React Hooks: useEffect() is called twice even if an empty array is used as an argument

If you’re working with React Hooks and have encountered a situation where the useEffect() hook is being called twice, even when you pass an empty array as the second argument, you’re not alone. This behavior can be confusing, but fortunately, there are a few solutions to this problem.

Solution 1: Ensure the Dependency Array is Truly Empty

When using the useEffect() hook, the second argument is an array of dependencies. If this array is not provided or is empty, the effect will only run once, similar to the behavior of componentDidMount(). However, if the array is not truly empty, even if it contains a single element, the effect will be called whenever that element changes.

To fix the issue, make sure that the dependency array is truly empty:

useEffect(() => {
  // Your effect code here
}, []);

Solution 2: Check for Unintended Side Effects

If your dependency array is truly empty and the useEffect() hook is still being called twice, it’s possible that there are unintended side effects causing the re-render. One common cause is the usage of functions or objects as dependencies.

For example, if you pass a function as a dependency, it will be recreated on every render, causing the effect to be called again. To avoid this, you can use the useCallback() hook to memoize the function:

const myFunction = useCallback(() => {
  // Your function code here
}, []);

Similarly, if you pass an object as a dependency, it will also be recreated on every render. To prevent this, you can use the useMemo() hook to memoize the object:

const myObject = useMemo(() => {
  // Your object code here
}, []);

Solution 3: Check for Component Rerenders

If none of the above solutions work, it’s possible that your component is being rerendered unnecessarily. This can happen if the parent component re-renders, causing the child component to also re-render.

To prevent unnecessary rerenders, you can use the React.memo() higher-order component or the React.PureComponent class component. These will perform a shallow comparison of props and prevent rerenders if the props have not changed.

const MyComponent = React.memo((props) => {
  // Your component code here
});

Conclusion

If you’re experiencing the useEffect() hook being called twice, even when passing an empty array as the second argument, there are a few solutions you can try. First, ensure that the dependency array is truly empty. If that doesn’t work, check for unintended side effects caused by functions or objects as dependencies. Finally, investigate if unnecessary component rerenders are occurring and use React.memo() or React.PureComponent to prevent them.

By following these solutions, you should be able to resolve the issue of useEffect() being called twice and ensure that your code behaves as expected.