playing with a simple state machine

state_machine.js

function createState(props){
  if(!props.name){
    throw new Error('state name required');
  }
  
  const defaults = {
    canEnter(toState, fromState, appState){ return true; },
    onEnter(toState, fromState, appState){ console.log(`entering ${toState.name}`) },
    canExit(fromState, toState, appState){ return true; },
    onExit(fromState, toState, appState){ console.log(`exiting ${fromState.name}`) }
  };

  return {...defaults, ...props};
}

const appState = {
  count: 0
};

const stateMap = {
  init: createState({
    name: 'init',
    canEnter(toState, fromState, appState){ return appState > 0;}
  }),
  one: createState({
    name: 'one',
    onEnter(toState, fromState, appState){ appState.count++; }
  }),
  two: createState({
    name: 'two',
    canExit(fromState, toState, appState){ return appState.count >= 2; },
    onEnter(toState, fromState, appState){
      setTimeout(() => appState.count++, 3000);
    }
  }),
  three: createState({
    name: 'three',
    canEnter(toState, fromState, appState){ return fromState.name != 'one'; }
  })
}

let current = stateMap.init;
const stateHistory = [current.name];

function transitionTo(stateName){
  console.log('appState', appState);
  const nextState = {...stateMap[stateName]};
  if(nextState.name === current.name){
    console.log('prevent transition: same state');
    return;
  }
  if(!current.canExit(current, nextState, appState)){
    console.log('prevent transition: cant exit');
    return;
  }
  if(nextState.canEnter(nextState, current, appState)){
    current.onExit(current, nextState, appState);
    current = nextState;
    stateHistory.push(current.name);
    current.onEnter(nextState, current, appState)
  } else {
    console.log('prevent transition: cant enter');
  }
}