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Use UseEffect Effectively

Posted on:August 23, 2023 at 03:22 PM

When working with React and managing side effects using the useEffect hook, specifying dependencies is a crucial step to ensure your code behaves as expected and avoids unnecessary re-renders.

Alright, so first things first, the useEffect hook takes two arguments: a callback function and an array of dependencies. The callback function contains the code you want to run when the component mounts, updates, or unmounts. The dependencies array lets React know which variables or values the effect depends on.

Here’s the deal with specifying dependencies:

  1. Empty Dependency Array: If you pass an empty array as the second argument ([]), the effect will only run once when the component mounts, and it won’t re-run even if the component updates. This is handy for things like fetching data when the component loads and you don’t want the effect to trigger on subsequent updates.

  2. No Dependency Array: If you omit the second argument altogether, the effect will run every time the component renders or re-renders. This might lead to performance issues and unnecessary re-fetching of data.

  3. Array with Dependencies: When you provide an array of dependencies, the effect will run whenever any of those dependencies change between renders. This is super useful when you want to respond to specific changes, like updating a UI element when a particular prop or state changes.

For example, let’s say you have a component that displays a user’s profile based on their userId. You’d want the effect to run whenever the userId prop changes, so you’d specify [userId] as the dependency array. That way, the effect only runs when the userId changes and doesn’t run on every render.

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

function UserProfile({ userId }) {
  const [user, setUser] = useState(null);

  useEffect(() => {
    // Fetch user data based on userId
    fetchUserData(userId).then(data => setUser(data));
  }, [userId]); // Only re-run the effect if userId changes

  return (
      {user ? (
      ) : (

So, the key takeaway is to carefully choose the dependencies in the array to make sure your effect runs when it should and doesn’t cause unnecessary re-renders. It’s a bit of a balancing act, but once you get the hang of it, it’ll make your React components more efficient and responsive.