Does this.setState return promise in react

Question

I made my componentWillMount() async. Now I can using await with the setState.

Here is the sample code:

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Answer 1

setState is usually not used with promises because there's rarely such need. If the method that is called after state update (fetchRooms) relies on updated state (roomId), it could access it in another way, e.g. as a parameter.

setState uses callbacks and doesn't return a promise. Since this is rarely needed, creating a promise that is not used would result in overhead.

In order to return a promise, setState can be promisified, as suggested in this answer.

Posted code works with await because it's a hack. await ... is syntactic sugar for Promise.resolve(...).then(...). await produces one-tick delay that allows to evaluate next line after state update was completed, this allows to evaluate the code in intended order. This is same as:

this.setState({ roomId: room && room.roomId ? room.roomId : 0 } => {
  console.log(2)
})

setTimeout(() => {
  console.log(3)
});

There's no guarantee that the order will stay same under different conditions. Also, first setState callback isn't a proper place to check whether a state was updated, this is what second callback is for.