PUT and PATCH route handling in Express

Introduction

PUT and PATCH let your server update data sent by users. They help keep information fresh and correct.

When you want to replace an entire item with new data.

When you want to change only some details of an item.

When building APIs that let users edit their profiles or settings.

When updating a product's information in an online store.

When fixing or correcting data without deleting it.

Syntax

Express

app.put('/path/:id', (req, res) => {
  // Replace entire item with req.body
});

app.patch('/path/:id', (req, res) => {
  // Update parts of item with req.body
});

PUT expects the full new data for the item.

PATCH expects only the fields to change.

Examples

Use PUT to replace the entire user info.

Express

app.put('/users/:id', (req, res) => {
  // Replace whole user data
});

Use PATCH to update only some user details.

Express

app.patch('/users/:id', (req, res) => {
  // Update some user fields
});

PUT replaces the whole product data.

Express

app.put('/products/:id', (req, res) => {
  // Replace product info
});

PATCH changes just the price or other parts.

Express

app.patch('/products/:id', (req, res) => {
  // Change product price only
});

Sample Program

This example shows a simple server with users data. PUT replaces the whole user info except the id. PATCH changes only the fields sent in the request.

Express

import express from 'express';
const app = express();
app.use(express.json());

let users = [
  { id: '1', name: 'Alice', age: 25 },
  { id: '2', name: 'Bob', age: 30 }
];

// PUT replaces entire user
app.put('/users/:id', (req, res) => {
  const { id } = req.params;
  const index = users.findIndex(u => u.id === id);
  if (index === -1) {
    return res.status(404).send('User not found');
  }
  // Replace whole user data, keep id same
  users[index] = { id, ...req.body };
  res.send(users[index]);
});

// PATCH updates parts of user
app.patch('/users/:id', (req, res) => {
  const { id } = req.params;
  const user = users.find(u => u.id === id);
  if (!user) {
    return res.status(404).send('User not found');
  }
  // Update only given fields
  Object.assign(user, req.body);
  res.send(user);
});

app.listen(3000, () => console.log('Server running on port 3000'));

Important Notes

PUT is like replacing a whole book page; PATCH is like editing a few words.

Always check if the item exists before updating.

Use express.json() middleware to read JSON data from requests.

Summary

PUT replaces the entire resource with new data.

PATCH updates only specific fields of a resource.

Both are used to update data on the server in APIs.

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