); tries to parse req.body manually, which is already parsed by middleware. app.post('/sum', express.urlencoded(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); uses express.urlencoded() which is for form data, not JSON. app.post('/sum', (req, res) => { const numbers = req.body.numbers; const sum = numbers.reduce((a, b) => a + b, 0); res.send({ sum }); }); accesses req.body.numbers but expects array directly in body; also uses res.send() which may not set JSON headers properly.

4. Final Answer:

   app.post('/sum', express.json(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); -> Option A

5. Quick Check:

   express.json() + res.json() for JSON array sum [OK]

Hint: Use express.json() middleware and res.json() for JSON data [OK]

Common Mistakes:

• Not using express.json() to parse JSON body
• Using express.urlencoded() for JSON data
• Manually parsing req.body with JSON.parse
• Using res.send() instead of res.json() for JSON response