JavaScript is a dynamic, prototype-based programming language standardized as ECMAScript and used across browsers, servers (Node.js), and embedded runtimes. This cheat sheet focuses on the most-used language constructs, standard library APIs, and core Web/Node primitives you’ll encounter in real projects. Examples are intentionally minimal and runnable, emphasizing correct modern syntax (ES2015+), predictable semantics, and common pitfalls. Use the links in the first column as the “source of truth” for behavior, edge cases, and compatibility.