JavaScript (.js) files contain standard JavaScript code, the primary language executed by web browsers for dynamic behavior. TypeScript (.ts) files use TypeScript, a superset of JavaScript developed by Microsoft. The core difference is that TypeScript adds a robust static type system, enabling developers to explicitly define the expected data types (like numbers, strings, or custom objects) for variables, function parameters, and return values. While standard JavaScript code is valid TypeScript, TypeScript requires compilation into plain JavaScript before browsers can run it. This compilation stage checks the code against the type definitions, catching potential errors earlier.

Developers use TypeScript in complex web applications to improve code reliability and maintainability. For example, in a large Angular project, defining interfaces for data structures received from an API in a .ts file prevents accidental misuse of that data elsewhere. Node.js developers also use TypeScript (via tools like ts-node or compiling to .js) to catch type mismatches in server-side logic before runtime, such as ensuring a function only receives valid user IDs.

The key advantages of TypeScript include catching type-related bugs during development (improving quality), enhancing tooling support like intelligent code completion in editors like VSCode, and making large codebases easier to understand and refactor. Its main limitation is the additional overhead of learning types, defining them, and setting up compilation. While this can feel cumbersome for very small projects, the trade-off is widely considered beneficial for team-based or enterprise-level web development, driving significant adoption.