What is CORS and how do you handle it in Node?

Question

IT Interview Practice · Accepted Answer

**CORS** (Cross-Origin Resource Sharing) is a browser security mechanism that controls whether a web page from one **origin** can make requests to a server on a **different origin**. By default, browsers block cross-origin requests; the server must explicitly allow them via response headers.

## The same-origin policy and the problem

```text
Origin = scheme + host + port. These are DIFFERENT origins:
  https://app.example.com   →  https://api.example.com   (different host)
  http://localhost:3000     →  http://localhost:4000      (different port)

Browser blocks the cross-origin request UNLESS the server sends CORS headers allowing it.
```

So a React app on `localhost:3000` calling an API on `localhost:4000` is cross-origin — the browser blocks it unless the API opts in.

## The key response headers

```text
Access-Control-Allow-Origin: https://app.example.com   ← who is allowed
Access-Control-Allow-Methods: GET, POST, PUT, DELETE   ← allowed methods
Access-Control-Allow-Headers: Content-Type, Authorization
Access-Control-Allow-Credentials: true                 ← allow cookies/auth
```

The server controls access by sending these headers. The browser reads them and decides whether to allow the page to see the response.

## Handling CORS in Express

```js
import cors from "cors";

// ❌ allow everything — convenient but insecure for credentialed/private APIs
app.use(cors());

// ✅ restrict to specific trusted origins
app.use(cors({
  origin: ["https://app.example.com", "http://localhost:3000"], // allowlist
  methods: ["GET", "POST", "PUT", "DELETE"],
  credentials: true, // allow cookies — REQUIRES a specific origin (not "*")
}));
```

The `cors` middleware sets the headers for you. For production, **restrict `origin` to an allowlist** rather than `*` — and note that allowing credentials (`credentials: true`) is incompatible with the wildcard `*`.

## Preflight requests

```text
For "non-simple" requests (PUT/DELETE, custom headers, JSON), the browser first
sends an OPTIONS "preflight" request to check permissions. The cors middleware
handles responding to it automatically.
```

## Common misconception

```text
CORS is enforced by the BROWSER, not the server. It does NOT protect your API
from non-browser clients (curl, Postman, other servers) — those ignore CORS.
It only governs what browser JavaScript on other origins can do.
```

## Why it matters

CORS is one of the most common stumbling blocks in web development — nearly every front-end-to-API setup hits it (especially in local dev with different ports).

Understanding *why* it exists (browser same-origin security), how the server opts in via headers, how to configure it safely (allowlist origins, careful credential handling), and the crucial fact that it's a *browser* mechanism (not API authorization) is essential for correctly and securely connecting front-ends to Node APIs without either being blocked or over-exposing the server.

The same-origin policy and the problem

The key response headers

Handling CORS in Express

Preflight requests

Common misconception

Why it matters

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