[acdlite]

When you call setState on a component, only that component's subtree is reconciled and re-rendered.

[rukuu001]

That's in the virtual DOM. React will diff the VDOM and browser DOM and only render actual differences.

The React API will also let a component decide for itself if it should be rendered.

[amelius]

Okay. But how do you know what components to call setState on then? Is state spread over components, or is it stored centrally?

[acdlite]

setState is a public API, usually triggered in response to a user event like a click or an input change event.

State in React is local to a component, but it can be passed down to a component's children in the form of props.

Centralized state can be achieved using a library like Redux, where an individual component subscribes to an external data store's changes. Redux abstracts the details away, but under the hood it's still just setState.