What is TypeScript?

The TypeScript language looks like JavaScript; it is JavaScript with type annotations added to it. The TypeScript compiler has two main features: it is a transpiler and a type checker. A transpiler is a special form of compiler that outputs source code. In case of the TypeScript compiler, TypeScript source code is compiled to JavaScript code. A type checker searches for contradictions in your code. For instance, if you assign a string to a variable, and then use it as a number, you will get a type error.

The compiler can figure out some types without type annotations; for others you have to add type annotations. An additional advantage of these types is that they can also be used in editors. An editor can provide completions and refactoring based on the type information. Editors such as Visual Studio Code and Atom (with a plugin, namely atom-typescript) provide such features.