What is JavaScript?
JavaScript is a programming language used to make applications dynamic.
In the browser, JavaScript makes pages interactive. In Node.js, JavaScript runs backend code on the server.
Why JavaScript exists
HTML gives structure. CSS gives styling. JavaScript gives behavior.
Example:
- open a dropdown,
- validate a form,
- fetch products from an API,
- update UI without refreshing the page,
- run backend logic in Node.js.
Where JavaScript runs
JavaScript can run in:
- browsers like Chrome, Firefox, Safari,
- backend runtimes like Node.js,
- desktop apps through tools like Electron,
- mobile frameworks like React Native.
JavaScript in MERN
MERN uses JavaScript across the stack:
| Layer | Tool |
|---|---|
| Frontend | React |
| Backend runtime | Node.js |
| Backend framework | Express |
| Database access | MongoDB driver or Mongoose |
Interview answer
JavaScript is a programming language used for dynamic behavior in web applications. It runs in browsers and also on servers through Node.js. In MERN, JavaScript is used across the stack: React on the frontend, Node and Express on the backend, and MongoDB access through JavaScript libraries.