Programming TypeScript

Chapter 3. All About Types

In the last chapter I introduced the idea of type systems, but I never defined what the type in type system really means.

Type

A set of values and the things you can do with them.

If that sounds confusing, let me give a few familiar examples:

• The boolean type is the set of all booleans (there are just two: true and false) and the operations you can perform on them (like ||, &&, and !).

• The number type is the set of all numbers and the operations you can perform on them (like +, -, *, /, %, ||, &&, and ?), including the methods you can call on them like .toFixed, .toPrecision, .toString, and so on.

• The string type is the set of all strings and the operations you can perform on them (like +, ||, and &&), including the methods you can call on them like .concat and .toUpperCase.

When you see that something is of type T, not only do you know that it’s a T, but you also know exactly what you can do with that T (and what you can’t). Remember, the whole point is to use the typechecker to stop you from doing invalid things. And the way the typechecker knows what’s valid and what’s not is by looking at the types you’re using and how you’re using them.

In this chapter we’ll take a tour of the types available in TypeScript and cover the basics of what you can do with each of them. Figure 3-1 gives an overview.

Figure 3-1. TypeScript’s type hierarchy