What is TypeScript?
TypeScript is a superset to JavaScript, developed and maintained by Microsoft. TypeScript, also known as TS, transpiles down to JavaScript, and enables the developer to write almost JavaScript, but with some extensions. The main extension TS brings in, are types. As TypeScript is a superset, pure, vanilla JavaScript can be written - you can bring in TypeScript-exclusive stuff in your code whenever you want.
TS found its way into Angular, Node.js, React and others. It even serves as the basis for the programming language AssemblyScript.
Author's notes
Learning TypeScript is absolutely worth it. Yet, many developers fall into the trap of using TypeScript to spam their code with type definitions. While bringing in types is the idea behind TypeScript, it is capable of much more than this.
For getting started with TS, write JavaScript code at first, and bring in TS-exclusive stuff when you feel comfortable with it and understand, what you are actually trying. As TS is just a superset of JavaScript, pure JS code runs perfectly in TypeScript.
Sources
As the sources for the following pages, Maximilian Schwarzmüllers Udemy course and the official documentation are used.