Question 72708 in Reactjs Question posted in
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Use effect runs on the next value check of the dependency condition [count == 2] – Reactjs

As can see in below code useEffect log for 0, 2 and 3 values of count. So zero is understood that runs on component initial render but then it should only run for count value for 2 but after that it runs for 3 as well.

useEffect(() => {
   console.warn(count);
}, [count == 2]);

Demo: https://codesandbox.io/p/sandbox/unruffled-kepler-o0ehc6?file=README.md

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2

Answers


  1. 0

    The reason why your useEffect is running every time even though you passed [count == 2] as the dependency array is because the expression count == 2 evaluates to a boolean value (true or false).

    In React, the dependency array should only contain values that the effect depends on. When the effect is triggered, React will compare each value in the dependency array to its previous value. If any of the values have changed, the effect will be re-run. However, in your case, you are passing a boolean value (true or false) instead of the value of count, which is not what you intended.

    To conditionally run the useEffect hook only when a counter is equal to two, you can include the counter in the dependency array of the useEffect hook. This way, whenever the counter value changes, the useEffect hook will run.

    Here’s an example code snippet that demonstrates this:

    import { useState, useEffect } from 'react';

function MyComponent() {
    const [counter, setCounter] = useState(0);

    useEffect(() => {
        if (counter === 2) {
            // do something when counter is equal to 2
        }
    }, [counter]);

    const incrementCounter = () => {
        setCounter(counter + 1);
    };

    return (
        <div>
            <p>Counter: {counter}</p>
            <button onClick={incrementCounter}>Increment counter</button>
        </div>
    );
}
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  2. 0

    The effect run every time the dependencies changes. So from 0 to 2 we move from false to true, and from 2 to 3 we move from true to false, hence the reason why it runs for 3 too.

    So, simply add the condition on the effect itself.

    useEffect(() => {
  if (count !== 2) {
    return;
  }
  
  console.warn(count);
}, [count]);

    The effect run every time the count change, but you only execute things depending on its value.

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