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Use useEffecthooks to listen to state changes in React. We can add the state variables we want to track to the dependencies array of the hook, and useEffectthe logic in the hook will run every time the state variable changes.

import {useEffect, useState} from 'react';

const App = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('useEffect ran. count is: ', count);
  }, [count]); // 👈️ 添加要跟踪的状态变量

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

export default App;

The second argument we pass to useEffectthe hook is an array of dependencies. The function we pass to the hook will be called when any of the dependencies change.

We countadd the state variable to the hook's dependencies array, so any time something changes, the logic in countour hook will be re-run.useEffect

Add all the state variables you want to track useEffectto the hook's dependencies array.

Note that this hook is also called when the component is mounted.

If we don't want to run the logic in the hook on the initial render useEffect, but only when a specific state variable changes, use a ref to return early on the initial render.

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

const App = () => {
  const [count, setCount] = useState(0);

  const isFirstRender = useRef(true);

  useEffect(() => {
    if (isFirstRender.current) {
      isFirstRender.current = false;
      return; // 👈️ return early if first render
    }

    console.log('useEffect ran. count is: ', count);
  }, [count]); // 👈️ 添加要跟踪的状态变量

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

export default App;

When running the hook on mount useEffect, we use ref to exit early.

Use this method if we want to listen to state changes but need to skip the first render.

It is important to note that if you useEffectupdate a specific state variable in the hook, and that variable exists in the hook's dependencies array, you will cause an infinite re-rendering loop.

Here is an example that demonstrates this problem.

import {useEffect, useState} from 'react';

const App = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('useEffect ran. count is: ', count);

    setCount(count + 1);  // 👈️ 这会导致无限循环
  }, [count]);

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

export default App;

The problem is that we have added the count state variable to useEffectthe dependencies array of the hook, but we are also updating the count in the hook.

Each useEffecttime you run , the value of the count variable changes, which re-runs the hook again because count is in its dependencies array.