React18: Batch Process Problem

Programming
,
1 
useEffect(() => {
  console.log("render");
});

const handleClick = () => {
  setC1((c) => c + 1);

  Promise.resolve().then(() => {
    setC1((c) => c + 1);
  });
};

const handleClick2 = () => {
  Promise.resolve().then(() => {
    setC1((c) => c + 1);
  });

  setC1((c) => c + 1);
};

Why do the handleClick and handleClick2 methods produce different outputs?

I expect the output of both methods to be the same. Can someone explain why the results differ?

2

The handleClick and handleClick2 methods produce different outputs because they have different orderings of state updates and promise resolutions.

In handleClick, the setC1 function is called first to update the state immediately. Then, a promise is resolved, and inside the promise’s then callback, setC1 is called again to update the state. Since the promise resolution is asynchronous, the second state update happens after the first one.

In handleClick2, the promise is resolved first, and inside the promise’s then callback, setC1 is called to update the state. Then, immediately after the promise, setC1 is called again to update the state. Since the promise resolution is asynchronous, the first state update happens after the second one.

As a result, the final state values will be different between the two methods due to the different order of state updates.

To ensure consistent behavior, you can use the async/await syntax to make the promise resolution synchronous and update the state in the desired order.