Transition states

machine.js

const machine = {
  initial: 'cart',
  states: {
    cart: {
      on: {
        CHECKOUT: 'shipping',
      },
    },
    shipping: {
      on: {
        NEXT: 'contact',
      },
    },
    contact: {
      on: {},
    },
  },
};

transition.js

function transition(state, event) {
  return machine.states[state].on?.[event.type] ?? state;
}

console.log(transition('cart', { type: 'CHECKOUT' }));
// shipping

console.log(transition('cart', { type: 'UNKNOWN' }));
// cart