useEffect is a hook in React that is used for performing side effects. It is invoked after the initial render of the component. It is equivalent to the componentDidUpdate method in Class components. useEffect takes a callback that will be triggered after each render of the component, and it has access to props and state. The callback will not be triggered if the props or state did not change. To guarantee that the callback gets invoked each time, regardless of any updates, pass [] (empty array) as the second argument.

Example:

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useEffect(() => {
  // perform side-effects
}, [])

The useEffect hook can be used in three major cases:

1. To run custom effects when React state or props change such as calls to APIs for data fetching or state updating.
2. To perform side effects such as setting up subscriptions, and cleanup tasks such as canceling those subscriptions.
3. To increase the app’s performance by preventing unnecessary re-renders of components.

For more detailed information visit React's official documentation on useEffect.

The useEffect hook is an important feature in React. It enables developers to perform side effects in a functional component in the same way that lifecycle methods can be used in class components. The useEffect hook is triggered after every render, meaning that it contains code that will run after the render is committed to the screen. The useEffect hook takes as an argument a function that will be run after the initial render, as well as any other consequent re-renders. Inside the function, you can define any number of operations on the data being rendered. This could include changes to the DOM, data fetching, subscriptions, or manually changing the state of a React component. For example, if you wanted to fetch data from an API after every re-render, you could use the useEffect hook. The code would look something like this:

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React.useEffect(() => {
    fetchData();
}, []);

The second argument of the useEffect hook is an array of values which, if changed, will cause the useEffect hook to run. To make the above example more specific, you can add a value inside the array that will make it run only when that value is changed. For example, if you wanted to make the useEffect hook run only when the input value changes, you could do the following:

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const [inputValue, setInputValue] = React.useState('');

React.useEffect(() => {
    fetchData();
}, [inputValue]);

This example will make the useEffect hook run every time the inputValue changes.

For more information on React hooks, please checkout the official React documentation.

React's useEffect hook can be used to avoid unnecessary API calls. To do this, you can check to see if the data from the API call is already present in state or props before making a new API call. If the data is not present, you can set up an effect that calls the API and sets the state.

For example:

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const MyComponent = () => {
  const [data, setData] = useState({});

  useEffect(() => {
    if (!data.name) {
      // Make API call and set data
    }
  }, [data]);

  return <MyComponent />;
};

In the code above, an API call would only be made if data did not already contain a value for name.