TypeScript Cookbook

Chapter 3. The Type System

In the previous chapter you learned about the basic building blocks that allow you to make your JavaScript code more expressive. But if you are experienced in JavaScript, you understand that TypeScript’s fundamental types and annotations cover only a small set of its inherent flexibility.

TypeScript is supposed to make intentions in JavaScript clearer, and it wants to do so without sacrificing this flexibility, especially since it allowed developers to design fantastic APIs used and loved by millions. Think of TypeScript more as a way to formalize JavaScript, rather than restrict it. Enter TypeScript’s type system.

In this chapter, you will develop a mental model for how to think about types. You will learn how to define sets of values as widely or as narrowly as you need, and how to change their scope throughout your control flow. You will also learn how to leverage a structural type system and when to break with the rules.

This chapter marks the line between TypeScript foundations and advanced type techniques. But whether you are an experienced TypeScript developer or just starting out, this mental model will be the baseline for everything to come.

3.1 Modeling Data with Union and Intersection Types

Problem

You have an elaborate data model you want to describe in TypeScript.

Solution

Use union and intersection types to model your data. Use literal types to define specific variants.

Discussion

Suppose you are creating a data model for a toy shop. ...