Lasting skills
Understand it, don't copy it
AI tools change often. The useful habit does not: read the code, test it, and check it against a reliable source. This page sets that habit out, with facts from MDN.
When you load a web page, the HTML, CSS and JavaScript run inside an execution environment, the browser tab. MDN: What is JavaScript? Each browser tab has its own separate bucket for running code, which is a security measure: in most cases, code in one tab cannot directly affect code in another tab or on another website. MDN: What is JavaScript?
MDN describes JavaScript as a lightweight interpreted programming language: the browser receives the code in its original text form and runs it from that. Modern engines may compile it just in time, but it is still considered interpreted because that compilation happens at run time. MDN: What is JavaScript? JavaScript is used most often for dynamic client-side scripts on web pages, but it is also often used on the server, with a runtime such as Node.js, Deno or Bun. MDN Glossary: JavaScript
Server-side code runs on the server, and its results are then downloaded and displayed in the browser. JavaScript can also be used on the server, for example in Node.js. MDN: What is JavaScript?
The standardized JavaScript language is called ECMAScript and is specified under ECMA-262, a living standard that is constantly updated. MDN Glossary: JavaScript
A site that needs regular review
AI tools change often. This site teaches lasting skills (reading code, testing it, and checking it) rather than any one product. The facts were checked against MDN on 14 November 2026 and should be reviewed again every few months.
Sample assistant replies
The "assistant" replies on this site were written by us for the lesson. They are not live results from an AI tool, and nothing you type is sent to one. That way every visitor sees the same lesson.
The next pages put that into practice, with sample replies we wrote for the lessons and checkers that tell you whether you spotted the problem.