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 * Thanks for this FSM implementation to Zakhar Ovcharov (
 * I did small adjustments to meet requirements of the specific task

class FSM {
   * Each instance of this class will have a certain alphabet, a list of states and transitions, and could be used to test some value against these params
   * @param {String} alphabet
   * @param {String[]} states
   * @param {String} startState
   * @param {String[]} finiteStates
   * @param {Object} transitions
  constructor(alphabet, states, startState, finiteStates, transitions) {
    this.alphabet = alphabet
    this.states = states
    this.startState = startState
    this.transitions = transitions
    this.finiteStates = finiteStates
    this.currentState = null

   * Check if the symbol belongs to the specified alphabet
   * @param {String} symbol
   * @returns {Boolean}
  _checkIsBelongAlphabet(symbol) {
    return this.alphabet.includes(symbol)

   * Change the current state depending on the symbol. FSM should abort if there is no transition for the current symbol on the current state
   * @param {String} symbol
  _changeState(symbol) {
    if (
      this.transitions[this.currentState] &&
    ) {
      this.currentState = this.transitions[this.currentState][symbol]
    } else {
      throw new Error(`No transition from ${this.currentState} by ${symbol}`)

   * Test some value against specified params. Accepts value if the finiteStates array includes the last current state and rejects otherwise
   * @param {String} value
   * @returns {String}
  test(value) {
    this.currentState = this.startState

    for (let symbol of value) {
      if (this._checkIsBelongAlphabet(symbol)) {
      } else {
        return false

    return this.finiteStates.includes(this.currentState)

// Params in order: alphabet, states, initial state, finite states, transitions
// We have to define transitions as an object, where:
// 1. Key is the state
// 2. Value is the object, where:
// 2.1. Key is the symbol
// 2.2. Value is the target state
const fsm = new FSM('01', ['q0', 'q1'], 'q0', ['q0'], {
  q0: {
    0: 'q0',
    1: 'q0'
  q1: {
    0: 'q1',
    1: 'q1'

console.log(fsm.test('111')) // true
console.log(fsm.test('0111')) // true
console.log(fsm.test('1112')) // false, 2 doesn't belong to the alphabet '01'