JavaScript is a programming language created in 1995 by Brendan Eich at Netscape, which became the only language run natively by every web browser, making it the language of interactivity on the web.
Together with HTML (structure) and CSS (styling), it forms the core trio of front-end development: it responds to clicks, updates the page without reloading it, and talks to servers. It is standardized as ECMAScript and evolves every year.
Since Node.js, JavaScript also runs on the server, making it possible to use a single language across an entire application. Its ecosystem is immense: React, Vue.js, Angular for the interface, Express or NestJS for the back-end, and millions of packages on npm.
Despite its name, it has nothing to do with Java. Many teams now use it through TypeScript, which adds static typing.
Why it matters when hiring
Almost every web developer writes JavaScript, but few master its fundamentals: asynchrony, how closures work, memory management. Those are the people who hold up against hard bugs. In the interview, step outside the framework: a candidate who can only do React without understanding the language beneath is a risk. Proficiency in TypeScript is now a good sign of rigor.