Why in React, to change useState, many people write like this setState(pre => pre = data), and not just like this setState(data)?

Question

React

Good day dear developers. I am a junior developer writing React js, I wrote a couple of projects. Every React developer will work with the useState hook. To change the state of the hook, I write setState(data), somewhere I saw that some people write setState(pre => pre = data) like this. And of course I want to know why it is written that way. Maybe it optimizes the application?

Answer 1

Sometimes it can optimize. for example

const [x, setX] = useState(0);
const f = useCallback(() => setX(x => x + 1), []);

Here f is the same, not recreated for each new x. If f is passed to the props of a memo component, it will not be updated unless it depends on x.

Answer 2

setState(pre => pre = data)- this is nonsense nonsense.

setState(pre => {
  const data = <что-делаем с pre>;
  return data;
})

- another thing. Such a function does not depend on external data, only on the previous state.

Answer 3

The way you present it doesn't make sense. Why the function is used is described in the documentation.
https://reactjs.org/docs/hooks-reference.html#func...

Functional updates
If the new state is computed using the previous state, you can pass a function to setState. The function will receive the previous value, and return an updated value. Here's an example of a counter component that uses both forms of setState: