JavaScript is a programming language that runs in the browser (and on servers through runtimes such as Node.js) to add behavior to web pages. It can react to clicks, change page content and fetch data without reloading.
Why it matters
Most modern sites depend on it for menus, forms, analytics, ads and entire single-page applications. It is also a common cause of slow pages and errors: large bundles delay interactivity, and a script failing to load can break a feature while the server still reports a healthy status.
Frameworks such as React or Vue compile into JavaScript bundles, so bundle size and caching directly affect how quickly a page becomes usable.
Example
<button id="hi">Say hi</button>
<script>
document.getElementById('hi').addEventListener('click', () => {
alert('Hello');
});
</script>Common mistakes
- Loading many third-party scripts synchronously in the head, blocking rendering.
- Assuming JavaScript always runs; crawlers, privacy tools and failed downloads can prevent it.
- Confusing JavaScript with Java, which is a different language.
Related
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