simple finite state machine runner

run-fsm.js

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