Simple FSM Based on Redux.js Style of State Management - Features Timewarping

fsm.js

import { merge, mergeWith } from 'lodash-es';

const history = [];         // history of all states
let timePeriod = 0;         // index for history, used to timewarp throughout the history
let state = Object.freeze({});     // current state

/**
 * Recursively merges all objects, and overwrites
 * all other values.
 * @param  {*} destObj Current state
 * @param  {*} srcVal  Any value to be merged
 * @return {*}         Resolved value
 */
export function customizer(destObj, srcVal) {
  if (isObj(srcVal)) {
    return merge(destObj, srcVal);
  } else {
    return srcVal;
  }
}

/**
 * Fast forward history by one iteration.
 * @return {Object} History state
 */
export function fastForwardTime() {
  timePeriod = Math.min(history.length - 1, ++timePeriod);
  return getHistory(timePeriod);
}

/**
 * Freeze the new state to prevent mutations
 * @param {Object} candidate New State candidate
 * @return {Object}          New Immutable State
 */
export function freezeState(candidate) {
  return Object.freeze(candidate);
}

/**
 * Get the history for a specific time period
 * @param  {Number} period  History index
 * @return {*}              History state
 */
export function getHistory(period) {
  return history[period];
}

/**
 * Get current state
 * @return {Object} Current state
 */
export function getState() {
  return state;
}

/**
 * Is a value an object?
 * @param  {*}  val   Any JavaScript value
 * @return {Boolean}  True, if value is an object; false, otherwise.
 */
export function isObj(val) {
  return typeof val === 'object' && !(val instanceof Array);
}

/**
 * Timewarp function that sets the current time period
 * to the present or most recent state
 * @return {Object} Latest history state
 */
export function presentTime() {
  timePeriod = history.length - 1;
  return getHistory(timePeriod);
}

/**
 * Rewind history by one iteration
 * @return {Object} History state
 */
export function rewindTime() {
  timePeriod = Math.max(0, --timePeriod);
  return getHistory(timePeriod);
}

/**
 * Set a new state
 * @param {Object} newState New state to be merged
 * @return {Object}         New state candidate that has been merged with previous state
 */
export function setState(newState) {
  const candidate = mergeWith(state, newState, customizer);
  state = freezeState(candidate);
  history.push(state);
  timePeriod = history.length - 1;
  return state;
}

export default {
  getState,
  fastForwardTime,
  presentTime,
  rewindTime,
  setState
};