Request body transformation in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware?

Consider this Express middleware that modifies the request body before passing control to the next handler. What will be the value of req.body in the next middleware?

Express

app.use(express.json());
app.use((req, res, next) => {
  if (req.body && req.body.name) {
    req.body.name = req.body.name.trim().toUpperCase();
  }
  next();
});
app.post('/test', (req, res) => {
  res.json({ transformedName: req.body.name });
});

A If sent {"name": " alice "}, response is {"transformedName": "alice"}

B If sent {"name": " alice "}, response is {"transformedName": null}

C If sent {"name": " alice "}, response is {"transformedName": " alice "}

D If sent {"name": " alice "}, response is {"transformedName": "ALICE"}

💡 Hint

Think about how the middleware modifies the req.body.name before the final handler.

The middleware trims whitespace and converts the name to uppercase. So sending {"name": " alice "} results in "ALICE".

📝 Syntax

intermediate

Which option correctly parses JSON body and adds a new field?

You want to parse JSON request bodies and add a new field timestamp with the current time before handling the request. Which code snippet correctly does this in Express?

A

app.use((req, res, next) => {
  req.body = JSON.parse(req.body);
  req.body.timestamp = Date.now();
  next();
});

B

app.use(express.json());
app.use((req, res, next) => {
  req.body.timestamp = Date.now();
  next();
});

C

app.use(express.urlencoded());
app.use((req, res, next) => {
  req.body.timestamp = Date.now();
  next();
});

D

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💡 Hint

Remember to parse JSON before accessing req.body as an object.

Option B and D use express.json() middleware to parse JSON bodies, then add the timestamp and call next(). Option A tries to parse req.body again causing error. Option C uses urlencoded parser which is wrong for JSON.

🔧 Debug

advanced

Why does this middleware not modify the request body as expected?

This middleware tries to uppercase a field in the request body but the change is not reflected in the final handler. Why?

Express

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

A Because <code>res.json()</code> is called before the middleware.

B Because <code>toUpperCase()</code> returns a new string but does not change <code>req.body.name</code> in place.

C Because <code>next()</code> is called before modifying <code>req.body.name</code>.

D Because <code>req.body</code> is undefined without JSON parsing middleware.

💡 Hint

Remember how string methods like toUpperCase() work in JavaScript.

Strings are immutable in JavaScript. toUpperCase() returns a new string but does not change the original. The middleware must assign the result back to req.body.name.

❓ state_output

advanced

What is the final value of req.body after this middleware chain?

Given these middlewares, what will req.body contain when the final handler runs?

Express

app.use(express.json());
app.use((req, res, next) => {
  req.body = { ...req.body, added: true };
  next();
});
app.use((req, res, next) => {
  delete req.body.removed;
  next();
});
app.post('/data', (req, res) => {
  res.json(req.body);
});

A If sent {"name":"bob","removed":123}, response is {"added":true}

B If sent {"name":"bob","removed":123}, response is {"name":"bob","removed":123,"added":true}

C If sent {"name":"bob","removed":123}, response is {"name":"bob","added":true}

D If sent {"name":"bob","removed":123}, response is {"name":"bob"}

💡 Hint

Look at how the object is copied and the removed property is deleted.

The first middleware copies req.body and adds added: true. The second middleware deletes removed property. So the final object has name and added, but no removed.

🧠 Conceptual

expert

Which option best explains why modifying req.body directly in middleware can be risky?

In Express, why might directly changing req.body in middleware cause unexpected issues in later handlers or middleware?

A Because <code>req.body</code> might be shared by other middleware expecting the original structure, so changes can break assumptions.

B Because <code>req.body</code> is immutable and cannot be changed once parsed.

C Because Express automatically resets <code>req.body</code> after each middleware.

D Because modifying <code>req.body</code> requires asynchronous operations to be safe.

💡 Hint

Think about how middleware chains share the same request object.

Middleware share the same req object. Changing req.body directly can break other middleware or handlers that expect the original data format.