1. Primitives

JavaScript has three very commonly used primitives: string, number, and boolean. Each has a corresponding type in TypeScript. As you might expect, these are the same names you'd see if you used the JavaScript typeof operator on a value of those types:

The type names String, Number, and Boolean (starting with capital letters) are legal, but refer to some special built-in types that will very rarely appear in your code. Always use string, number, or boolean for types.

The TypeScript Handbook: Everyday Types

Exercises