Sometimes you need to react to changes of the state. You might want to run a sequence related to a change of the state or maybe some logic inside a component. Reactions allows you to express this kind of logic.

Reactions in modules 

When you create a module you can attach a reaction to it:

import { Module } from 'cerebral'
import * as reactions from './reactions'

export default Module({

So you would typically create your reactions in a separate file, reactions.js:

import { Reaction } from 'cerebral'
import { state, sequences } from 'cerebral'

export const pageChanged = Reaction(
    page: state.currentPage
  ({ page, get }) => {
    get(sequences.openPage)({ page })

This reaction will react to whenever the current page is changed and fire off the openPage signal, with the new page.

Reactions in views 

You can also create reactions inside views, here shown with React:

import * as React from 'react'
import { connect } from '@cerebral/react'
import { state, sequences } from 'cerebral'

export default connect(
    inputValue: state.inputValue,
    changeInputValue: sequences.changeInputValue
  class MyComponent extends React.Component {
    componentDidMount() {
          error: state.usernameError
        ({ error }) => error && this.input.focus()
    render() {
      return (
          ref={(node) => {
            this.input = node
          onChange={(event) =>
            this.props.changeInputValue({ value: })

These reactions are destroyed when the view is unmounted.

Also the view reaction receives the get property allowing you to grab arbitrary state etc. A reaction can also depend on a computed, allowing you to react to computed updates.