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State, Actions & Reducer
The Record API is in feature-freeze. For the newest features and better support going forward, please consider migrating to the new function components.

Finally, we're getting onto stateful components!

ReasonReact stateful components are like ReactJS stateful components, except with the concept of "reducer" (like Redux) built in. If that word doesn't mean anything to you, just think of it as a state machine. If that word does mean something to you, just think: "Woah this is great".

To declare a stateful ReasonReact component, instead of ReasonReact.statelessComponent("MyComponentName"), use ReasonReact.reducerComponent("MyComponentName").

Here's a complete, working, stateful ReasonReact component. We'll refer to it later on.

/* State declaration */
type state = {
  count: int,
  show: bool,

/* Action declaration */
type action =
  | Click
  | Toggle;

/* Component template declaration.
   Needs to be **after** state and action declarations! */
let component = ReasonReact.reducerComponent("Example");

/* greeting and children are props. `children` isn't used, therefore ignored.
   We ignore it by prepending it with an underscore */
let make = (~greeting, _children) => {
  /* spread the other default fields of component here and override a few */

  initialState: () => {count: 0, show: true},

  /* State transitions */
  reducer: (action, state) =>
    switch (action) {
    | Click => ReasonReact.Update({...state, count: state.count + 1})
    | Toggle => ReasonReact.Update({...state, show: !})

  render: self => {
    let message =
      "You've clicked this " ++ string_of_int(self.state.count) ++ " times(s)";
      <button onClick=(_event => self.send(Click))>
      <button onClick=(_event => self.send(Toggle))>
        (ReasonReact.string("Toggle greeting"))
          ? ReasonReact.string(greeting)
          : ReasonReact.null


ReactJS' getInitialState is called initialState in ReasonReact. It takes unit and returns the state type. The state type could be anything! An int, a string, a ref or the common record type, which you should declare right before the reducerComponent call:

type state = {count: int, show: bool};

let component = ReasonReact.reducerComponent("Example");

let make = (~onClick, _children) => {
  initialState: () => {count: 0, show: true},
  /* ... other fields */

Since the props are just the arguments on make, feel free to read into them to initialize your state based on them.

Actions & Reducer

In ReactJS, you'd update the state inside a callback handler, e.g.

  /* ... other fields */
  handleClick: function() {
    this.setState({count: this.state.count + 1});
  handleSubmit: function() {
  render: function() {
    return (
        onSubmit={this.handleSubmit} />

In ReasonReact, you'd gather all these state-setting handlers into a single place, the component's reducer! Please refer to the first snippet of code on this page.

Note: if you ever see mentions of self.reduce, this is the old API. The new API is called self.send. The old API's docs are here.

A few things:

  • There's a user-defined type called action, named so by convention. It's a variant of all the possible state transitions in your component. In state machine terminology, this'd be a "token".
  • A user-defined state type, and an initialState. Nothing special.
  • The current state value is accessible through self.state, whenever self is passed to you as an argument of some function.
  • A "reducer"! This pattern-matches on the possible actions and specifies what state update each action corresponds to. In state machine terminology, this'd be a "state transition".
  • In render, instead of self.handle (which doesn't allow state updates), you'd use self.send. send takes an action.

So, when a click on the dialog is triggered, we "send" the Click action to the reducer, which handles the Click case by returning the new state that increments a counter. ReasonReact takes the state and updates the component.

Note: just like for self.handle, sometimes you might be forwarding send to some helper functions. Pass the whole self instead and annotate it. This avoids a complex self record type behavior. See Record Field send/handle Not Found.

State Update Through Reducer

Notice the return value of reducer? The ReasonReact.Update part. Instead of returning a bare new state, we ask you to return the state wrapped in this "update" variant. Here are its possible values:

  • ReasonReact.NoUpdate: don't do a state update.
  • ReasonReact.Update(state): update the state.
  • ReasonReact.SideEffects(self => unit): no state update, but trigger a side-effect, e.g. ReasonReact.SideEffects(_self => Js.log("hello!")).
  • ReasonReact.UpdateWithSideEffects(state, self => unit): update the state, then trigger a side-effect.

Important Notes

Please read through all these points, if you want to fully take advantage of reducer and avoid future ReactJS Fiber race condition problems.

  • The action type's variants can carry a payload: onClick=(data => self.send(Click(
  • Don't pass the whole event into the action variant's payload. ReactJS events are pooled; by the time you intercept the action in the reducer, the event's already recycled.
  • reducer must be pure! Aka don't do side-effects in them directly. You'll thank us when we enable the upcoming concurrent React (Fiber). Use SideEffects or UpdateWithSideEffects to enqueue a side-effect. The side-effect (the callback) will be executed after the state setting, but before the next render.
  • If you need to do e.g. ReactEvent.BlablaEvent.preventDefault(event), do it in self.send, before returning the action type. Again, reducer must be pure.
  • Feel free to trigger another action in SideEffects and UpdateWithSideEffects, e.g. UpdateWithSideEffects(newState, (self) => self.send(Click)).
  • If your state only holds instance variables, it also means (by the convention in the instance variables section) that your component only contains self.handle, no self.send. You still need to specify a reducer like so: reducer: ((), _state) => ReasonReact.NoUpdate. Otherwise you'll get a variable cannot be generalized type error.


Cram as much as possible into reducer. Keep your actual callback handlers (the self.send(Foo) part) dumb and small. This makes all your state updates & side-effects (which itself should mostly only be inside ReasonReact.SideEffects and ReasonReact.UpdateWithSideEffects) much easier to scan through. Also more ReactJS fiber async-mode resilient.

Async State Setting

In ReactJS, you could use setState inside a callback, like so:

setInterval(() => this.setState(...), 1000);

In ReasonReact, you'd do something similar:

Js.Global.setInterval(() => self.send(Tick), 1000)