a tiny functional state machine

Usage no npm install needed!

<script type="module">
  import fnMachine from 'https://cdn.skypack.dev/fn-machine';



A tiny, functional, state machine utility


npm install --save fn-machine


fn-machine consists of 3 functions. The first two are used to define a machine:

machine([State], 'initialState', initialContextObj, stateChangeCallback, loggerFn)

state('name', transitionsObj, enterFunction, exitFunction)

The third function is what would traditionally be called a send() function. This function is returned whenever machine(...) is called.

Setting up a machine

// import the setup functions
import {machine, state} from 'fn-machine';

// initial context object
const initialContext = {
  loading: false,
  users: []

function loadUsers() {
  // simulate a network request
  setTimeout(() => {
    // once the request completes, we can call `myMachine` (the 'send' function).
    myMachine('loaded', {users:['foo', 'bar']})
  }, 1000);

// initialize a machine
const myMachine = machine([
  state('initial', {
    // each method on this object represents a transition for this particular state.
    loadData: (detail, context) => {
      // a transition method should return the new state, as well as the optional context.
      // here we return {state:'loadingData'} to signify we want the state to now be 'loadingData', and
      // that the context.loading property should be true.
      return {
        context: {...context, ...{loading: true}}
  state('loadingData', {
    loaded: (detail, context) => {
      return {
        state: 'loadedData',
        context: {...context, ...detail, ...{loading: false}}
  }, context => {// call loadUsers when this state is entered
  state('loadedData', {}) // 'loaded' is an empty state. There are no transitions.
], 'initial', initialContext, newState => {
  console.log('myMachine state changed:', newState.state, newState.context);
}, console.log);// pass an optional logger function

As you can see in the loadUsers() function above, we invoke the third function provided by fn-machine, which is the send function. The send function takes a string as the first parameter, which is the name of a transition we'd like to invoke, and optionally a detail object, which might contain some data we want the machine to work with.

You can also define transitions using a short-hand syntax like so:

state('myState', {
  someAction: 'newState',

which is equivelent to:

state('myState', {
  someAction: (detail, context) => {
    return {
      state: 'newState',
      context: {...context, ...detail},

More examples

There is an example in this repo, or you can play around with this codepen that shows a basic integration with LitElement.


There are two utility functions to convert to and from mermaid syntax.

toMermaid([state('on', {powerOff: 'off'}, state('off', {powerOn: 'on'}))], 'off');

produces a string like that you can process with mermaidjs to visualize your machine:

[*] --> off
on --> off: powerOff
off --> on: powerOn

Or, you can take a mermaid string and output some stub javascript:

const mermaidStr = `
[*] --> off
on --> off: powerOff
off --> on: powerOn

which produces:

[state('on', {powerOff: 'off'}, state('off', {powerOn: 'on'}))]

These are useful for visualization and initial creation of your machines, but beware that if your machine transitions contain logic, that logic would be lost should you try to go full circle: machine -> mermaid -> machine.


Yes! PR's are welcome. Tests are written in mocha. Run with npm run test or yarn test. Typechecking is provided by typescript via JSDoc annotations.