notes.txt

API QUICK NOTES
====================================

1. CORE CONCEPTS
================
- API: Bridge between frontend and backend using HTTP
- REST API: Uses HTTP methods, stateless, works with resources
- Endpoint: URL where API is accessed (e.g., api.example.com/users)
- JSON: Key-value data format {"name": "John", "age": 25}

2. HTTP METHODS (CRUD)
======================

Method    Purpose           Body?   Success Code
------    -------           -----   ------------
GET       Read data         NO      200
POST      Create new        YES     201
PUT       Replace all       YES     200
PATCH     Partial update    YES     200
DELETE    Remove            NO      200/204

PUT vs PATCH:
- PUT: All fields required (missing fields become null)
- PATCH: Only fields you want to change

3. STATUS CODES
===============

Success (2xx):
- 200 OK: Request successful
- 201 Created: New resource created (POST)
- 204 No Content: Success, no response body

Client Errors (4xx):
- 400: Bad request (invalid data)
- 401: Authentication missing/failed (no token)
- 403: No permission (logged in but forbidden)
- 404: Resource not found
- 422: Validation failed

Server Errors (5xx):
- 500: Internal server error
- 503: Service unavailable

4. AUTH vs AUTHORIZATION
========================

Authentication              Authorization
--------------              -------------
WHO are you?                WHAT can you do?
Login process               Permission check
Error: 401                  Error: 403
Username + Password         Admin vs User roles

Auth goes in HEADER, not body:
Authorization: Bearer token_here

5. FETCH API
============

GET Request:
const response = await fetch(url);
const data = await response.json();

POST Request:
const response = await fetch(url, {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(userData)
});

With Error Handling:
try {
  const response = await fetch(url);
  if (!response.ok) throw new Error('Failed');
  const data = await response.json();
} catch (error) {
  console.error(error);
}

6. AXIOS
========

GET:
const response = await axios.get(url);
console.log(response.data);

POST:
const response = await axios.post(url, userData);

With Auth:
axios.defaults.headers.common['Authorization'] = 'Bearer token';

Fetch vs Axios:
- Fetch: Need .json(), manual error handling
- Axios: Auto JSON, better errors, cleaner syntax

7. ERROR HANDLING
=================

try {
  const response = await fetch(url);
  
  if (!response.ok) {
    switch (response.status) {
      case 401: throw new Error('Please login');
      case 403: throw new Error('No permission');
      case 404: throw new Error('Not found');
      default: throw new Error(`Error: ${response.status}`);
    }
  }
  
  const data = await response.json();
  return data;
  
} catch (error) {
  console.error('Error:', error.message);
  return null;
}

8. POSTMAN TESTING
==================

Test API: https://jsonplaceholder.typicode.com

1. GET: Select GET → Enter URL → Send
2. POST: Select POST → Add Header Content-Type: application/json 
         → Body (raw JSON) → Send
3. Check: Status code, Response time, Response body

9. TOP VIVA QUESTIONS
=====================

Q: PUT vs PATCH?
A: PUT replaces entire resource (missing fields become null), 
   PATCH updates only specified fields.

Q: 401 vs 403?
A: 401 = Not logged in, 403 = Logged in but no permission.

Q: Why Content-Type header?
A: Tells server data format (application/json for JSON).

Q: Why body not in GET?
A: GET retrieves data, doesn't send. Data goes in URL as query params.

Q: Successful POST status?
A: 201 Created.

Q: Authentication vs Authorization?
A: Authentication = WHO (identity), Authorization = WHAT (permissions).

Q: What is JSON?
A: Lightweight data format, key-value pairs, human readable.

Q: Why try-catch?
A: Handle network failures, prevent crashes, show user-friendly errors.

QUICK CHECKLIST
===============
- 5 HTTP methods + CRUD mapping
- Status codes: 200, 201, 400, 401, 403, 404, 500
- PUT vs PATCH difference
- Auth header syntax
- Fetch GET + POST syntax
- Error handling with try-catch
- Postman basic testing