Nonlinear Programming Methods
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Monitos (Spanish for little monkeys) are state machines. Their transitions can be either predetermined or have certain randomness described by saving throws.

A saving throw is a roll of 20-sided dice to determine if a specific transition should be taken or not.


let chimp = new Monito({ 
    initialState: 'register', 
    states: {
        register: (next) => {
            next(null, 'getProfile');   // Go straight to the state "getProfile"
        getProfile: (next) => {
            next(null, {
                browse: 4               // If your dice rolls 4 or more, go to "browse"
            }, 'shop');                 // Otherwise, by default, go to "shop"
        browse: (next) => {
            next(null, {
                browse: monito => 6     // You can also use functions
            }, 'shop');                 
        shop: (next) => {
            next(null, 'logout');       // Go straight to the state "logout"
        logout: (next) => {
            next();                     // This will be the last state
chimp.on('transition', data => {
    console.log(data.previousState + ' -> ' +data.nextState);
chimp.on('end', data => {
    console.log('Stopped in state ' + data.finalState);
chimp.on('error', data => {
    console.log('An error occurred in state ' + data.currentState);

See the full code of the example.



let monito = new Monito(options);
monito.on('transition', data => { });
monito.on('end', data => { });


The options object expects the following properties:

  • initialState (String, mandatory) - Name of the initial state.
  • states (Object, mandatory) - The state descriptor. An object where every key is the name of a state and its value is a function with signature (Function callback), whose scope is the monito instance itself. The function callback has the following signature:
    • callback([Object error[, Object savingThrows[, String defaultNextState]]])
      • error (Object, optional) - Like in most of the callback signatures, an optional errorsis the first argument. null if everything went fine.
      • savingThrows (Object, optional) - An object where the key is the name of a possible next state candidate and its value is the difficulty of the saving throw to go into that state.
      • defaultNextState (String, optional) - Name of the default next state, in case savingThrows is present and none of those savings are passed.

There are three possible ways to make this callback: * callback(Object error, Object savingThrows, String defaultNextState) - Performs the saving throws described in savingThrows; if all of them fail, the next state will be defaultNextState. * callback(Object error, String nextState) - Goes right away to nextState. * callback(Object error) and callback() - Ends the state machine.


  • error(Object data) - Whenever an error occurred. The object data contains the following properties:
    • err (Error)
    • currentState (String) - The state in which the error took place.
  • transition(Object data) - Fired when there is a transition to a new state. The object data contains the following:
    • previousState (String) - Name of the previous state, undefined if this is the first transition of the state machine.
    • nextState (String) - Name of state the machine is transitioning to.
  • end(Object data) - Fired when the state machine comes to an end. The object data contains the following property:
    • finalState (String) - Name of the state in which the monito stopped.


  • monito.getTransitionChallenge() -> Function - Gets the function currently used as a transition challenge.
  • monito.setTransitionChallenge(Function challenge) - Sets a custom challenge to be used in transitions. If the function returns truthy, the challenged will be passed. Otherwise, it won't.
  • monito.start() - Starts the state machine.


Run all the tests (linting and unit tests) with the following npm task:

> npm test

You can run the linting and the unit tests individually:

> gulp test
> gulp lint




npm i monitos

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