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Some days ago I was working with React and, for the 1000th time, I got myself in trouble with the stale state problem.

What is the stale state problem?

The stale state problem is when you have a useState and update the corresponding state using the hook's given setState function. Until React rerenders the component, your state will be stale, meaning that it will not be updated.

Here's an example of a stale state:

const [count, setCount] = useState(0)
setCount(count + 1);
console.log(count); // Expecting 1, receiving 0 instead.
setTimeout(() => {
  setCount(count + 1);
}, 1000); // Expecting 2, receiving 1 instead.

As mentioned, the above code will not work as expected, because the state will be stale until React re-renders the component.

This is because React re-creates the component function at every re-render, effectively preventing a consistent reference for variables.

This is where useAtom comes in.

By using a mix of React's state mechanism, Proxy Pattern and Observer Pattern, useAtom allows the state to be completely and fully reactive, preventing the stale state issue mentioned above.

By doing this, we can be sure that the state is always up to date, even if React hasn't re-rendered the component yet.
Example:

const atomTest = useAtom(0);
console.log(atomTest.value); // 0, as expected
atomTest.value++;
console.log(atomTest.value); // 1, as expected
setTimeout(() => {
  atomTest.value++;
}, 1000); // 2, as expected

The above code will work as expected, because the state is not stale. The useAtom hook will update the state immediately, and the value property will be updated as well.

What can I put in an atom?

You can use the useAtom hook both with primitives (strings, numbers, etc..), arrays and objects. It cannot be used with functions.

Internally, the hook will create an atom for every primitive in the array or in the object.

  const atomObj = useAtom({
    a: 1,
    b: 10,
    c: 100,
  });

  atomObj.a.value = 2; // This will update the state
  console.log(atomObj.a.value); // 2, as expected

  const atomArr = useAtom([1, 2, 3, 4, 5]);

  atomArr[0].value = 10; // This will update the state
  console.log(atomArr[0].value); // 10, as expected

What is the difference between useAtom and useState?

The difference is that useAtom will update the state variable immediately thanks to its Proxy-based implementation, while useState will not update it until React re-renders the component.

What is the difference between useAtom and useRef?

The difference is that useAtom will re-render the component on change, while useRef will not.

How can I subscribe for changes?

If you want to respect the Observer Pattern, you can use the subscribe method to subscribe and unsubscribe for changes:

const atomTest = useAtom(0);
const atomObserver = (newValue, oldValue) => {
  console.log(`New value: ${newValue}, old value: ${oldValue}`);
};
atomTest.subscribe(atomObserver);
atomTest.value++;
atomTest.unsubscribe(atomObserver);

What if I want to keep using the useEffect hook?

If you instead prefer a more standard approach, the hook exposes a state property which contains the React state value, allowing you to use the useEffect hook as usual.

const atomTest = useAtom(0);
useEffect(() => {
  console.log(`New value: ${atomTest.state}`);
}, [atomTest.state]);
atomTest.value++;

How can I trigger my subscribe callback at first component mount?

You can pass a second parameter to the useAtom hook, which will contain the atom configuration.

You can use the triggerOnMount property to trigger the subscribe callback at first component mount.

This will work in a similar way to a Behaviour Subject in Reactive Programming and works just like the useEffect hook.

const atomTest = useAtom(0, {
  triggerSubscribeOnMount: true,
});
atomTest.subscribe((newValue, oldValue) => {
  console.log(`New value: ${newValue}, old value: ${oldValue}`);
}); // This will be logged at first component mount with initial value of 0.

Please, let me know what you think about my implementation and please, share some use cases in which the useAtom hook helped you!