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Middlewares are implicit methods to deal with side effects of your superstates.

Let's say we want to display an alert whenever a superstate changes. One simple way to achieve it would be through a simple Middleware:

function myMiddleware({ eventType }) {
if (eventType === 'after:change') {
alert('hello world!')


const count = superstate(0).use([myMiddleware])

count.set(10) // here your `alert()` would be called

And honestly, Middlewares are that simple!

Event Types

One of the most important aspects of a Middleware is the eventType property superstate passes to it. The eventType helps you decide what to do and when.

Here's a list of all the available event types:


The init event type

The init event type is executed once when a Middleware is successfully attached to a superstate:

function initMiddleware({ eventType }) {
if (eventType === 'init') {
alert('initMiddleware was installed successfully!')


const count = superstate(0).use([initMiddleware]) // the `alert()` would be called here.

May be important to note that init won't ever be called again despite this very first time.

before:change and after:change

Just like init, these change events are special, because it has two different instigators.

It'll be triggered when...

  • you .publish() successfully and/or,
  • you .set() successfully.

Note that the change event will not trigger for changes made to the draft. If you are interested on draft events, please use before:sketch and after:sketch.

Before/After events

As you may have noticed, some events are prefixed with before: or after:.

These events will only trigger when an action was successfully achieved, and only before/after the action itself.

To illustrate an example, before:publish occurs when you call .publish(), but before the publish method has published your changes; at the same time, after:publish also occurs when you call .publish(), except it will be executed after the publish method has published your changes. Both before:publish and after:publish will only occur if publishing was successful.

Advanced Middlewares

eventType is not the only property exposed to Middlewares. Actually, Middlewares have access to the exact same functions a superstate object has, such as publish, set, discard, etc (Learn more). To illustrate you an example:

function myMiddleware({ eventType, set, now }) {
if (eventType === 'after:publish') {

const count = superstate(0).use([myMiddleware])


The Middleware above will assign your draft with the value of now(), which is 5, every time you .publish() something.

Infinite loop!

If you call publish() inside a Middleware that does something after you publish(), expect an infinite loop. That said, don't do that!

When Middlewares make sense

From Wikipedia:

Middleware is a type of computer software that provides services to software applications beyond those available from the operating system. It can be described as "software glue".

Middleware makes it easier for software developers to implement communication and input/output, so they can focus on the specific purpose of their application.

To name you a practical example of where a Middleware can be useful, the localStorage adapter is a Middleware. Instead of having to manually write/read to/from localStorage, a Middleware was written to do that for us.

If you need to automate something in your superstates, Middlewares can be the answer.


function myMiddleware<T = number>(input: IMiddlewareInput<T>): void {
// ... your function

const count = superstate(0).use([myMiddleware])

Feel free to change T = number to whatever you need.