Estado

Simple, stateless JavaScript finite-state machines.

What is it? Estado is a tiny, framework-agnostic JS library for representing finite-state machines and hierarchical state machines, or Harel statecharts. Its main use is as a pure (extended) transition function of the form (state, action) -> state.

Why? (Article coming soon!) TL;DR: Finite state machines are extremely useful for representing the various states your application can be in, and how each state transitions to another state when an action is performed. Also, declaring your state machine as data (Estado language parses to pure JSON, and is SCXML-compatible) means you can use your state machine, well, anywhere. Any language.

(Example coming soon!)

Getting Started

1. Install via NPM: npm install estado --save
2. Import the state machine creator into your project:

import { machine } from 'estado';

let lightMachine = machine`
  green -> yellow (TIMER)
  yellow -> red (TIMER)
  red -> green (TIMER)
`;

// Pure, stateless transition functions
let currentState = lightMachine.transition('green', 'TIMER');
// => 'yellow'

let newState = lightMachine.transition(currentState, 'TIMER');
// => 'red'

// Initial state
let initialState = lightMachine.transition();
// => 'green'

The Estado language

Estado allows you to parse an easy-to-learn DSL for declaratively writing finite state machines.

####States

A state is just an alphanumeric string (underscores are allowed) without quotes or spaces: some_valid_state. Final states are appended with an exclamation point: someFinalState!.

By default, the first state declared in a state group is an initial state.

####Transitions

A transition (edge) between states is denoted with an arrow: ->. A state can transition to itself with a reverse arrow: <-. In the example below, state1 transitions to state2 on the FOO event. state2 transitions to itself on the BAR event.

state1 -> state2 (FOO)
state2 <- (BAR)

####Actions

An action is also an alphanumeric string (underscores allowed), just like states. They are contained in parentheses after a transition: state1 -> state2 (SOME_EVENT), or after a self-transition: state3 <- (AN_EVENT). Actions are optional (but encouraged for proper state machine design).

####Nested States

States can be hierarchical (nested) by including them inside brackets after a state declaration. They can be deeply nested an infinite amount of levels. This is useful for implementing statecharts.

state1 {
  nestedState1 -> nestedState2 (FOO)
  nestedState2!
} -> state2 (BAR)
  -> state3 (BAZ)
state2!
state3!

Here's a more pragmatic example:

green -> yellow (TIMER)
yellow -> red (TIMER)
red {
  walk -> countdown (COUNTDOWN_START)
  countdown -> stop (COUNTDOWN_STOP)
  stop!
} -> green (TIMER)

When you enter the red state from yellow, you immediately go into red.walk (which allows pedestrians to walk). Upon reaching red.stop (which disallows pedestrians from walking), actions are handled from the parent red state, so the TIMER going off would transition it back to the green state.

####Formatting / Best Practices

• Indent transitions on a new line for each transition.
• Always declare all states used in the state machine (be explicit!)
• Keep actions on the same line as their transition.
• Indent nested states on a new line for each nested state.

###AST / State Machine Schema Read here if you're curious to the AST and State Machine Schema that this language produces.

API

machine(data, options = {})

Creates a new Machine() instance with the specified data (see the schema above) and options (optional).

• data: (object | string) The definition of the state machine.
• options: (object) Machine-specific options:
  • deterministic: (boolean) Specifies whether the machine is deterministic or nondeterministic (default: true)

Machine.transition(state = null, action = null)

Returns the next state, given a current state and an action. If no state nor action is provided, the initial state is returned.

Note: This is a pure function, and does not maintain internal state.

• state: (string) The current state ID.
• action: (string | Action) The action that triggers the transition from the state to the next state.

Example:

lightMachine.transition();
// => 'green'

lightMachine.transition('green', 'TIMER');
// => 'yellow'

lightMachine.transition('yellow', { type: 'TIMER' });
// => 'red'

lightMachine.transition('yellow');
// => 'yellow'