Meiosis Documentation

Table of Contents

The Fundamental Meiosis Setup

We now have an understanding of streams. In this section, we will build the fundamental Meiosis setup. As you will see, it's very simple. It's the starting point for data flow management. After this, we can further improve the setup in various ways, but there isn't a single "best" way because that depends on each project, developer preferences, and so on. That's the good news about the setup: there is a foundation, but you are free to adapt it to what works best for you.

Also, know that the setup code is something you write just once for your web application. It is the top-level glue code that wires up a reactive loop through which our data will flow, and by which our UI automatically refreshes.

The Top-Level Model

The first thing we'll do in Meiosis is have a single source of truth. This is the top-level model. This model contains the data that we need to produce our user interface, or "view".

Let's build a simple example where we have a Temperature value that we can increase by pressing a button. The model for our example is:

  value: 20

Pretty simple. Next, let's look at actions.


Actions are given an update callback to signal updates to the model. The way an action signals an update is by invoking the callback and passing it a function that gets the current model and returns the updated model containing the changes.

const createActions = update => ({
  increase: () => update(model => {
    model.value = model.value + 1;
    return model;

We've created an actions object with an increase function that ion invokes the update callback and passes it a function that increases the model's value by 1, and returns the updated model.

The View

The view is created by passing the actions object. In return, we get a function of the model that renders the view. The view will signal updates simply by calling action functions.

Here is our function that creates the view:

const createView = actions => model => (
    <span>Temperature: {model.value}&deg;C </span>
    <button className="btn btn-default"

I am using React in this example, but this will work just as well with a React alternative (Preact, Inferno), any other virtual DOM library (Mithril, Snabbdom, etc.), or also other libraries such as lit-html and hyperHTML.

The view renders the temperature value with model.value and calls the increase action when the user clicks on the button.

The Meiosis Setup

So far, we've written application code, code that defines the model and the view. In this section, we'll write setup code, which is something that we only need to do once in an application. We'll set up a stream for model updates, produce a stream of models, and render the views:

const update =;
const models = flyd.scan((model, func) => func(model),
  { value: 20 }, update);

const view = createView(createActions(update));
const element = document.getElementById("app"); => ReactDOM.render(view(model), element));

The code above is the fundamental Meiosis setup:

And there we have it. The Meiosis setup is to have a stream of functions that update the model, a stream of models by scanning the model updates, and a re-rendering of the view by mapping the stream of models and calling the virtual DOM library's render function.

Note that the last line of code is the only change you need to make to use a different virtual DOM library (besides changing the view function of course.) For example, with Mithril you would use => m.render(element, view(model, update)));

Now, you will see the Temperature initially at 20, and go up by 1 every time you press the Increase button.

You can see the full code example and experiment with it below:

Principles / Takeaways

You've seen a simple example of the Meiosis setup. Here are some takeaway principles:

Up Next

Next, we'll look at structuring code into components.

Table of Contents
