Why React useState with functional update form is needed? - reactjs

Question

I'm reading React Hook documentation about functional updates and see this quote:
The ”+” and ”-” buttons use the functional form, because the updated
value is based on the previous value
But I can't see for what purposes functional updates are required and what's the difference between them and directly using old state in computing new state.
Why functional update form is needed at all for updater functions of React useState Hook? What are examples where we can clearly see a difference (so using direct update will lead to bugs)?
For example, if I change this example from documentation
function Counter({initialCount}) {
const [count, setCount] = useState(initialCount);
return (
<>
Count: {count}
<button onClick={() => setCount(initialCount)}>Reset</button>
<button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
<button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
</>
);
}
to updating count directly:
function Counter({initialCount}) {
const [count, setCount] = useState(initialCount);
return (
<>
Count: {count}
<button onClick={() => setCount(initialCount)}>Reset</button>
<button onClick={() => setCount(count + 1)}>+</button>
<button onClick={() => setCount(count - 1)}>-</button>
</>
);
}
I can't see any difference in behaviour and can't imagine case when count will not be updated (or will not be the most recent). Because whenever count is changing, new closure for onClick will be called, capturing the most recent count.

Answer 1

State update is asynchronous in React. So it is possible that there would be old value in count when you're updating it next time. Compare, for example, result of these two code samples:
function Counter({initialCount}) {
const [count, setCount] = useState(initialCount);
return (
<>
Count: {count}
<button onClick={() => setCount(initialCount)}>Reset</button>
<button onClick={() => {
setCount(prevCount => prevCount + 1);
setCount(prevCount => prevCount + 1)}
}>+</button>
</>
);
}
and
function Counter({initialCount}) {
const [count, setCount] = useState(initialCount);
return (
<>
Count: {count}
<button onClick={() => setCount(initialCount)}>Reset</button>
<button onClick={() => {
setCount(count + 1);
setCount(count + 1)}
}>+</button>
</>
);
}

Answer 2

I stumbled into a need for this recently. For example let's say you have a component that fills up an array with some amount of elements and is able to append to that array depending on some user action (like in my case, I was loading a feed 10 items at a time as the user kept scrolling down the screen. the code looked kind of like this:
function Stream() {
const [feedItems, setFeedItems] = useState([]);
const { fetching, error, data, run } = useQuery(SOME_QUERY, vars);
useEffect(() => {
if (data) {
setFeedItems([...feedItems, ...data.items]);
}
}, [data]); // <---- this breaks the rules of hooks, missing feedItems
...
<button onClick={()=>run()}>get more</button>
...
Obviously, you can't just add feedItems to the dependency list in the useEffect hook because you're invoking setFeedItems in it, so you'd get in a loop.
functional update to the rescue:
useEffect(() => {
if (data) {
setFeedItems(prevItems => [...prevItems, ...data.items]);
}
}, [data]); // <--- all good now

Answer 3

The “state update is asynchronous in React” answer is misleading, as are some comments below it. My thinking was also wrong until I dug into this further. You are right, this is rarely needed.
The key idea behind functional state updates is that state you depend on for the new state might be stale. How does state get stale? Let’s dispel some myths about it:
Myth: State can be changed under you during event handling.
Fact: The ECMAScript event loop only runs one thing at a time. If you are running a handler, nothing else is running alongside it.
Myth: Clicking twice fast (or any other user action happening quickly) can cause state updates from both handler calls to be batched.
Fact: React is guaranteed to not batch updates across more than one user-initiated event. This is true even in React 18, which does more batching than previous versions. You can rely on having a render in between event handlers.
From the React Working Group:
Note: React only batches updates when it’s generally safe to do. For example, React ensures that for each user-initiated event like a click or a keypress, the DOM is fully updated before the next event. This ensures, for example, that a form that disables on submit can’t be submitted twice.
So when do you get stale state?
Here are the main 3 cases I can think of:
Multiple state updates in the same handler
This is the case already mentioned where you set the same state multiple times in the same handler, and depend on the previous state. As you pointed out, this case is pretty contrived, because this clearly looks wrong:
<button
onClick={() => {
setCount(count + 1);
setCount(count + 1);
}}
>+</button>
A more plausible case is calling multiple functions that each do updates on the same state and depend on the previous state. But that’s still weird, it’d make more sense to do all the calculations then set the state once.
Async state updates in a handler
For example:
<button
onClick={() => {
doSomeApiCall().then(() => setCount(count + 1));
}}
>+</button>
This is not so obviously wrong. The state can be changed in between you calling doSomeApiCall and when it resolves. In this case, the state update really is async, but you made it that way, not React!
The functional form fixes this:
<button
onClick={() => {
doSomeApiCall().then(() => setCount((currCount) => currCount + 1));
}}
>+</button>
Updating state in useEffect
G Gallegos's answer pointed this out for useEffect in general, and letvar's answer pointed this out for useEffect with requestAnimationFrame. If you're updating state based on previous state in useEffect, putting that state in the dependency array (or not using a dependency array) is a recipe for infinite loops. Use the functional form instead.
Summary
You don’t need the functional form for state updates based on previous state, as long as you do it 1. in a user-triggered-event handler 2. once per handler per state and 3. synchronously. If you break any of those conditions, you need functional updates.
Some people might prefer to always use functional updates, so you don’t have to worry about those conditions. Others might prefer the shorter form for clarity when it’s safe to do so, which is true for many handlers. At that point it’s personal preference / code style.
Historical note
I learned React before Hooks, when only class components had state. In class components, “multiple state updates in the same handler” doesn’t look so obviously wrong:
<button
onClick={() => {
this.setState({ count: this.state.count + 1 });
this.setState({ count: this.state.count + 1 });
}}
>+</button>
Since state is an instance variable instead of a function parameter, this looks fine, unless you know that setState batches calls when in the same handler.
In fact, in React <= 17, this would work fine:
setTimeout(() => {
this.setState({ count: this.state.count + 1 });
this.setState({ count: this.state.count + 1 });
}, 1000);
Since it’s not an event handler, React re-renders after each setState call.
React 18 introduces batching for this and similar cases. This is a useful performance improvement. There is the downside that it breaks class components that rely on the above behavior.
References
React Working Group discussion
ehab’s answer, which also mentions the two cases where functional updates are needed.

Answer 4

I have answered a similar question like this and it was closed because this was the canonical question - that i did not know of, upon looking the answers i decided to repost my answer here since i think it adds some value.
If your update depends on a previous value found in the state, then you should use the functional form. If you don't use the functional form in this case then your code will break sometime.
Why does it break and when
React functional components are just closures, the state value that you have in the closure might be outdated - what does this mean is that the value inside the closure does not match the value that is in React state for that component, this could happen in the following cases:
1- async operations (In this example click slow add, and then click multiple times on the add button, you will later see that the state was reseted to what was inside the closure when the slow add button was clicked)
const App = () => {
const [counter, setCounter] = useState(0);
return (
<>
<p>counter {counter} </p>
<button
onClick={() => {
setCounter(counter + 1);
}}
>
immediately add
</button>
<button
onClick={() => {
setTimeout(() => setCounter(counter + 1), 1000);
}}
>
Add
</button>
</>
);
};
2- When you call the update function multiple times in the same closure
const App = () => {
const [counter, setCounter] = useState(0);
return (
<>
<p>counter {counter} </p>
<button
onClick={() => {
setCounter(counter + 1);
setCounter(counter + 1);
}}
>
Add twice
</button>
</>
);
}

Answer 5

Another use case for using functional updates with setState - requestAnimationFrame with react hooks. Detailed information is available here - https://css-tricks.com/using-requestanimationframe-with-react-hooks/
In summary, handler for requestAnimationFrame gets called frequently resulting in incorrect count value, when you do setCount(count+delta). On the other hand, using setCount(prevCount => prevCount + delta) yields correct value.