TypeScript is a superset of JavaScript. But what does that mean?

According to math, a superset is a set that contains all the elements of another set:

A = {1, 2, 3}
B = {1, 2, 3, 4, 5}  // 👈 B is a superset of A

In other words, TypeScript is JavaScript plus some extra features.

What are these extra features? TypeScript adds a complete type system with optional static typing and type checking.