TypeScript is a programming language developed by Microsoft, and is used primarily as a JavaScript substitute for development. The language adds features and enhancements to the JavaScript language that help in creating and maintaining large code bases.

TypeScript compiles (or, more correctly, transpiles) to JavaScript, which can then be interpreted by browsers or any other JavaScript runtime environments.

TypeScript adds static typing capabilities to JavaScript, such as type declaration and interfaces. In addition, it enables developers to use new JavaScript syntax from the latest, or even future, ECMAScript releases, and compile them to JavaScript, which can be interpreted by today's web browsers.

TypeScript has become very popular in the last couple of years, and is even used to develop the Angular framework itself. Though it is not required for SPA-based application development, it makes code clearer and more maintainable, which is why it is highly popular among developers.