PUT and PATCH route handling in Express - Performance & Optimization

Performance: PUT and PATCH route handling

MEDIUM IMPACT

This affects server response time and client perceived latency when updating resources via HTTP routes.

Updating a resource partially versus fully replacing it

Express

app.patch('/user/:id', (req, res) => {
  // expects partial user data
  const partialUpdate = req.body;
  // update only provided fields
  db.updateUserFields(req.params.id, partialUpdate);
  res.sendStatus(200);
});

PATCH sends only changed fields, reducing payload size and server processing time, improving response speed.

📈 Performance Gainreduces network payload size by up to 80% for small updates; faster server processing

Updating a resource partially versus fully replacing it

Express

app.put('/user/:id', (req, res) => {
  // expects full user object
  const updatedUser = req.body;
  // replace entire user in DB
  db.updateUser(req.params.id, updatedUser);
  res.sendStatus(200);
});

PUT expects full resource replacement, sending large payloads even for small changes, causing unnecessary processing and network load.

📉 Performance Costblocks response for full data processing; larger payloads increase network latency

Performance Comparison

Pattern	Payload Size	Server Processing	Network Latency	Verdict
PUT full update	Large (full resource)	High (process entire object)	High (larger data sent)	[X] Bad
PATCH partial update	Small (only changes)	Low (process few fields)	Low (smaller data sent)	[OK] Good

Rendering Pipeline

When a PUT or PATCH request is made, the server processes the payload, updates the database, and sends a response. The client then updates the UI based on the response. Smaller payloads and faster processing reduce the time before the UI updates, improving interaction responsiveness.

→Network Transfer

→Server Processing

→Client Rendering

⚠️ BottleneckServer Processing time increases with larger payloads and full resource replacements.

Core Web Vital Affected

INP

This affects server response time and client perceived latency when updating resources via HTTP routes.

Optimization Tips

1Use PATCH for partial updates to minimize data sent and processed.

2Avoid using PUT for small changes to reduce network and server load.

3Monitor request payload size and response time in DevTools Network panel.

Performance Quiz - 3 Questions

Which HTTP method is better for sending only changed data to update a resource?

A GET

B PATCH

C PUT

D DELETE

DevTools: Network

How to check: Open DevTools, go to Network tab, perform the update request, and inspect the request payload size and response time.

What to look for: Look for smaller request payloads and faster response times indicating efficient PATCH usage.

Practice

1. What is the main difference between PUT and PATCH methods in Express route handling?

easy

A. PUT updates only specified fields, while PATCH replaces the entire resource.

B. PUT replaces the entire resource, while PATCH updates only specified fields.

C. PUT is used only for creating resources, PATCH for deleting.

D. PUT and PATCH behave exactly the same in Express.

Solution

1. Step 1: Understand HTTP method semantics

   PUT is designed to replace the entire resource with the new data sent in the request.

2. Step 2: Understand PATCH behavior

   PATCH updates only the fields specified in the request, leaving other fields unchanged.

3. Final Answer:

   PUT replaces the entire resource, while PATCH updates only specified fields. -> Option B

4. Quick Check:

   PUT replaces, PATCH updates partial [OK]

Hint: Remember: PUT replaces all, PATCH changes parts only [OK]

Common Mistakes:

• Confusing PUT and PATCH behavior
• Thinking PATCH deletes resources
• Assuming PUT only updates parts

2. Which of the following is the correct way to define a PATCH route in Express to update a user by ID?

easy

A. app.get('/user/:id', (req, res) => { /* update logic */ });

B. app.put('/user/:id', (req, res) => { /* update logic */ });

C. app.patch('/user/:id', (req, res) => { /* update logic */ });

D. app.post('/user/:id', (req, res) => { /* update logic */ });

Solution

1. Step 1: Identify the HTTP method for partial updates

   PATCH is used to update parts of a resource, so the route should use app.patch.

2. Step 2: Check route syntax

   The route path includes /:id to specify which user to update, and the handler function is correctly defined.

3. Final Answer:

   app.patch('/user/:id', (req, res) => { /* update logic */ }); -> Option C

4. Quick Check:

   PATCH route uses app.patch() [OK]

Hint: PATCH routes use app.patch(path, handler) syntax [OK]

Common Mistakes:

• Using app.put instead of app.patch for partial updates
• Using app.get or app.post incorrectly
• Missing route parameter syntax

3. Given this Express code snippet, what will be the response after a PATCH request to /product/5 with JSON body {"price": 20}?

const products = { 5: { name: "Pen", price: 10, stock: 100 } };
app.patch('/product/:id', (req, res) => {
  const id = req.params.id;
  Object.assign(products[id], req.body);
  res.json(products[id]);
});

medium

A. {"name":"Pen","price":20,"stock":100}

B. {"name":"Pen","price":10,"stock":100}

C. {"price":20}

D. Error: Cannot read property 'price' of undefined

Solution

1. Step 1: Understand the initial product data

   Product with ID 5 has name "Pen", price 10, and stock 100.

2. Step 2: Analyze the PATCH handler logic

   The handler uses Object.assign to update the product with fields from req.body. The JSON body contains {"price": 20}, so only the price changes.

3. Final Answer:

   {"name":"Pen","price":20,"stock":100} -> Option A

4. Quick Check:

   PATCH updates price only, other fields stay same [OK]

Hint: PATCH merges fields; unchanged fields remain [OK]

Common Mistakes:

• Expecting full replacement of product data
• Ignoring unchanged fields
• Assuming error due to missing fields

4. What is wrong with this Express PUT route handler?

app.put('/item/:id', (req, res) => {
  const id = req.params.id;
  items[id] = req.body;
  res.send('Item updated');
});

Assuming items is an object storing item data.

medium

A. The route path is missing a trailing slash.

B. The route should use PATCH instead of PUT.

C. The response should be JSON, not plain text.

D. It does not parse JSON body, so req.body will be undefined.

Solution

1. Step 1: Check if JSON body parsing middleware is used

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

2. Step 2: Identify the impact on the handler

   Assigning items[id] = req.body will set the item to undefined, causing data loss or errors.

3. Final Answer:

   It does not parse JSON body, so req.body will be undefined. -> Option D

4. Quick Check:

   Missing express.json() causes undefined req.body [OK]

Hint: Always use express.json() to read JSON request bodies [OK]

Common Mistakes:

• Forgetting to add express.json() middleware
• Confusing PUT and PATCH usage
• Ignoring response content type

5. You want to implement a PUT route to replace a user profile but keep the id field unchanged. Which approach correctly handles this in Express?

hard

A. Overwrite the entire user object with req.body except manually set id back after assignment.

B. Use PATCH instead of PUT to update only the fields except id.

C. Reject the PUT request if id is present in req.body.

D. Delete the user first, then create a new user with req.body including id.

Solution

1. Step 1: Understand PUT semantics

   PUT replaces the entire resource, so the user object will be overwritten by req.body.

2. Step 2: Preserve the id field

   Since id should not change, after overwriting, manually set user.id back to the original value.

3. Final Answer:

   Overwrite the entire user object with req.body except manually set id back after assignment. -> Option A

4. Quick Check:

   PUT replaces all; preserve id manually [OK]

Hint: PUT replaces all; reset id manually after update [OK]

Common Mistakes:

• Using PATCH when full replacement is needed
• Allowing id to be overwritten unintentionally
• Deleting and recreating user unnecessarily