cors middleware setup in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is CORS in web development?

CORS stands for Cross-Origin Resource Sharing. It is a security feature that allows or blocks web pages from making requests to a different domain than the one that served the web page.

beginner

How do you install the CORS middleware in an Express app?

You install it using npm with the command <code>npm install cors</code>. Then you import it in your app with <code>import cors from 'cors'</code> or <code>const cors = require('cors')</code>.

beginner

How do you enable CORS for all routes in an Express app?

You use the CORS middleware globally by adding app.use(cors()) before your routes. This allows all origins to access your server.

intermediate

How can you restrict CORS to allow only specific origins?

You pass an options object to the CORS middleware like app.use(cors({ origin: 'https://example.com' })). This allows only requests from that origin.

intermediate

What is the purpose of the optionsSuccessStatus option in CORS middleware?

It sets the status code for successful OPTIONS preflight requests. Some browsers expect a 200 status instead of the default 204, so you can set optionsSuccessStatus: 200 to fix issues.

What does the CORS middleware do in an Express app?

A Allows or blocks cross-origin requests

B Handles database connections

C Manages user sessions

D Serves static files

CORS middleware controls which external domains can access your server resources.

How do you apply CORS middleware to all routes in Express?

A app.post(cors())

B app.get(cors())

C app.use(cors())

D app.listen(cors())

Using app.use(cors()) applies the middleware globally to all routes.

Which npm command installs the CORS middleware?

A npm install express

B npm install helmet

C npm install body-parser

D npm install cors

The CORS middleware is installed with npm install cors.

How do you restrict CORS to only allow requests from 'https://myapp.com'?

A app.use(cors({ credentials: true }))

B app.use(cors({ origin: 'https://myapp.com' }))

C app.use(cors({ methods: 'https://myapp.com' }))

D app.use(cors({ headers: 'https://myapp.com' }))

The origin option specifies which domain is allowed.

What HTTP method is commonly used for CORS preflight requests?

A OPTIONS

B POST

C GET

D PUT

Browsers send OPTIONS requests to check permissions before the actual request.

Explain how to set up CORS middleware in an Express app to allow all origins.

Think about the steps from installation to applying middleware globally.

Describe how to configure CORS middleware to restrict access to a specific domain.

Focus on the options object passed to the middleware.