Why 404 Not Found in Rest API? - Purpose & Use Cases

The Big Idea

Ever wondered why some websites tell you a page is missing while others just leave you guessing?

The Scenario

Imagine you are browsing a website and click a link, but instead of seeing the page you want, you get a confusing blank screen or a generic error message.

The Problem

Without a clear way to handle missing pages, users get lost and frustrated. Manually checking every URL and showing a helpful message is slow and easy to forget, leading to bad user experience.

The Solution

The 404 Not Found response tells the browser and user clearly that the page does not exist. It helps websites handle missing pages gracefully and guide users back on track.

Before vs After

✗ Before

if page not found:
    show blank page or crash

✓ After

if page not found:
    return '404 Not Found' with friendly message

What It Enables

It enables websites and APIs to communicate clearly when a resource is missing, improving user trust and navigation.

Real Life Example

When you mistype a URL on a shopping site, the 404 page helps you know the item isn't there and offers links to popular products instead of leaving you confused.

Key Takeaways

404 Not Found signals a missing resource clearly.

It prevents confusion and improves user experience.

It helps websites guide users back to useful content.

Practice

1. What does the HTTP status code 404 Not Found indicate in a REST API?

easy

A. The requested resource does not exist on the server.

B. The server has encountered an internal error.

C. The request was successful and data is returned.

D. The client is not authorized to access the resource.

Solution

1. Step 1: Understand HTTP status codes

   HTTP status codes communicate the result of a client's request to the server.

2. Step 2: Identify meaning of 404

   404 means the server cannot find the requested resource or URL.

3. Final Answer:

   The requested resource does not exist on the server. -> Option A

4. Quick Check:

   404 = Resource not found [OK]

Hint: 404 means resource missing, not server error [OK]

Common Mistakes:

• Confusing 404 with 500 internal server error
• Thinking 404 means unauthorized access
• Assuming 404 means request succeeded

2. Which of the following is the correct way to send a 404 Not Found response in a REST API using HTTP headers?

easy

A. HTTP/1.1 403 Forbidden\r\nContent-Type: application/json\r\n\r\n

B. HTTP/1.1 200 OK\r\nContent-Type: application/json\r\n\r\n

C. HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n

D. HTTP/1.1 500 Internal Server Error\r\nContent-Type: text/html\r\n\r\n

Solution

1. Step 1: Identify correct HTTP status line for 404

   The status line must start with HTTP version and '404 Not Found' to indicate missing resource.

2. Step 2: Check headers format

   Headers like Content-Type follow the status line, separated by CRLF, then a blank line before body.

3. Final Answer:

   HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n -> Option C

4. Quick Check:

   Status line with 404 and proper headers = HTTP/1.1 404 Not Found\r\nContent-Type: application/json\r\n\r\n [OK]

Hint: 404 response starts with 'HTTP/1.1 404 Not Found' [OK]

Common Mistakes:

• Using 200 OK status for missing resource
• Mixing 404 with 500 or 403 status codes
• Incorrect header formatting or missing blank line

3. Consider this Python Flask code snippet for a REST API endpoint:

from flask import Flask, jsonify, abort
app = Flask(__name__)

@app.route('/items/')
def get_item(item_id):
    items = {1: 'apple', 2: 'banana'}
    if item_id not in items:
        abort(404)
    return jsonify({'item': items[item_id]})

What will be the HTTP status code if a client requests /items/3?

medium

A. 404 Not Found

B. 200 OK

C. 500 Internal Server Error

D. 400 Bad Request

Solution

1. Step 1: Check if item_id 3 exists in items dictionary

   Items only have keys 1 and 2, so 3 is missing.

2. Step 2: Understand abort(404) behavior

   Calling abort(404) sends a 404 Not Found response to the client.

3. Final Answer:

   404 Not Found -> Option A

4. Quick Check:

   Missing item triggers abort(404) = 404 Not Found [OK]

Hint: abort(404) sends 404 status when resource missing [OK]

Common Mistakes:

• Assuming 200 OK even if item missing
• Confusing abort(404) with server error
• Thinking 400 Bad Request applies here

4. You have this Express.js route handler:

app.get('/users/:id', (req, res) => {
  const users = {1: 'Alice', 2: 'Bob'};
  const user = users[req.params.id];
  if (!user) {
    res.status(404);
  }
  res.send(user);
});

What is the problem with this code when a user is not found?

medium

A. It correctly sends a 404 status with no body.

B. It sends a 404 status but still sends undefined as response body.

C. It throws a syntax error due to missing semicolon.

D. It sends a 500 Internal Server Error automatically.

Solution

1. Step 1: Analyze response when user not found

   res.status(404) sets status but does not end response or send body.

2. Step 2: Check what res.send(user) does

   res.status(404) sets the status, but execution continues to res.send(undefined), which sends the string 'undefined' as body with 404 status.

3. Final Answer:

   It sends a 404 status but still sends undefined as response body. -> Option B

4. Quick Check:

   Missing res.send after status causes wrong response [OK]

Hint: Always send response body or end response after setting status [OK]

Common Mistakes:

• Setting status without sending response body
• Assuming status alone ends response
• Not checking for undefined response body

5. You are designing a REST API for a library. When a client requests a book by ID that does not exist, you want to return a 404 Not Found with a helpful JSON message. Which of the following Express.js code snippets correctly implements this behavior?

hard

A. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.sendStatus(404);
}
res.json({book});
});

B. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.json({error: 'Book not found'});
}
res.status(404).json({book});
});

C. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.status(404);
}
res.json({book});
});

D. app.get('/books/:id', (req, res) => {
const books = {1: '1984', 2: 'Brave New World'};
const book = books[req.params.id];
if (!book) {
res.status(404).json({error: 'Book not found'});
return;
}
res.json({book});
});

Solution

1. Step 1: Check how 404 and message are sent

   app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404).json({error: 'Book not found'}); return; } res.json({book}); }); sets status 404 and sends JSON with error message, then returns to avoid sending more.

2. Step 2: Verify other options

   app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.json({error: 'Book not found'}); } res.status(404).json({book}); }); sends JSON without status 404 first, then tries to send 404 with book JSON (wrong order). app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404); } res.json({book}); }); sets status but does not send response properly. app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.sendStatus(404); } res.json({book}); }); sends 404 status but then sends book JSON anyway.

3. Final Answer:

   Sets status(404).json({error: 'Book not found'}); return; to send 404 with helpful JSON message and end response. -> Option D

4. Quick Check:

   Set status, send JSON, and return to end response = app.get('/books/:id', (req, res) => { const books = {1: '1984', 2: 'Brave New World'}; const book = books[req.params.id]; if (!book) { res.status(404).json({error: 'Book not found'}); return; } res.json({book}); }); [OK]

Hint: Set status, send JSON error, and return immediately [OK]

Common Mistakes:

• Not returning after sending 404 response
• Sending response body after status without ending
• Mixing order of status and response methods