const increment = context => context.count + 1;
const decrement = context => context.count - 1;
const reset = context => context.count = 0;

const counterMachine = Machine({
  initial: 'active',
  context: {
    count: 0
  },
  states: {
    active: {
      on: {
        INC: { actions: assign({ count: increment }) },
        DEC: { actions: assign({ count: decrement }) },
        RESET: { actions: assign({ count: reset }) }
      }
    }
  }
});