What is JSX?
Why Interviewers Ask This
Foundational questions like this help interviewers calibrate the rest of the interview. A confident, accurate answer signals that you have solid React.js basics — a prerequisite for any developer role.
Answer
JSX (JavaScript XML) is a syntax extension for JavaScript that lets you write HTML-like markup directly inside JavaScript code. JSX is not valid JavaScript — it must be transformed by a compiler (Babel or the new React compiler) into regular JavaScript function calls. For example, <h1>Hello, {name}!</h1> compiles to React.createElement("h1", null, "Hello, ", name, "!"). Key JSX rules: every element must be closed (self-closing tags like <img />); use className instead of class and htmlFor instead of for; JavaScript expressions go inside { }; components must start with an uppercase letter to distinguish them from HTML elements; adjacent JSX elements must be wrapped in a single parent (or a Fragment). JSX is optional — you can use React without it — but it is universally adopted because it makes component markup far more readable and maintainable.
Common Mistake
Candidates often give textbook answers here. Interviewers are more impressed when you relate the concept to a specific problem you solved in a real React.js project.