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When we use the useEffect hook in our code but forget to import it, it will generate “Uncaught ReferenceError: useEffect is not defined”. To fix the error, you need to import the hook before using it import {useEffect} from 'react'.

Here is an example that produces this error

import {useState} from 'react';

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

  useEffect(() => {
    console.log('Count is: ', count);
  }, [count]);

  const handleClick = event => {
    setCount(current => current + 1);
  };
  return (
    <div>
      <h2>Count: {count}</h2>
      <button onClick={handleClick}>Increment</button>
    </div>
  );
};

export default App;

In the above code, we can see that we did not import useEffect before using it. If we run the above code, the following error will be generated

To fix this error, we just need to import it before using useEffect.

import {useEffect, useState} from 'react';

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

  useEffect(() => {
    console.log('Count is: ', count);
  }, [count]);

  const handleClick = event => {
    setCount(current => current + 1);
  };
  return (
    <div>
      <h2>Count: {count}</h2>
      <button onClick={handleClick}>Increment</button>
    </div>
  );
};

export default App;

The useEffect hook can be imported from the react package using named imports.

import {useEffect} from 'react';

The useEffect hook takes a function and an array of dependencies as arguments.

Every time one of the dependencies changes, the function is rerun.

If we only want to call the function when the component mounts, pass an empty dependencies array to the hook.

useEffect(() => {
  console.log('Only runs on mount');
}, []); // 👈️ 空依赖数组

If we need to run some logic when the component unmounts, return a function from useEffect.

useEffect(() => {
  console.log('Only runs on mount');

  return () => {
    console.log('Only runs on unmount')
  }
}, []); // 👈️ 空依赖数组

Please note that there are some rules when using React Hooks.

As the documentation states:

• Hooks are only called from React function components or custom hooks.
• Only call hooks at the top level
• Do not call hooks in loops, if conditions or nested functions
• Always use hooks at the top level of a React function, before any early returns