Full Stack developer interview questions

A component is a function that returns UI from props and state.

The virtual DOM is React's in-memory tree of UI.

Class components use this.state and lifecycle methods.

key helps React identify which item changed, was added, or was removed.

Authentication proves who the user is, usually with a password, OTP, or OAuth login.

A JSON Web Token is a signed string with a header, payload, and signature.

Hashing turns a password into a one-way digest so a database leak does not reveal real passwords.

The React form posts credentials to Express.

HTTP-only cookies are harder for stolen scripts to read because JavaScript cannot access them.

You build React into static files, run the Node API as a long-lived process, and put both on a public URL.

Development builds include source maps, hot reload, and extra warnings.

Staging is a production-like environment used to test a release before customers see it.

PM2 keeps a Node process running, restarts it on crash, and can run multiple instances.

Unit tests check one function or component in isolation.

Jest is a JavaScript test runner for React components and Node unit tests.

An assertion checks that actual output matches what you expect, such as a 200 status or a rendered heading.

API tests lock in status codes, validation, and auth rules so refactors do not silently break the React client.

CORS is a browser rule that blocks a page on one origin from reading another origin unless the server allows it.

Cross-site scripting injects malicious JavaScript into a page other users load.

API keys, database URIs, and JWT secrets in Git history can be cloned or leaked forever.