Type System React

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TypeScript's type system is incredibly powerful. It can represent bizarrely complex interdependent JavaScript types and comes with a Turing-complete set of logical conditions. But this is a React conference, right? Let's implement a primitive React purely in the type system. For fun!

Josh Goldberg
Josh Goldberg
21 min
12 Dec, 2023

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Video Summary and Transcription

This Talk is about implementing a limited subset of the type equivalent of React, specifically its JSX engine, in the TypeScript type system with no runtime code. The speaker demonstrates how to use TypeScript features like constrained types and template literal strings to infer and render JSX elements in the type system. They also show how to render headings and children using a utility called 'render component'. The Talk concludes with additional resources for learning TypeScript and related topics.

Available in Español: Sistema de Tipos React

1. Introduction to Type System React

Short description:

Hello and welcome to Type System React with me, Josh Goldberg. I am a full-time open source maintainer in the TypeScript ecosystem. We're going to implement a limited subset of the type equivalent of React, just its JSX engine in the type system with no runtime code. Let's begin in TypeScript lang dot org slash play.

Hello and welcome to Type System React with me, Josh Goldberg. I am a full-time open source maintainer in the TypeScript ecosystem. I work on general projects that help you write TypeScript a little better, most notably TypeScript ESLint, the tool that lets you run standard JavaScript tools, such as ESLint and Prettier on TypeScript code.

I'm also the author of the learning TypeScript book through O'Reilly, so I like talking about TypeScript. Everything we're going to chat about today is on GitHub and open source under Type System React, a repo in my Josh Goldberg user, but I want to note that this is not normal TypeScript. You don't need to follow along precisely and get everything fully to be a TypeScript dev or to work proficiently in TypeScript. This is all shenanigans today, and while the concepts I'm going to show are actually really useful for working in the type system if you do it a lot, it's not stuff that you should be using day-to-day. It's all silly stuff, all the weird shenanigans.

Because what we're going to do is implement a very limited subset of the type equivalent of React, really just it's JSX engine in the type system with no runtime code. We're going to make a type, let's say a component registry. We're going to make a helper type render that takes in a string and gives back the JSX results, the rendered results, and we're going to print that out to the developer tooling stuff that's running TypeScript for us. So very much not practical. You would never satisfy any user needs with this, but I think it's a cute way to explore the type system.

So let's begin. You can find all the code I'm going to send my live code under the repo under source slash fun, but I'm going to be working in TypeScript lang dot org slash play, which is a really nice playground available on the TypeScript website. You can type in TypeScript code on the left and get back JavaScript code or any errors, declaration files on the right. Now on the left, I have just some standard TypeScript code. I have a type component registry and a console log, and we can see that the type system representation just includes the type and the JS representation just includes the JavaScript. But I mentioned that today is purely in the type system. So I'm just going to go ahead and remove our console log. No JavaScript just types. But that brings up the question of how are we going to print things. So I'm going to make this little temporary type print me here. I'm going to say I want to print component registry. And there's actually a nice little TypeScript playground and other TypeScript editors feature called to slash assertions. You can get an extension of VS Code to do this. But if you write a comment that has this little caret and a question mark, it will ask it to print to the screen what you would have gotten if you were to hover your mouse there. So here we got type print me as the component registry type. And printed out, we got the stringified version of that. Emoji is sparkling heart.

2. Exploring Tag Rendering and JSX in TypeScript

Short description:

I'm going to switch to sparkles and heading is H1 with children. I want to be able to get a tag name and render out the contents under that tag. TypeScript has a feature called a constrained type to ensure we're only passing in one of the actual keys. I'm going to extract that out to a component type so that whenever I say component, what I really mean is one of the keys of the registry. I want to be able to have JSX and self-closing tags, which is not supported currently.

I'm actually going to switch to sparkles, a little less glaring, and heading is H1 with children. Hooray. Printing in the type Printing in the type system.

But I need to do more than that. I want to be able to, let's say, get a particular tag name and render out the contents under that tag. And you can do that in TypeScript with this index signature lookup, this little array type. Here we're saying give me under component registry the type under the emoji property name, which in this case, yes is sparkling hearts, pardon me, sparkles. If we were to switch that to heading, we'd get H1 children slash H1. If we were to switch that to, I don't know, ASDF WAT, some gibberish, we'd get Red Squiggly's property ASDF WAT does not exist in type, component registry.

Cool. And that little constraint there, the fact that it needs to be one of the actual keys of the type is useful because I want to write a render type which takes in a type parameter and gives me back component registry of that type parameter, making a kind of dynamic version of what I'm doing here. If I were to say want to render emoji, this fun fact actually works. This is what's called a generic type or type with a type parameter. It's kind of like a function in the type system. We take in a tag, say emoji, the string, and then we do something. We create a new type with that tag. Here we're making component registry of tag as a result, but we're getting the complaint type tag cannot be used to index type component registry. In fact, if we switch to the errors tab here, we can see that. Well, that makes sense because what if I passed in asdf? We need some way of making sure we're only ever passing in one of the actual keys of this type. And TypeScript has a feature for that, it's called a constrained type. With the extends keyword, we can extend key of component registry. Voila, no more red squigglies. Print me is happily the sparkling emoji, and here we're saying tag extends or must be one of the keys of component registry. I'm actually going to extract that out to a component type here so that whenever I say component, what I really mean is one of the keys of the registry. If we were to switch this to heading, yep, it renders the heading nicely. Rendering, it's a start. But I want to be able to do more than just take in a tag name and render under the tag. I want to be able to have JSX. I want my little self-closing tags. I want to be able to do something like emojis self-closing, which right now is not supported.

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