Built-in middleware (json, urlencoded, static) in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending JSON data with express.json() middleware?

Consider this Express server code snippet:

const express = require('express');
const app = express();
app.use(express.json());
app.post('/data', (req, res) => {
  res.json({ received: req.body });
});
// Server listens on port 3000

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 SyntaxError: Unexpected token n in JSON at position 1

D {"received":null}

💡 Hint

Think about what express.json() middleware does to the request body.

The express.json() middleware parses the incoming JSON string into a JavaScript object and assigns it to req.body. So the server responds with the parsed object inside the 'received' key.

📝 Syntax

intermediate

Which option correctly uses express.urlencoded() middleware?

You want to parse URL-encoded form data in an Express app. Which code snippet correctly sets up the middleware?

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

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

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

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

💡 Hint

The 'extended' option must be a boolean.

express.urlencoded() requires the 'extended' option to be true or false. Option D correctly sets it to true, enabling rich objects and arrays to be encoded.

🔧 Debug

advanced

Why does static middleware not serve files as expected?

Given this Express setup:

app.use(express.static('public'));
app.get('/', (req, res) => {
  res.send('Home page');
});

The 'public' folder contains 'index.html'. When visiting '/', the browser shows 'Home page' instead of the static file. Why?

A express.static requires an absolute path, 'public' is invalid.

B The static middleware must be placed after the route handler to work.

C The static middleware only serves files for matching paths, '/' matches the route handler first.

D Static middleware only serves files with .js or .css extensions by default.

💡 Hint

Think about middleware order and route matching.

Express matches routes in order. The route handler for '/' matches the root path and sends 'Home page' before static middleware can serve 'index.html'.

❓ state_output

advanced

What is the response when sending URL-encoded data without urlencoded middleware?

Consider this Express app:

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

If a client sends a POST request to /submit with Content-Type application/x-www-form-urlencoded and body name=Bob&age=30, what will the response be?

Express

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

A {"data":null}

B {"data":{}}

C SyntaxError: Unexpected token n in JSON at position 1

D {"data":{"name":"Bob","age":"30"}}

💡 Hint

What happens if you don't use urlencoded middleware for form data?

Without urlencoded middleware, Express does not parse URL-encoded form data, so req.body remains an empty object.

🧠 Conceptual

expert

Which middleware order correctly serves static files and parses JSON and URL-encoded bodies?

You want to set up an Express app that:
- Serves static files from 'public'
- Parses JSON request bodies
- Parses URL-encoded form data

Which middleware order is correct to ensure all features work as expected?

A

app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(express.static('public'));

B

app.use(express.static('public'));
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

C

app.use(express.urlencoded({ extended: true }));
app.use(express.static('public'));
app.use(express.json());

D

app.use(express.urlencoded({ extended: true }));
app.use(express.json());
app.use(express.static('public'));

💡 Hint

Static files should be served after body parsing middleware to avoid conflicts.

Middleware order matters. Parsing middleware should come before static middleware to ensure request bodies are parsed for routes, while static files are served for matching paths.

Practice

1. Which built-in Express middleware is used to parse incoming JSON request bodies?

easy

A. express.json()

B. express.urlencoded()

C. express.static()

D. express.bodyParser()

Solution

1. Step 1: Understand middleware purpose

   express.json() is designed to parse JSON data sent in request bodies.

2. Step 2: Compare with other middleware

   express.urlencoded() parses URL-encoded form data, express.static() serves files, and express.bodyParser() is not a built-in middleware.

3. Final Answer:

   express.json() -> Option A

4. Quick Check:

   JSON parsing = express.json() [OK]

Hint: JSON data uses express.json() middleware [OK]

Common Mistakes:

• Confusing urlencoded with json middleware
• Using express.static() for parsing data
• Assuming bodyParser is built-in

2. Which of the following is the correct way to use Express middleware to parse URL-encoded form data with support for rich objects and arrays (extended: true)?

easy

A. app.use(express.urlencoded);

B. app.use(express.urlencoded());

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

D. app.use(express.urlencoded(true));

Solution

1. Step 1: Recall middleware syntax

   express.urlencoded() is a function that returns middleware and accepts an options object.

2. Step 2: Identify correct usage

   The correct usage includes calling the function with an options object, commonly { extended: true } to support rich objects and arrays.

3. Final Answer:

   app.use(express.urlencoded({ extended: true })); -> Option C

4. Quick Check:

   Call urlencoded() with options object [OK]

Hint: Call express.urlencoded() with options object [OK]

Common Mistakes:

• Forgetting parentheses after express.urlencoded
• Passing boolean directly instead of options object
• Using express.urlencoded without calling it

3. Given the code below, what will be the output when a GET request is made to /public/image.png if the file exists?

const express = require('express');
const app = express();
app.use(express.static('public'));
app.get('/', (req, res) => res.send('Home'));
app.listen(3000);

medium

A. 404 Not Found error because no route matches /public/image.png.

B. The server will respond with 'Home' text.

C. Syntax error due to missing middleware.

D. The image file image.png will be served from the public folder.

Solution

1. Step 1: Understand express.static middleware

   express.static('public') serves files from the 'public' folder when requested by path.

2. Step 2: Analyze request path

   A GET request to /public/image.png looks for public/public/image.png which does not exist. But since the middleware serves files relative to the folder, the correct URL to access image.png is /image.png.

3. Step 3: Check the actual request path

   Requesting /public/image.png will cause 404 because the middleware serves files at root, not under /public.

4. Final Answer:

   404 Not Found error because no route matches /public/image.png. -> Option A

5. Quick Check:

   express.static serves files at root URL [OK]

Hint: Static files served at root URL, not under folder name [OK]

Common Mistakes:

• Assuming static serves files under /folder path
• Expecting route handler to serve static files
• Confusing middleware path with URL path

4. Identify the error in the following Express setup code:

const express = require('express');
const app = express();
app.use(express.json);
app.use(express.urlencoded({ extended: false }));
app.listen(3000);

medium

A. express.json is not a valid middleware.

B. Missing parentheses after express.json middleware call.

C. Incorrect option passed to express.urlencoded.

D. app.listen should be called before middleware.

Solution

1. Step 1: Check middleware usage syntax

   Middleware functions like express.json() must be called with parentheses to return the middleware function.

2. Step 2: Identify the error

   The code uses express.json without parentheses, so it passes the function itself, not the middleware, causing an error.

3. Final Answer:

   Missing parentheses after express.json middleware call. -> Option B

4. Quick Check:

   Call express.json() with () [OK]

Hint: Always call middleware functions with parentheses [OK]

Common Mistakes:

• Forgetting () after express.json
• Passing wrong options to urlencoded
• Calling app.listen before middleware

5. You want to serve static files from the folder named assets under the URL path /static. Which Express middleware setup achieves this correctly?

hard

A. app.use(express.static('assets/static'));

B. app.use(express.static('/static/assets'));

C. app.use('/assets', express.static('static'));

D. app.use('/static', express.static('assets'));

Solution

1. Step 1: Understand express.static usage with mount path

   To serve files from a folder under a specific URL path, use app.use('/urlpath', express.static('folder')).

2. Step 2: Match folder and URL path

   Here, the folder is assets and the URL path is /static, so app.use('/static', express.static('assets')) is correct.

3. Step 3: Analyze incorrect options

   Other options either swap folder and path or use incorrect paths causing errors or wrong URLs.

4. Final Answer:

   app.use('/static', express.static('assets')); -> Option D

5. Quick Check:

   Mount static folder on desired URL path [OK]

Hint: Mount static folder with URL path as first argument [OK]

Common Mistakes:

• Swapping folder and URL path arguments
• Using absolute paths incorrectly
• Not specifying URL path for static files