setState() callback function: potential confusion of call order

This paragraph from the React.Component reference page:

setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied.

can create the impression that instead of doing:

this.setState({ value: 5 });
console.log(this.state.value); // => not necessarily 5

you can use the callback function to safely read the state back:

this.setState({ value: 5 }, () => console.log(this.state.value)); // => "guaranteed" 5

(especially if you draw a parallel between the callback function and the updater function)

and it may actually work in some cases where there's no setState batching, but as more calls get batched in the future, these constructs may generate unintended side-effects.

Instead, we could clarify that all callback functions for a batch of setState() calls get called after the entire batch has been processed.

Issue でリンクされている React.Component リファレンスページの setState セクションから始めてください。callback と batching に関する説明を見直し、callback の実行タイミングと batch 後に利用できる state が明確になるように更新してください。完了の条件は、例から callback が自分自身の個別の更新だけを確認するように読み取れないことです。