Simple redux-style state machine via a generator function

state-generator.js

function *stateGenerator(state, reducer) {
    let action;
    while (true) {
        if (action) {
            state = reducer(state, action);
        }
        action = yield state;
    }
}

// Usage:

const iter = stateGenerator({count: 0}, (state, action) => {
    switch (action.type) {
        case 'increment':
            return {count: state.count + 1};
        case 'decrement':
            return {count: state.count - 1};
        default:
            return state;
    }
});

iter.next();                    //=> {value: {count: 0}, done: false}
iter.next({type: 'increment'}); //=> {value: {count: 1}, done: false}
iter.next({type: 'increment'}); //=> {value: {count: 2}, done: false}
iter.next({type: 'decrement'}); //=> {value: {count: 1}, done: false}
iter.next({type: 'increment'}); //=> {value: {count: 2}, done: false}
iter.next({type: 'increment'}); //=> {value: {count: 3}, done: false}