About updating state in React

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Why does the value of count1 quadruple instead of doubling?
Nested functional updates quadruple the value Why is setCount1 executed twice in a row?
I'm curious about the processing.

export default function Test3() {
    const [count, setCount] = useState(0);
    const [count1, setCount1] = useState(1);
    const [t, setT] = useState(false);
    function handleClick() {
        setT(!t);
        console.log('t', t);
    }
    function handleClick2() {
        setT(!t);
        setCount(0);
        setCount1(1);
    }
    useEffect(() => {
        setCount(prev => {
            if (prev < 11 && t) {
                setCount1(pr => {
                    return pr * 2;
                })
                return prev + 1;
            }
            else return prev
        })
    }, [t,])


    return (
        <div className='flex flex-col items-center'>
            <h1>Counters that update together</h1>
            <div>count: {count}</div>
            <div>count1: {count1}</div>
            <button onClick={handleClick} >button</button>
            <button onClick={handleClick2}>reset</button>
        </div>
    );
}```

[ZoroXCode]

Question:

Why does the value of count1 quadruple instead of doubling? Why is setCount1 executed twice in a row?

---

Answer:

The behavior occurs because of nested functional updates and the way React batches state updates for performance optimization. In your useEffect hook, setCount1 is called inside the functional update of setCount. This causes setCount1 to be executed twice during the same render cycle:

1. React schedules the first update for setCount1(pr => pr * 2), which doubles the value of count1.
2. After the first state update, React re-runs the state logic again due to the nested functional updates, causing setCount1 to execute a second time. This doubles the value of count1 again, resulting in it quadrupling.

---

Root Cause:

The key issue lies in calling multiple state updates (setCount1) inside another functional state update (setCount). React processes each state update independently but batches them within the same render cycle, leading to multiple executions.

---

Solution:

To avoid this behavior, separate the updates in useEffect so that setCount1 is only executed once during each render cycle. Here's the updated code:

import React, { useState, useEffect } from 'react';

export default function Test3() {
    const [count, setCount] = useState(0);
    const [count1, setCount1] = useState(1);
    const [t, setT] = useState(false);

    function handleClick() {
        setT(!t);
        console.log('t', t);
    }

    function handleClick2() {
        setT(false);
        setCount(0);
        setCount1(1);
    }

    useEffect(() => {
        if (t && count < 11) {
            // Update count
            setCount(prev => prev + 1);

            // Update count1 separately
            setCount1(pr => pr * 2);
        }
    }, [t, count]);

    return (
        <div className="flex flex-col items-center">
            <h1>Counters that update together</h1>
            <div>count: {count}</div>
            <div>count1: {count1}</div>
            <button onClick={handleClick}>button</button>
            <button onClick={handleClick2}>reset</button>
        </div>
    );
}

Hope it helped.💚