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Comment by fendy3002 | Hacker News Reader

What's wrong with setstate? I have used it without problem.

There's nothing wrong with `setState`, in my opinion. However, it does do magic. Although it might feel like it immediately sets the state to be what you provide it as argument, it actually schedules a state update to be performed later when React feels like it. That's why it's best practice to provide an updated function rather than a plain object: if the new state depends on the previous one, and you read the previous state before calling `setState`, then that can lead to problematic behaviour.

(And of course, there's the magic of the returned object not being set as the new state, but rather being merged into the previous one.)

I think the more relevant analogy between hooks and setState is that setState uses essentially the exact same technique as hooks to keep track of which component is calling it, namely, React keeps track of which component it’s rendering, and setState mutates some “global” state. A lot of people seem to think that setState stores state in the instance of the React component class, but that’s not the case (and that wouldn’t work for a lot of React features and optimizations).

> setState mutates some “global” state

Is this true? Maybe I'm reading it wrong but this.setState in a React.Component just transparently calls out to `this.updater`[1] where `this.updater` is injected by the particular platform library (eg react-dom). At least in react-dom/server's case, a unique `updater` object exists per-component[2], so it's effectively a private instance field; and those state changes apply to a similarly per-component `inst.state`[3]

You can also prove this by calling setState with the wrong `this`: `this.setState.call({}, {x:1})` blows up. So the `this` is required, unlike how `useState` works.

[1]: https://github.com/facebook/react/blob/aa9423701e99a194d65a8... [2]: https://github.com/facebook/react/blob/aa9423701e99a194d65a8... [3]: https://github.com/facebook/react/blob/aa9423701e99a194d65a8...

I actually don’t know that much about the implementation, but it sounds like you’re right. I believe I was recalling this article by Dan Abramov:

https://medium.com/@dan_abramov/making-sense-of-react-hooks-... (Ctrl-f “where React keeps the state for Hooks”)

Which is talking about where the state is stored, not the implementation detail of how setState works.

Which is also something beginner React users often discover the hard way when they start with class instance variables, wonder why things don't work as they expect, and then have to learn to use setState.

At least from the principle of least surprise, Hooks don't have that particular learning hump of trying to use a built-in language feature (class instance variables) and finding out the hard way that they don't work as expected.

Ah yes, that is a more relevant analogy.

> it actually schedules a state update to be performed later when React feels like it.

I thought it updates the state and the re-render is what is included in a scheduled function?

> it's best practice to provide an updated function rather than a plain object

I'm guessing you mean for a global store with shared state? Local components aren't singletons.

> I thought it updates the state and the re-render is what is included in a scheduled function?

See here: https://reactjs.org/docs/react-component.html#setstate

> setState() enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state.

(Emphasis mine.)

> I'm guessing you mean for a global store with shared state? Local components aren't singletons.

No, local state, the one you set using `setState`. The offical docs don't actually recommend always using an updater function, but it's the primary method they explain, and my inclination is to better be safe than sorry:

> If the next state depends on the current state, we recommend using the updater function form, instead

Apparently nothing, but from how I read it a lot of magic is going on underneath the covers (which I cannot really imagine, if react is as simple as it claims).

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