In the last chapter I introduced the idea of type systems, but I never defined what the type in type system really means.
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
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month, and much more.
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.
Julian F.
Head of Cybersecurity
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.
Addison B.
Field Engineer
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.
Amir M.
Data Platform Tech Lead
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.
