Understanding the useEffect Hook in React

The 'useEffect' hook in React is used to manage side effects in your functional components. It plays the role of componentDidMount, componentDidUpdate, and componentWillUnmount combined in class components, thus managing the lifecycle of components in React apps.

Generally, operations that cause side effects are positioned outside the body of components. These operations could include asynchronous data fetching, manual DOM manipulations, or subscription setups - basically any operation that doesn't affect the output of the render directly, but needs to happen during the render lifecycle.

React's useEffect hook can be seen as a means to take that extra care of actions that you want to happen after your component has been rendered or updated or just before it’s about to unmount.

Below is a basic example of how the useEffect hook can be used in a functional component:

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

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

In the code above, the useEffect hook tells React that the component has to do something after render. In this case, it is setting the document title, but it could be any side-effect operation as per the application's requirements. Essentially, this allows you to establish and clean up subscriptions or other side effects in the lifecycle of your component in a cleaner and readable way without writing different lifecycle methods.

By default, React runs the effects after every render. However, you can also choose to fire them only when certain values have changed by passing those values in the second argument array to useEffect().

React’s useEffect hook is an incredibly powerful tool that can optimize and simplify your code once you get the hang of it. Understanding and using this function effectively can greatly enhance your React development experience.