How do you implement CORS in a Node.js application?

Question

Accepted Answer

**CORS (Cross-Origin Resource Sharing)** — allows/denies cross-origin HTTP requests.

**Using cors middleware:**
```javascript
const cors = require("cors");

// Allow all origins
app.use(cors());

// Specific configuration
app.use(cors({
    origin: "https://example.com",
    methods: ["GET", "POST", "PUT"],
    allowedHeaders: ["Content-Type", "Authorization"],
    credentials: true,
    maxAge: 86400,
}));

// Dynamic origin
app.use(cors({
    origin: (origin, callback) => {
        const allowed = ["https://example.com"];
        if (!origin || allowed.includes(origin)) {
            callback(null, true);
        } else {
            callback(new Error("Not allowed by CORS"));
        }
    }
}));
```

**Manual CORS headers:**
```javascript
app.use((req, res, next) => {
    res.setHeader("Access-Control-Allow-Origin", "https://example.com");
    res.setHeader("Access-Control-Allow-Methods", "GET, POST");
    res.setHeader("Access-Control-Allow-Headers", "Content-Type");
    next();
});
```

Node.js: How do you implement CORS in a Node.js application?