Learning TypeScript

Chapter 2. The Type System

JavaScript’s power

Comes from flexibility

Be careful with that!

I talked briefly in Chapter 1, “From JavaScript to TypeScript” about the existence of a “type checker” in TypeScript that looks at your code, understands how it’s meant to work, and lets you know where you might have messed up. But how does a type checker work, really?

What’s in a Type?

A “type” is a description of what a JavaScript value shape might be. By “shape” I mean which properties and methods exist on a value, and what the built-in typeof operator would describe it as.

For example, when you create a variable with the initial value "Aretha":

let singer = "Aretha";

TypeScript can infer, or figure out, that the singer variable is of type string.

The most basic types in TypeScript correspond to the seven basic kinds of primitives in JavaScript:

• null

• undefined

• boolean // true or false

• string // "", "Hi!", "abc123", …​

• number // 0, 2.1, -4, …​

• bigint // 0n, 2n, -4n, …​

• symbol // Symbol(), Symbol("hi"), …​

For each of these values, TypeScript understands the type of the value to be one of the seven basic primitives:

• null; // null

• undefined; // undefined

• true; // boolean

• "Louise"; // string

• 1337; // number

• 1337n; // bigint

• Symbol("Franklin"); // symbol

If you ever forget the name of a primitive, you can type a let variable with a primitive value into the TypeScript Playground or an IDE and hover your mouse over the variable’s name. The resultant popover ...