Kea 2.6 will be the last 2.x of Kea. Version 3.0 is just around the corner with big changes, and version 2.6 is here to provide a smoother upgrade path.

• Update version requirements of peer dependencies: reselect 4.1+, redux 4.2+, react-redux 7+ and react 16.8+. If you're using React 18, upgrade react-redux to version 8+.

• React 18 support: Add a "redux listener silencing store enhancer", which prevents Redux's useSelectors from updating, when mounting a logic from within the body of a React component (e.g. with useValues). This effectively silences log spam in React 18 (Warning: Cannot update a component (Y) while rendering a different component (X). To locate the bad setState() call inside X, follow the stack trace as described.), and improves performance.

• Support custom selector memoization. Use memoizeOptions as the 4th selector array value, which is then passed directly to reselect:

• Set the autoConnectMountWarning option to true by default. Kea 2.0 introduced "auto-connect", and while it works great in reducers and selectors, automatically connecting logic in listeners turned out to be a bad idea. Thus, in Kea 2.6, when accessing values on an unmounted logic, you'll get a warning by default. In Kea 3.0, it will trigger an error.

Big news, we have a brand new logic testing framework!

Read all about it in the updated Testing guide!

Here's a teaser:

Oh, and Kea 2.5 is out as well, featuring logic.isMounted() and a bunch of fixes from the 2.4 series.

Finally, files generated with kea-typegen will automatically import any types they can, and add the rest as type arguments for kea<logicType<LocalType, LocalUser>>

You just need to add types to your actions and reducers.

Read the updated TypeScript guide to learn more.

Back in 2015, shortly after learning about React and Redux, I fell in love with the functional programming paradigms behind them because of what they enabled.

By following a few principles of immutability and purity, React frontends were generally better written, stabler and easier to debug, compared to contemporary alternatives such as Ember or Angular.

Having seen what a bit of functional programming did to JavaScript, I started looking into Clojure, the most popular functional language at the time, and into ClojureScript frontend frameworks: reagent, quiescent, om, om.next, and re-frame. Now there's also fulcro that wasn't around back then.

What stood out was how they all handled application state.

They all had developed at least three globally isolated layers:

No framework skipped the application state step. Nobody did this:

Yet that's exactly what you're doing when you store application state in React components.

The three layers#

All these frameworks reached the conclusion that the best way to convert API responses to DOM nodes was to first convert them to an intermediary representation: the global application state.

These three layers (input, data and view) have overlapping, yet separate and parallel hierarchies:

The structure of your input data is different from the structure of your application state, which is different from the structure of your DOM nodes.

The folks at Facebook/React agree: to build great user experiences, you benefit from having parallel data and view trees. They do use Relay after all for their state.

Yet the work with Concurrent Mode, Suspense, useTransition, startTransition, useDeferredValue, the suggestion to start fetching early in event callbacks, etc, is moving React in a direction where it's taking on more and more responsibilities from the data layer.

I think this is a clear step backwards for maintainability.

Put your data first#

I think it's time for a change. It's time for a paradigm shift.

It's time for a revolution in data-first frontend frameworks, which relegate React to what it does best: rendering and diffing DOM nodes. Values in, actions out. No useState. No useEffect.

In practice this means adopting a data layer as the core of your application.

It means going from:

to

Rob Pike puts it best:

Data dominates. If you've chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming.

In all my years of programming I have yet to see an exception to this rule. On the contrary, teaching this to junior developers has proven to be the single most impactful thing in improving their code quality.

Forget JSX templates. Forget algorithms. Get your data structures right, and the rest will follow:

From that moment forward, I would see this everywhere. If ever I felt like my program was getting too complicated or hard to read, it was almost always a data structure problem. Since then, every time I've been humbled by another programmer's code, it hasn't been by clever tricks or algorithms. It has always been by their ingenious insight into how their program's data ought to be organized.

It's time for a change. It's time for a paradigm shift. It's time for a revolution in data-first frontend frameworks that control the view layer and not the other way around.

Since you're reading this on the Kea blog, I'm obviously biased as to what's the best data layer for frontend developers. Suggesting alternatives is made even more complicated by the fact that most other tools position themselves as "state management" libraries, at the mercy of React.

Kea is one of the few frameworks for managing the complete lifecycle of your data. It uses React as the view layer and integrates nicely with the existing Redux ecosystem.

Go check it out and then start writing webapps the way they were meant to be written: data first.

Read the comments to this post on HN

What's that? A new release?!?

Kea 2.4 brings the following:

• Fixed crashes with React Fast Refresh.

• Changed the default path for logic without a path (or when not using the kea babel plugin) from kea.inline.2 to kea.logic.2. If you have ever hardcoded "kea.inline" anywhere, perhaps in tests, this will cause a bit of headache. If you need it set at kea.inline, use: resetContext({ defaultPath: ['kea', 'inline'] }).

• Added <Provider /> tag to simplify calling React-Redux's <Provider store={getContext().store} />.

Oh, and there's a new Kea CRA template you can use.

Starting a new CRA project with all the Kea tools configured is now as simple as running:

... and you're good to go!

TypeScript support in Kea just got a boost! Starting with kea-typegen 0.7.0 we also:

• Automatically add import { logicType } from './logicType' in logic files.
• Automatically add the logicType type to kea<logicType>().
• Separate changed files in the log with 🔥🔥🔥!

When using keyed logics it's easy to get into a situation where you need to pass props similar to this:

In Kea 2.3 you can use <BindLogic> to clear this up:

When you write <BindLogic logic={itemLogic} props={{ id: 12 }}>, Kea stores a specific build of itemLogic inside a React Context. Any nested children will get that when calling e.g: useValues(itemLogic).

Kea 2.2 adds support for TypeScript via kea-typegen and fixes a few bugs.

Check out the TypeScript Guide to learn more!

Even before Kea reached 1.0 last year, one topic kept popping up over and over again:

"Yeah it's great, but what about typescript?"

... or more eloquently:

"Unless the API has changed dramatically in the last few months it’s written in a way that ensure that it’s basically impossible to create effective typescript types to use it safely."

While that comment above is still technically true, as of version 2.2 (2.2.0-rc.1), Kea has full support for TypeScript!

The road there was long and winding... with plenty of dragons along the way.

Yet we prevailed!

But how?

TypeScript Support#

First up, it's relatively easy to add TypeScript to a project. Just install the deps, convert your files to .ts or .tsx, set compilerOptions in tsconfig.json to strict and add types until there aren't any anys left.

This already gives a lot!

For example an autocomplete for resetContext:

But we want more. This should work as well:

and this:

How on earth do we do that?

The Gallery of Failed Attempts#

As predicted by the Redditor quoted above:

"Unless the API has changed dramatically in the last few months it’s written in a way that ensure that it’s basically impossible to create effective typescript types to use it safely."

It turns out code like this is nearly impossible to type safely:

Who could have guessed?

There's a lot happening here:

1. We have a lot of keys (actions, reducers) inside one huge object literal {}
2. We have one action openBlog that takes an (id: number) and returns { id }
3. The reducers are specified as a function that gets the logic itself as its first parameter. That's some TS-killing loopy stuff right there!
4. The reducer blog uses the openBlog action (defined above in the same object!) to change its value
5. This reducer also depends on an action from a different logic, appLogic
6. The selector doubleBlog depends on the return type of the blog reducer
7. The selector tripleBlog depends on both blog and doubleBlog and their return types.

These are just a few of the complications.

This was going to be hard.

Yet I was determined to succeed, for I had on my side the strongest motivation on the planet: I had to prove someone wrong on the internet.

Attempt 1#

It immediately became clear that just getting rid of anys in the codebase wasn't going to be enough.

The JavaScript that converts kea(input) into a logic is just a bit too complicated for the TypeScript compiler to automatically infer types from it.

TypeScript Generics enter the game.

Just write a long TypeScript type that gets the kea(input) parameter's type, looks at its properties and morphs them into a LogicType. Write some functional loopy stuff in a funny markup language. No big deal.

So I thought.

The first attempt looked like this when stripped to its core:

This implementation gives us type completion when using the logic:

... but not when writing it:

The lines marked // ! are where this breaks down.

There's just no way to make the (logic: Logic<Input>) => any inside Input depend on the I extends Input that was passed to Logic<Input>.

Got all that? Me neither.

This kind of loopy stuff is just not possible with TypeScript:

With this attempt I got something to work, but ultimately without typing assistance inside the logic, it wouldn't prove someone on the internet wrong enough.

Back to the drawing board!

Attempt 2#

I first alluded to automatic type generation 10 months ago, yet it always seemed like a huge undertaking. There had to be an easier way.

What if I changed the syntax of Kea itself to something friendlier to TypeScript? Hopefully in a completely opt-in and 100% backwards-compatible way?

Surely there won't be any problems maintaining two parallel implementations and everyone using Kea will understand that this is Kea's hooks moment [1], right? Right?

Right?

Would something like this be easier to type?

I mean, it's just a slight alteration to this code that already works:

Surely not a big effort to refactor?

Unfortunately (or fortunately?), this approach didn't work either.

While this huge chain of type extensions sounds good in theory, you'll hit TypeScript's max instantiation depth limit eventually, as discovered by someone who was trying to add TS support to ... ehm, SQL?

I would experience the same. After a certain complexity the types just stopped working.

Definitely not ideal... and again won't prove someone on the internet wrong enough.

Attempt 3#

Attempt 3 was one more go at attempt 1, but by building out the types in the other direction:

So instead of:

I started with something like:

... only to fail even harder.

Attempt N+1#

There were many other experiments and types that I tried.

They all had their issues.

In the end, it appears that this kind of loopy syntax that Kea uses together with selectors that depend on each other just wouldn't work with TypeScript.

That's even before you take into account plugins and logic.extend(moreInput).

What now?#

I guess there's only one thing left to do.

My job now is to spend countless nights and weekends building kea-typegen, which will use the TypeScript Compiler API to load your project, analyse the generated AST, infer the correct types and write them back to disk in the form of logicType.ts files.

These logicTypes will then be fed back to the const logic = kea<logicType>() calls... and presto! Fully typed logic!

It's not ideal (ugh, another command to run), but it should work.

The stakes are high: If I fail or quit, the person on the internet will be proven right... and that is just not an option.

Automatic Type Generation#

Thus it's with great excitement that I can announce kea-typegen to the world!

It's still rough with a lot of things to improve, yet it's already really useful!

We've been using it in PostHog for about a week now, and it's working great!

Take that, random person on the internet!

Install the typescript and kea-typegen packages, run kea-typegen watch and code away!

Read the TypeScript guide for more details.

Rough Edges#

This is the very first version of kea-typegen, so there are still some rough edges.

1. You must manually import the logicType and insert it into your logic. This will be done automatically in the future.

2. You must manually hook up all type dependencies by adding them on the logicType in logic.ts. Kea-TypeGen will then put the same list inside logicType. This will also be done automatically in the future.

3. When connecting logic together, you must use [otherLogic.actionTypes.doSomething] instead of [otherLogic.actions.doSomething]

4. Sometimes you might need to "Reload All Files" in your editor at times... or explicitly open logicType.ts to see the changes.

5. Plugins aren't supported yet. I've hardcoded a few of them (loaders, router, window-values) into the typegen library, yet that's not a long term solution.

6. logic.extend() doesn't work yet.

These are all solvable issues. Let me know which ones to prioritise!

Alternative: MakeLogicType<V, A, P>#

At the end of the day, Kea's loopy syntax doesn't bode well with TypeScript and we are forced to make our own logicTypes and feed them to Kea.

However nothing says these types need to be explicitly made by kea-typegen. You could easily make them by hand. Follow the example and adapt as needed!

To help with the most common cases, Kea 2.2.0 comes with a special type:

Pass it a bunch of interfaces denoting your logic's values, actions and props... and you'll get a close-enough approximation of the generated logic.

The result is a fully typed experience:

You'll even get completion when coding the logic:

Thank you to the team at Elastic for inspiring this approach!

Closing words#

TypeScript support for Kea is finally here!

Well, almost. You can already use it in Kea v2.2.0-rc.1. The final v2.2.0 is not far away.

I've been building kea-typegen in isolation until now. I'd love to hear what the wider community thinks of it. Is it useful? What's missing? How can I improve the developer ergonomics? Can it work in your toolchain? Should I send the created logicTypes to GPT-3, so it would code the rest of your app? And who ate all the bagels?

Just open an issue and let's chat!

Also check out the samples folder in the kea-typegen repository for a few random examples of generated logic.

Finally here's a 12min video where I add TypeScript support to PostHog (we're hiring!):

Footnotes#

[1] Hooks Moment: A massive improvement in developer ergonomics at the cost of all old code becoming legacy overnight.

Kea 2.1 does two things:

• Continues the "let's make things simpler" trend started in Kea 2.0 by removing another bunch of squiggly bits that you will not need to type again: " ((((((()))))))===>>>{}"
• Adds support for accessing the state before an action was fired inside listeners.

It's also backwards compatible: Logic written for Kea version 1.0 will still run in 2.1.

The saga until now:#

This is Kea 1.0:

In Kea 2.0 we can skip [actions.] and { actions }:

You can still write [actions.] explicitly... and you do it mostly when using actions from another logic:

... but you save 41 keystrokes in the default case:

Changed in Kea 2.1:#

Why stop there?

There's another low hanging fruit we can eliminate: () => ({}).

Gone!

16 units of squiggly bits gone! Here they are, in chronological and ascending order:

They are there if you need them, of course. For example when using props in reducers:

What about the selectors? How can we simplify this?

Here's the simplest backwards-compatible change that went into Kea 2.1:

Goodbye another 14 spaces and squgglies:

If you're really feeling the minimalist vibe, you could also simplify the object in listeners and events, but:

You might get tired of writing thisLogic everywhere.

In general, the suggestion is to always write the simplest thing first:

... and only when needed, extend it into a function to pull in objects and evaluate lazily:

Previous State in Listeners#

There's a certain way listeners work:

The action will first update the reducers and only then run the listener.

What if you really need the state before the action ran?

You could set up a two step system (setFloorStart & setFloorUpdate) ... or you could use previousState, the new 4th argument to listeners:

Take the store's previousState (new 4th argument) and run it through any selector you can get your hands on. Every value has a selector, so you have plenty to choose from.