🌐 Code Snippet

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

🧠 Purpose

This code adds CORS (Cross-Origin Resource Sharing) support to your FastAPI app.

CORS defines which web apps or domains are allowed to make API requests to your server.
Without it, browsers block requests from other origins (for security reasons).

🔍 Line-by-Line Explanation

🧩 app.add_middleware(CORSMiddleware, ...)

This means:

Add a middleware layer that handles CORS rules before the request reaches your FastAPI routes.

🧱 Middleware = something that runs between the client request and your route logic.
It can modify requests, add headers, log data, or enforce security policies.

🌍 allow_origins=["*"]

This tells FastAPI:

Allow requests from any domain (i.e., * = all websites).

✅ Example:

• Requests from http://localhost:3000

• Requests from https://yourfrontend.com
  All will be accepted.

⚠️ Security Tip:
In production, you should replace "*" with specific domains:

allow_origins = ["https://yourfrontend.com"]

🔑 allow_credentials=True

This allows the browser to include:

• Cookies

• Authorization headers

• TLS client certificates

✅ Example:
If your frontend sends a request with a user session cookie, FastAPI will accept it.

🧾 allow_methods=["*"]

This means:

Allow all HTTP methods like GET, POST, PUT, DELETE, PATCH, etc.

✅ Example:
Your API can be accessed by any type of request from any frontend.

📦 allow_headers=["*"]

This allows:

All types of headers (like Content-Type, Authorization, X-Custom-Header, etc.)

✅ Without this, browsers may block requests that send custom headers.

⚡ In Short

🎯 Example Scenario

You built:

• A backend → FastAPI at http://127.0.0.1:8000

• A frontend → React app at http://localhost:3000

If you don’t enable CORS, the browser will block your React app from calling FastAPI due to “Cross-Origin Request Blocked” errors.

So, this middleware ensures your frontend can safely talk to your backend. 🔄