Type System React

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.

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.

In order to do that, I need to be able to reach into the provided type and grab something out. In this case, the emoji substring. And the the way you do that is with something called a conditional inferred type.

I'm going to actually just quickly type that up given a render, which maybe, maybe we're just going to start with objects. If the provided object matches some template, if raw extends, coincidentally the same extends keyword, text, let's call this string, then yay, otherwise, no. Here we see that okay, this conditional type is in fact saying that raw extends this object type text string. Yay, no. That's a conditional type.

Now we can add in this infer tag. So let's call this tag actually. Tag. Aha! Now, we are reaching into the provided type raw, grabbing out the tag, and then returning it kind of like again, a function of the type system. But we're able to do a little bit of logic almost here. We're checking whether this condition is met. And this is a ternary just like you would have a question mark and colon in the runtime space. But I mentioned earlier, I wanted not object shapes, but I wanted string shapes. So I want to be able to do this. Self-closing tag. And you can do that, actually. You can have template literal strings the type system, and you can infer within them. You can say, doesn't match this pattern. Infer tag. If so, give me back the tag. Emoji. But that's just the tag. So what I really want to do is get component registry of tag. And now we get that same complaint from before. Type tag cannot be used to index component registry, which means I again have to use the extends keyword to say, only match if the tag extends components. And voila. We are able to render a JSX element in the type system.

To recap before we move on and get more fancy, is we are checking, did this raw, and I'm just going to go ahead and say it should always be a string. Does this raw extend a string template like the following? Starts, some tag that's a component. Close. If so, give me back the contents under the registry of that tag. Otherwise, I'm going to call this invalid to make it clear.

Yay, we are halfway there. But I want more. I want to be able to, let's say, print hello world before the emoji, which right now isn't supported, because this does not strictly match the self-closing emoji tag. Fortunately, I can use these template literal strings in my results. I can add in another infer before, and print before, and that works. And I can do infer after, which in this case is an empty string. Let's say we want an exclamation mark there or something. Don't need that paint anymore. Let's scroll down a bit. It's beautiful. Hello world emoji. That works. But what happens if we have multiple emojis? Let's say I want to render three sparkles. Well, it doesn't right now, because when we infer the after, we're directly plopping it back as a string. We need to recursively render. We need to take whatever's after the component, registry into the tag, and give it back. We need to rerun the render on it, recursively rendering until there's nothing more, which actually does work. So that's awesome. But then we get invalid at the end, which actually I think makes sense, because once we're done rendering the last self-closing tag, what we have left is the empty string, which does not match this template. So instead of rendering invalid, we can render whatever we were given to begin with. There it is. Recursive descendant rendering.

But I want more. Always more. I want not just self-closing tags.

I want a heading. I want heading, hello world. Let's stop with these emojis quite so much. Let's do heading like this. There we go. Yeah, this isn't working. This is not detecting. So I'm going to need to add another condition.

Which, fun fact, you are allowed to do. I'm going to have to say if raw doesn't extend from the self-closing template, well, what if it extends from a different template? In this case, for children slash tag. So it's the same as before. We're adding another case where if it didn't match the self-closing template, but it does match a template for, let's see here, start tag, some inferred children, and end tag, then we do render, and it works. We see a heading rendered right here. Heading, and then we even still recurse onto the after. So we get the second, the sparkle. It's awesome, but I want children, which means I'm going to actually go ahead and write a utility here, because I want to render the children somewhere along ComponentRegistry.

So I'm going to write type render component, which takes in tag extends components, again, kind of like a function in the type system. And then I'm going to also take in children extends string. And I'm going to check, does components registry of tag extend some template that has before this indicator that I want children, then infer after if it does, then the result will be before the children and after. Otherwise, we're going to directly return component registry of tag, like nothing ever happened. And I'm going to use that here. Render components of tag. Ooh, and of children, and there we go. And voila, it works. Heading, which becomes the H1 and it renders the Hello World. If I were to ya-ya-ya, ya-ya-ya, it works. And it works because raw matches to this with children template, which means we then run to render component and component registry of, in this case, heading, does in fact match the template that contains children. So we effectively string replace the children with children. So that makes me happy. But a little bit more work, we're seeing that the children includes the raw emoji tag, but I actually wanted to render that.

So in the render component, I'm going to swap. Instead of directly returning component registry, I'm going to do render children, and that works. Hello World sparkles. Ah, now I suppose we could always render children in case the components refer to each other, but that's a little out of scope for this 20-minute talk.

So that's cool. And that's actually working pretty well. I'm just going to do a little bit of code cleanup. I'd like to use the same render component generic type here, but in the self-closing tag case it's an empty string for children, which means I can call onto a typed code feature default types to say in my render component if I don't have any children, just default them to the empty string. Making this code here I arguably think a little cleaner.

And there you have it folks, a working implementation of a very limited subset of React JSX rendering engine in the type system. Let's recap from the top before we finish up. I've declared a component registry type here, which is the concept of an object that has two properties. One emoji has the sparkle string emoji as its value. Not just any old string, but specifically that string. And also heading, which is h1, this indicator that we have children, slash h1. I then declared the utility type components here, which is any of the keys of component registry, practically speaking of emoji and heading. I then declare this render component type, which is a generic type. It takes in type parameters and returns a resultant type based on that. It takes in one of the components, so a tag. And then children, which is a string that defaults to the empty string. It checks. Does a component value under that tag extend from this template? So text before, the children indicator, text after. If it does extend from that template, then you render, or you return a result of before, the rendered children, and after, otherwise give back the component itself. That's used in our main, our driver, our worker type, render, which takes in some raw text and it checks two conditions. If the text is this template, any string before, the component tag inside a self-closing little area, and after, then we result in the before text, the render of the component, render of the after. Otherwise, if it's a component with a slightly different template, if it's before the lesson sign, the component itself with some children and then the component again with some after. Similarly, we render before. Render component, tag children. Render after. If the raw text didn't match either of those two templates, we return it directly.

And at the very end, print me is the result, where we take in this text, we recurse a couple free times, and we get back our string results. Yay. I feel very cool and strong doing that. And this makes me happy. It brings me a lot of joy to mess with TypeScript in this way. But I totally understand if this is not for you, if you've never played with TypeScript before or done deep things in the type system, that's totally fine.

Again, this is not something you need to use on a day-to-day TypeScript basis, it's just some cool stuff I think that is fun to explore with some of the type system techniques and foundations. If you want to get started with TypeScript, I do have most of the concepts you need for this in a different folder in the same repo source slash foundations.

So that was fun. Let's wrap up. Let's talk about a little bit of other stuff for now. If you want to learn more, typescriptblind.org and theplaygrounds slash play are fantastic. They're open source online. I highly recommend. I also mentioned I have a book, Learning TypeScript, which you can absolutely try out. Its website has a lot of free open source articles and projects to help you flesh out your TypeScript understanding. And if books aren't for you, or if you just want more or another resource, I really like mattpocockstotaltypescript.com. If you're wondering, this is all cool stuff, what on earth am I going to do with it? Well, a couple places you can go after this, definitelytyped is one of the largest public open source repositories in the world.

It is the place that all third party types, so packages that don't describe how they look to TypeScripts, you can describe how they look in this repo exist. So if you want to say, look at React's type, because React has definitely typed types, you can go to definitelytyped, types, react, index DTS, and see a whole bunch of syntax that actually uses a lot of the features we played with today. So if we ever have a problem with a third party type, or there's a package you use that doesn't have type information, definitely typed is a place you can go to make progress on that, to make it better for everyone. Secondly, type challenges is a great repository. Also, they've got a really cute little homepage, type challenge that links to the GitHub. It's got a whole bunch of starting small, getting much more complex type challenges to flex the concepts we showed today. Type system is incredibly powerful and could actually model not just React but TypeScript itself. People have implemented TypeScript in the TypeScript type system. If you want to be one of those ridiculous people, this is a way to get yourself there. Or you can just use the easy and medium challenges to flesh out your basic TypeScript and foundational TypeScript type system features. Everything we've talked about today is, at the very least, built towards my book Learning TypeScript. Highly recommended. I think it's pretty good. Also, because I'm a full-time open source maintainer, anything you can send my way, in terms of love, appreciation, money, issues filed, pull requests sent will help me work for all of y'all. Similarly, TypeScript VS Lint is an independent project. We're not associated with any one company. So, anything you can do to help us to file bugs, to send code and send us money would really be appreciated, to help us continue to make TypeScript better for everyone. With all that being said, I had a great time with this. I hope you enjoyed it. You can find me online as JoshuakGoldberg on most sites, including by joshuakgoldberg.com.