PUT and PATCH route handling in Express - Mini Project: Build & Apply

PUT and PATCH Route Handling in Express

📖 Scenario: You are building a simple Express server to manage a list of books. Each book has an id, title, and author. You want to allow users to update book details either fully or partially.

🎯 Goal: Create two routes in Express: a PUT route to fully replace a book's details, and a PATCH route to update only some fields of a book.

📋 What You'll Learn

Create an array called books with three book objects, each having id, title, and author.

Create a variable app by calling express() and use express.json() middleware.

Create a PUT route at /books/:id that replaces the entire book object with the request body if the book exists.

Create a PATCH route at /books/:id that updates only the provided fields of the book if it exists.

💡 Why This Matters

🌍 Real World

APIs often need to let users update data fully or partially. PUT replaces the whole item, PATCH changes only some parts.

💼 Career

Understanding PUT and PATCH routes is essential for backend developers building RESTful APIs with Express.

1

Create the initial books array

Create an array called books with these exact objects: { id: 1, title: '1984', author: 'George Orwell' }, { id: 2, title: 'Brave New World', author: 'Aldous Huxley' }, and { id: 3, title: 'Fahrenheit 451', author: 'Ray Bradbury' }.

Express

# Create the books array with 3 book objects
# Your code here

Hint

Use const books = [ ... ] with three objects inside.

2

Set up Express app and JSON middleware

Create a variable called app by calling express(). Then add JSON parsing middleware by calling app.use(express.json()).

Express

const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' },
  { id: 3, title: 'Fahrenheit 451', author: 'Ray Bradbury' }
];

// Create app and add JSON middleware
# Your code here

Hint

Remember to import express first with const express = require('express').

3

Create the PUT route to replace a book

Create a PUT route at /books/:id. Inside, find the book with id matching Number(req.params.id). If found, replace the entire book object in books with req.body. Use res.status(200).send() to send the updated book. If not found, send status 404.

Express

const express = require('express');
const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' },
  { id: 3, title: 'Fahrenheit 451', author: 'Ray Bradbury' }
];

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

// Create PUT route at /books/:id to replace a book
# Your code here

Hint

Use findIndex to locate the book, then replace it with req.body.

4

Create the PATCH route to update part of a book

Create a PATCH route at /books/:id. Find the book with id matching Number(req.params.id). If found, update only the fields present in req.body on the book object. Send the updated book with res.status(200).send(). If not found, send status 404.

Express

const express = require('express');
const books = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'Brave New World', author: 'Aldous Huxley' },
  { id: 3, title: 'Fahrenheit 451', author: 'Ray Bradbury' }
];

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

app.put('/books/:id', (req, res) => {
  const id = Number(req.params.id);
  const index = books.findIndex(book => book.id === id);
  if (index !== -1) {
    books[index] = req.body;
    res.status(200).send(books[index]);
  } else {
    res.status(404).send();
  }
});

// Create PATCH route at /books/:id to update part of a book
# Your code here

Hint

Use Object.assign to update only the fields sent in req.body.

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