A minimalistic finite state machine library for browser and node implemented using promises.
📢 For Visual Studio Code users: checkout the UML visualization extension.
- How to use
- Configuring promise library
- Create finite state machine
- Callbacks
- Common Methods
- Emitted Events
- Handling Errors
- Recipes
- Tooling
- Contributing
- License
- Credits
Run npm install fsm-as-promised to get up and running. Then:
// ES5
const StateMachine = require('fsm-as-promised');
// ES6
import StateMachine from 'fsm-as-promised';StateMachine.Promise = YourChoiceForPromiseYou can choose from the following promise libraries:
If the environment does not provide Promise support, the default implementation is es6-promise.
The library works also with the promise implementation bundled with es6-shim.
A state machine object can be created by providing a configuration object:
const fsm = StateMachine({
events: [
{ name: 'wait', from: 'here'},
{ name: 'jump', from: 'here', to: 'there' },
{ name: 'walk', from: ['there', 'somewhere'], to: 'here' }
],
callbacks: {
onwait: function () {
// do something when executing the transition
},
onleavehere: function () {
// do something when leaving state here
},
onleave: function () {
// do something when leaving any state
},
onentersomewhere: function () {
// do something when entering state somewhere
},
onenter: function () {
// do something when entering any state
},
onenteredsomewhere: function () {
// do something after entering state somewhere
// transition is complete and events can be triggered safely
},
onentered: function () {
// do something after entering any state
// transition is complete and events can be triggered safely
}
}
});The state machine configuration contains an array of event that convey information about what transitions are possible. Typically a transition is triggered by an event identified by name, and happens between from and to states.
The state machine configuration can define callback functions that are invoked when leaving or entering a state, or during the transition between the respective states. The callbacks must return promises or be thenable.
You can define the initial state by setting the initial property:
const fsm = StateMachine({
initial: 'here',
events: [
{ name: 'jump', from: 'here', to: 'there' }
]
});
console.log(fsm.current);
// hereotherwise the finite state machine's initial state is none.
You can define the final state or states by setting the final property:
const fsm = StateMachine({
initial: 'here',
final: 'there', //can be a string or array
events: [
{ name: 'jump', from: 'here', to: 'there' }
]
});An existing object can be augmented with a finite state machine:
const target = {
key: 'value'
};
StateMachine({
events: [
{ name: 'jump', from: 'here', to: 'there' }
],
callbacks: {
onjump: function (options) {
// accessing target properties
console.log(target.key === this.key);
}
}
}, target);
target.jump();You can override the default library error handler by setting the error property:
const fsm = StateMachine({
initial: 'red',
events: [
{ name: 'red', from: 'green', to: 'red' }
],
error: function customErrorHandler(msg, options) {
throw new Error('my error');
}
});The value of the error property is a function that expects two arguments:
- msg a string containing the error reason
- options an object havin as properties the
nameof the transition and thefromstate when the error occurred.
The following arguments are passed to the callbacks:
const fsm = StateMachine({
events: [
{ name: 'jump', from: 'here', to: 'there' }
],
callbacks: {
onjump: function (options) {
// do something with jump arguments
console.log(options.args);
// do something with event name
console.log(options.name);
// do something with from state
console.log(options.from);
// do something with to state
console.log(options.to);
return options;
}
}
});
fsm.jump('first', 'second');You can define synchronous callbacks as long as the callback returns the options object that is going to be passed to the next callback in the chain:
const fsm = StateMachine({
events: [
{ name: 'jump', from: 'here', to: 'there' }
],
callbacks: {
onjump: function (options) {
// do something
return options;
}
}
});
fsm.jump();You can define asynchronous callbacks as long as the callback returns a new promise that resolves with the options object when the asynchronous operation is completed. If the asynchronous operation is unsuccessful, you can throw an error that will be propagated throughout the chain.
const fsm = StateMachine({
events: [
{ name: 'jump', from: 'here', to: 'there' }
],
callbacks: {
onjump: function (options) {
return new Promise(function (resolve, reject) {
// do something
resolve(options);
});
}
}
});
await fsm.jump();The callbacks are called in the following order:
| callback | state in which the callback executes |
|---|---|
| onleave{stateName} | from |
| onleave | from |
| on{eventName} | from |
| onenter{stateName} | from |
| onenter | from |
| onentered{stateName} | to |
| onentered | to |
A state is locked if there is an ongoing transition between two different states. While the state is locked no other transitions are allowed.
If the transition is not successful (e.g. an error is thrown from any callback), the state machine returns to the state in which it is executed.
By default, each callback in the promise chain is called with the options object.
Callbacks can pass values that can be used by subsequent callbacks in the promise chain.
const fsm = StateMachine({
initial: 'one',
events: [
{ name: 'start', from: 'one', to: 'another' }
],
callbacks: {
onleave: function (options) {
options.foo = 2;
},
onstart: function (options) {
// can use options.foo value here
if (options.foo === 2) {
options.foo++;
}
},
onenter: function (options) {
// options.foo === 3
}
}
});This also includes callbacks added to the chain by the user.
fsm.start().then(function (options) {
// options.foo === 3
});The options object can be hidden from the promises added by the end user by setting the options.res property. This way the subsequent promises that are not part of the state machine do not receive the options object.
const fsm = StateMachine({
initial: 'one',
events: [
{ name: 'start', from: 'one', to: 'another' }
],
callbacks: {
onstart: function (options) {
options.res = {
val: 'result of running start'
};
}
}
});
const result = await fsm.start();
console.log(result);By default, the callback names start with on. You can omit the prefix by setting it to empty string or assign any other prefix:
StateMachine.callbackPrefix = 'customPrefix';The library adds the following utilities to the finite state machine object:
can(event)checks if the event can be triggered in the current state,cannot(event)checks if the event cannot be triggered in the current state,is(state)checks if the state is the current state,isFinal(state)checks if the state is final state. If no state is provided the current state is checked.hasState(state)checks if the finite state machine has the state.instanceIdreturns the uuid of the instance
The finite state machine object is an EventEmitter. By default, the library emits state event whenever the state machine enters a new state.
You can define and emit new events.
Errors thrown by any of the callbacks called during a transition are propagated through the promise chain and can be handled like this:
fsm.jump().catch(function (err) {
// do something with err...
// err.trigger - the event that triggered the error
// err.current - the current state of the state machine
// err.message - described bellow...
// err.pending - an object containing the description of intra-state pending transitions
});The library throws errors with the following messages:
| message | explanation | note |
|---|---|---|
| Ambigous transition | The state machine has one transition that starts from one state and ends in multiple | must be fixed during design time |
| Previous transition pending | The previous intra-state transition(s) is in progress preventing new ones until it has completed | - |
| Previous inter-state transition started | Inter-state transition started | - |
| Invalid event in current state | The state machine is in a state that does not allow the requested transition | - |
It is not advisable to let the errors that can be handled gracefully at callback level to propagate to the end of the promise chain.
The following is an example where the error is handled inside a synchronous callback:
const fsm = StateMachine({
initial: 'green',
events: [
{ name: 'warn', from: 'green', to: 'yellow' }
],
callbacks: {
onwarn: function (options) {
try {
throw new Error('TestError');
} catch (err) {
// handle error
return options;
}
}
}
});
await fsm.warn()
fsm.current === 'yellow';
// trueThe same inside an asynchronous callback:
const fsm = StateMachine({
initial: 'green',
events: [
{ name: 'warn', from: 'green', to: 'yellow' }
],
callbacks: {
onwarn: function (options) {
return new StateMachine.Promise(function (resolve, reject) {
reject(new Error('TestError'));
}).catch(function (err) {
// handle error
return options;
});
}
}
});
await fsm.warn()
fsm.current === 'yellow';
// trueThe library provides a way to define conditional transitions:
StateMachine({
events: [
{ name: 'conditional',
from: 'init',
to: ['one', 'two'],
condition: function (options) {
return 0; // transition to state 'one'
}
}
]
});The above is equivalent to:
StateMachine({
events: [
{ name: 'conditional',
from: 'init',
to: ['one', 'two'],
condition: function (options) {
return 'one'; // transition to state 'one'
}
}
]
});The condition callback must return the to Array's index of the selected state, the name of the selected state, or a promise which resolves to either. The condition callback is executed after on{eventName} callback.
If the above is not suitable, complex conditional transitions can be achieved through transitioning explicitly to a pseudo state where the condition is checked, then the appropriate event is triggered:
StateMachine({
events: [
{ name: 'trigger', from: 'existing', to: 'pseudo' },
{ name: 'triggerOptionA', from: 'pseudo', to: 'option-a' },
{ name: 'triggerOptionB', from: 'pseudo', to: 'option-b' }
],
callbacks: {
onenteredpseudo: function () {
if (condition) {
this.triggerOptionA();
} else {
this.triggerOptionB();
}
}
}
});If your pseudo state's callback returns a Promise, you must return the call to the event function; e.g. return this.triggerOptionA().
You can visualize the state machine as a UML diagram in vscode using the Finite state machine viewer extension.
The state machine definitions can be visualized as UML diagrams using fsm2dot.
Install fsm2dot and graphviz, then:
fsm2dot -f fsm.js -o fsm.dot
dot -Tpdf fsm.dot -o fsm.pdfInstall the library and run tests:
npm install
npm test
The library is available under the MIT license.
The framework is heavily influenced by Jake Gordon's javascript-state-machine.
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