function Reducer(counter : number = 0, action : Action) {
  counter++; // Should trigger an error but it doesn't.
  return counter + 1;
}

function NestedReducer(nestedCounter : { counter : number } = { counter : 0 }, action : Action) {
  nestedCounter = { counter : 0 }; // Doesn't trigger an error.
  nestedCounter.counter++; // This does trigger an error.
  return { counter : nestedCounter.counter + 1 };
}

let a = { counter : 0 };
let b = a;
console.log(`${a.counter}, ${b.counter}`);
b.counter++; // This changes a.counter as well since both a and b reference the same object.
console.log(`${a.counter}, ${b.counter}`);
b = { counter : 0 }; // This does not change a or a.counter, now a and b point to different objects.
console.log(`${a.counter}, ${b.counter}`);

0, 0
1, 1
1, 0