Hooked on React: Mastering useEffect Comparisons

Photo by Steve Johnson

Recognizing the Need for Comparison

React's useEffect hook runs after every render, including the initial mount, but sometimes we want to fire off our effects only when certain values change. Here lies the art of comparison—identifying exactly when our component's props or state have altered their tune from the previous render.

useEffect(() => {
  // This runs after every render – but we want more control!
}, []);

The Usual Suspects: Dependencies Array

Your first pitstop in mastering useEffect is the dependencies array, that vigilant list that tells React which values to keep an eye on.

useEffect(() => {
  // Code that runs when `value` changes
}, [value]);

🔎 Yet, this doesn't show us how value has changed, which brings us to our next tool in the toolbox.

useRef: The Keeper of History

To compare the past with the present, we need a memory, and useRef does that without the drama of triggering re-renders.

const valueRef = useRef();

useEffect(() => {
  if (value !== valueRef.current) {
    // `value` has changed since the last render. Let's do something about it!
    valueRef.current = value;
  }
}, [value]);

Crafting a Custom Hook

Developers love dry humor and DRY code. So why repeat comparison logic? A custom hook can rescue you from the monotony.

import { useEffect, useRef } from 'react';

function usePrevious(value) {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
}

// Usage in a component
const MyComponent = ({ value }) => {
  const prevValue = usePrevious(value);

  useEffect(() => {
    if (prevValue !== value) {
      // Time for a celebration 🎉, `value` has evolved!
    }
  }, [value, prevValue]);

  // ...
};

When to Use Your Comparison Superpowers

So, you've got the power to compare like a wizard. But with great power comes... you know the drill. Use comparison gracefully and only when it's logical, like for fetching data on prop change or managing side effects that depend on specific state variables.

useEffect(() => {
  if (!isEqual(prevProps, props)) {
    // If only there was a function called `useWiseEffect`...
    fetchData();
  }
}, [props]);

Edge Cases and Gotchas

Keep in mind that comparing objects and arrays could lead to an endless loop of "gotcha!" if you're not careful. Always compare their contents, not their identities, or the joke will be on you.

const prevPropsStringified = JSON.stringify(prevProps);
const propsStringified = JSON.stringify(props);

useEffect(() => {
  if (prevPropsStringified !== propsStringified) {
    // Objects in strings walking into a bar might sound like a bad joke, but it gets the job done here.
    fetchData();
  }
}, [propsStringified, prevPropsStringified]);

Conclusion: Write Once, Compare Wisely

Master the art of comparison within useEffect to fine-tune your components' responsiveness to change without falling into the traps that lead to unnecessary renders or infinite loops. Remember, you're not trying to recreate the Battle of Waterloo here—keep it friendly, keep it efficient.

Code smart, compare intelligently, and your React components will be as smooth as a perfectly paved Dutch bike lane. Happy coding! 👨‍💻

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