Testing POST with request body in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending a JSON body to this Express POST route?

Consider this Express route that handles POST requests with JSON body parsing enabled.

const express = require('express');
const app = express();
app.use(express.json());

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

If a client sends a POST request to /data with JSON body {"name": "Alice"}, what will the server respond with?

Express

const express = require('express');
const app = express();
app.use(express.json());

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

A {"received":{"name":"Alice"}}

B {"received":"{\"name\": \"Alice\"}"}

C {"received":null}

D SyntaxError

💡 Hint

Remember that express.json() middleware parses JSON body into an object.

The express.json() middleware parses the JSON body and sets req.body as an object. The response sends back this object inside received. So the output is a JSON object with received key holding the parsed object.

📝 Syntax

intermediate

Which option correctly parses URL-encoded POST body in Express?

You want to parse URL-encoded form data sent in a POST request body in Express. Which middleware setup is correct?

Express

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

// Which middleware to use here?

app.post('/submit', (req, res) => {
  res.send(req.body);
});

A app.use(express.json());

B app.use(express.urlencoded({ extended: true }));

C app.use(bodyParser.json());

D app.use(express.text());

💡 Hint

URL-encoded form data is not JSON. Use the middleware that parses URL-encoded bodies.

express.urlencoded({ extended: true }) parses URL-encoded bodies like form submissions. express.json() parses JSON bodies. bodyParser.json() is deprecated in modern Express. express.text() parses plain text bodies.

🔧 Debug

advanced

Why does this Express POST route not receive the JSON body?

Look at this Express app code:

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

app.post('/api', (req, res) => {
  res.json(req.body);
});

app.use(express.json());

When sending a JSON POST request to /api, req.body is undefined. Why?

Express

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

app.post('/api', (req, res) => {
  res.json(req.body);
});

app.use(express.json());

A res.json() cannot send objects

B express.json() middleware is missing

C express.json() middleware is used after the route, so body is not parsed yet

D req.body is always undefined in POST routes

💡 Hint

Middleware order matters in Express.

Middleware like express.json() must be used before routes that need to access req.body. Here, the middleware is added after the route, so the body is not parsed when the route runs.

❓ state_output

advanced

What is the value of req.body after sending a plain text POST request?

Given this Express setup:

const express = require('express');
const app = express();
app.use(express.text());

app.post('/text', (req, res) => {
  res.send(req.body);
});

If a client sends a POST request to /text with body Hello World (plain text), what will req.body contain?

Express

const express = require('express');
const app = express();
app.use(express.text());

app.post('/text', (req, res) => {
  res.send(req.body);
});

A "Hello World"

B {"Hello World"}

C undefined

D SyntaxError

💡 Hint

express.text() parses plain text bodies as strings.

The express.text() middleware parses the request body as a string and assigns it to req.body. So req.body will be the string Hello World.

🧠 Conceptual

expert

Which middleware combination correctly handles JSON and URL-encoded POST bodies simultaneously?

You want your Express app to accept POST requests with either JSON or URL-encoded form data bodies. Which middleware setup correctly supports both?

Express

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

// Middleware setup here

app.post('/submit', (req, res) => {
  res.json(req.body);
});

A app.use(express.raw()); app.use(express.urlencoded({ extended: true }));

B app.use(express.urlencoded({ extended: false })); app.use(express.json());

C app.use(express.text()); app.use(express.json());

D app.use(express.json()); app.use(express.urlencoded({ extended: false }));

💡 Hint

Order of middleware matters, but both JSON and URL-encoded parsers must be included.

To handle both JSON and URL-encoded bodies, include both express.json() and express.urlencoded() middleware. The order usually does not matter, but the common pattern is to add JSON parser first. Options A and D use text or raw parsers which do not parse JSON or URL-encoded data properly.

Practice

1. What is the main purpose of testing a POST request with a request body in Express?

easy

A. To verify the server correctly receives and processes the sent data

B. To check if the server can handle GET requests

C. To test the server's response time only

D. To confirm the server ignores the request body

Solution

1. Step 1: Understand POST request role

   POST requests send data to the server, so testing ensures data is received and handled.

2. Step 2: Identify testing goal

   Testing POST with a body checks if the server processes the data correctly, not just the response time or ignoring data.

3. Final Answer:

   To verify the server correctly receives and processes the sent data -> Option A

4. Quick Check:

   POST body testing = verify data handling [OK]

Hint: POST tests check data handling, not just response [OK]

Common Mistakes:

• Confusing POST with GET request testing
• Ignoring the request body in tests
• Testing only response time without data validation

2. Which of the following is the correct way to send JSON data in a POST request test using Supertest in Express?

easy

A. request(app).post('/api').body({ name: 'John' })

B. request(app).post('/api').send({ name: 'John' })

C. request(app).post('/api').query({ name: 'John' })

D. request(app).get('/api').send({ name: 'John' })

Solution

1. Step 1: Identify correct HTTP method and data sending

   POST requests use .post() and .send() to include JSON body data in Supertest.

2. Step 2: Evaluate options

   The option with request(app).post('/api').send({ name: 'John' }) uses .post() and .send() correctly. Using .get() is wrong for sending body data. Using .query() sends URL params, not body data. Using .body() is not a Supertest method.

3. Final Answer:

   request(app).post('/api').send({ name: 'John' }) -> Option B

4. Quick Check:

   POST + .send() = correct syntax [OK]

Hint: Use .post() with .send() to send JSON body [OK]

Common Mistakes:

• Using GET instead of POST for sending body data
• Using .query() instead of .send() for body
• Trying to use .body() method which doesn't exist

3. Given this Express route and test code, what will be the response body when the test runs?

app.use(express.json());
app.post('/user', (req, res) => {
  res.json({ received: req.body.name });
});

request(app)
  .post('/user')
  .send({ name: 'Alice' })
  .then(res => console.log(res.body));

medium

A. { "received": "Alice" }

B. { "name": "Alice" }

C. {}

D. Error: req.body is undefined

Solution

1. Step 1: Understand route response

   The route sends back JSON with key 'received' set to req.body.name.

2. Step 2: Analyze test request body

   The test sends { name: 'Alice' }, so req.body.name is 'Alice'.

3. Final Answer:

   { "received": "Alice" } -> Option A

4. Quick Check:

   Response body = { received: 'Alice' } [OK]

Hint: Response echoes req.body.name as 'received' key [OK]

Common Mistakes:

• Expecting original body instead of transformed response
• Forgetting to parse JSON body in Express (missing middleware)
• Assuming empty response or error without middleware

4. You wrote this test for your Express POST route but it fails with an empty req.body. What is the likely cause?

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

request(app)
  .post('/data')
  .send({ key: 'value' })
  .expect(200, { key: 'value' });

medium

A. The test expects wrong status code

B. Using .send() instead of .query() to send data

C. The route should use GET instead of POST

D. Missing middleware to parse JSON body like express.json()

Solution

1. Step 1: Identify why req.body is empty

   Express needs middleware like express.json() to parse JSON body; without it, req.body is empty.

2. Step 2: Check test and route setup

   The test sends JSON with .send(), which is correct; route uses POST correctly; status 200 is expected.

3. Final Answer:

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

4. Quick Check:

   Missing express.json() = empty req.body [OK]

Hint: Add express.json() middleware to parse JSON body [OK]

Common Mistakes:

• Not adding express.json() middleware
• Confusing .send() with .query() for body data
• Expecting req.body without parsing middleware

5. You want to test an Express POST route that accepts a JSON array in the request body and returns the array length. Which test code correctly sends the array and checks the response?

app.use(express.json());
app.post('/items', (req, res) => {
  res.json({ count: req.body.length });
});

hard

A. request(app).post('/items').send({ length: 3 }).expect(200).then(res => {
if(res.body.count !== 3) throw new Error('Wrong count');
});

B. request(app).post('/items').send('{1,2,3}').expect(200).then(res => {
if(res.body.count !== 3) throw new Error('Wrong count');
});

C. request(app).post('/items').send([1, 2, 3]).expect(200).then(res => {
if(res.body.count !== 3) throw new Error('Wrong count');
});

D. request(app).post('/items').query([1, 2, 3]).expect(200).then(res => {
if(res.body.count !== 3) throw new Error('Wrong count');
});

Solution

1. Step 1: Understand route expects JSON array body

   The route reads req.body.length, so the body must be an array.

2. Step 2: Check test sending array correctly

   request(app).post('/items').send([1, 2, 3]).expect(200).then(res => { if(res.body.count !== 3) throw new Error('Wrong count'); }); sends an actual array with .send(), which is correct. request(app).post('/items').send('{1,2,3}').expect(200).then(res => { if(res.body.count !== 3) throw new Error('Wrong count'); }); sends a string, not JSON array. request(app).post('/items').send({ length: 3 }).expect(200).then(res => { if(res.body.count !== 3) throw new Error('Wrong count'); }); sends an object, not array. request(app).post('/items').query([1, 2, 3]).expect(200).then(res => { if(res.body.count !== 3) throw new Error('Wrong count'); }); uses .query() which sends URL params, not body.

3. Final Answer:

   request(app).post('/items').send([1, 2, 3]).expect(200).then(res => { if(res.body.count !== 3) throw new Error('Wrong count'); }); -> Option C

4. Quick Check:

   Send array with .send() to test array length [OK]

Hint: Send array directly with .send() for JSON body tests [OK]

Common Mistakes:

• Sending array as string instead of JSON
• Using .query() instead of .send() for body data
• Sending object with length property instead of array