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If the useEffect return function returns a function, the returned function is called cleanup. cleanup is run before the effect is reached (to clean up effects from previous renders).
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The useEffect returned function runs according to the dependencies array:
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- if it's empty [], on unmount.
- if it has dependencies [value1,value2], on dependencies
change (shallow comparison) and unmount.
- if it has no dependencies (no 2nd argument for useEffect)
it runs on every render and unmount.
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In React, the useEffect hook is used to synchronize a component with an
external system. It allows you to handle side effects in function components.
The useEffect hook takes two arguments: a callback function that contains
the logic for the side effect, and a dependencies array that determines when
the effect should be run.
The callback function passed to useEffect will be called after the component
is rendered.
The useEffect function returns a cleanup function which will be called when
the component is unmounting or when the effect is re-run. This cleanup
function can be used to perform any necessary cleanup, such as canceling
a network request or removing an event listener.