Full code example

full-example.js

//define the finite state machine data structure...
let fsm = {
    "checkLogin" : {"valid" : "login",
                    "invalid" : "showError"}
};

//create the handler functions...
function checkLogin(state){
    if(state.data.username == "admin"){
        return {
            ...state,
            transitionTo: "valid"
        };
    }else {
        return {
            ...state,
            transitionTo: "invalid"
        };
    }
}

function showLogin(state){
    return {
        data : {
            ...state.data,
            message: "Logged in as " + state.data.username,
        },
        transitionTo: "done"
    };
}

function showError(state){
    return {
        data : {
            ...state.data,
            message: "Username " + state.data.username + " is invalid"
        },
        transitionTo: "done"
    };
}

//create a mapping between state names and state functions
let fsmRunner = {
    "checkLogin": checkLogin,
    "showLogin" : showLogin,
    "showError" : showError
};

//create the runner
function run(fsm, stateRunner, state){
    let currentStateFn = stateRunner[state.currentState];
    return Promise.resolve(currentStateFn(state)).then(function(transition){
        if(transition.transitionTo === "done"){
            return transition.data;
        }
        return run(fsm, stateRunner, {
            ...transition,
            currentState: fsm[state.currentState][transition.transitionTo]
        });
    });
}

function runLogin(initialData){
    return run(fsm, fsmRunner, {
        data : initialData,
        currentState: "checkLogin"
    });
}

runLogin({"username": "admin"}).then(function(state){
    console.log(state.message);
});