📘 React `useEffect` Hook

The useEffect Hook allows you to perform side effects in your React components.

What Are Side Effects?

Side effects are operations that affect something outside the scope of the function being executed. Examples include:

Fetching data from an API
Directly updating the DOM
Setting up timers (e.g., setTimeout, setInterval)
Subscribing to events

Basic Syntax

useEffect(<function>, <dependency_array?>)

The first argument is the function that runs the side effect.
The second argument is optional, and it controls when the effect should run.

Timer Example

import { useEffect, useState } from "react";
 
function Timer() {
  const [count, setCount] = useState(0);
 
  useEffect(() => {
    setTimeout(() => {
      setCount((count) => count + 1);
    }, 1000);
  }, []); // 👈 Runs only on first render
 
  return <h1>I've rendered {count} times!</h1>;
}

To make the effect run only once (on mount), pass an empty dependency array ([]):

Timer (Wrong Way)

import { useEffect, useState } from "react";
 
function Timer() {
  const [count, setCount] = useState(0);
 
  useEffect(() => {
    setTimeout(() => {
      setCount((count) => count + 1);
    }, 1000);
  });
 
  return <h1>I've rendered {count} times!</h1>;
}

What's wrong?

This code keeps running setTimeout on every render, causing the counter to update infinitely.

Run useEffect When Dependencies Change

useEffect(() => {
  setTimeout(() => {
    setCount((count) => count + 1);
  }, 1000);
  // Runs on initial render AND when `count` changes
});
}, [count]);

Tips

Always specify the dependencies clearly to avoid bugs.
Clean up side effects when necessary to prevent memory leaks.
Avoid using async directly in useEffect - use an inner function instead.

useEffect(() => {
  const fetchData = async () => {
    // your async code
  };
  fetchData();
}, []);

📘 React useEffect Hook