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.

Comece pela seção setState da página de referência de React.Component vinculada na issue. Revise a redação sobre callbacks e batching e atualize-a para que o momento em que o callback é executado e o state disponível após um batch sejam inequívocos; considera-se concluído quando os exemplos não puderem sugerir que um callback observa apenas sua atualização individual.