Q) Let’s say their is an API and I want you run it on the first page load , how can you do it ?

Ans. Yes! We need useEffect for that .

useEffect is something which helps us to perform side effects in our react application

Now what is side effect?

Side effect is something which gives unpredictable output / result or actions which are performed by the outside world.

Some common example of side effect are –

• making an API request
• unpredictable async functions call , etc.

Not only this, side effect don’t hinder the rendering or performance of the react function while interacting with the outside world.

Let’s see useEffect practically

Basic Syntax –

import { useEffect } from 'react';

function MyComponent() {
 
  useEffect(() => {}, []);
  
  // return ...
}

Component mounted for the first time –

Replacement for “componentDidMount“

import React, { useEffect } from "react";

const A = () => {

  console.log("component mounted"); //first

  useEffect(() => {
    console.log("use effect called"); //second
  });

  return <div>A</div>;
};

export default A;

In the above function , the data under useEffect will be printed once component is mounted for the first time.

Component update using useEffect and dependency array

Replacement for “componentDidUpdate“

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

const A = () => {
  var [counter, setCounter] = useState(0);
  var [counter2, setCounter2] = useState(0);

  console.log("component mounted");
  console.log("component did updated");

  useEffect(() => {
    console.log("use effect called");
  }, [counter]);

  return (
    <div>
      <h1> Component A  </h1>
      <h2>{counter}</h2>
      <button onClick={() => setCounter(counter + 1)}>Click me</button>
      <br />
      <h2>{counter2}</h2>

      <button onClick={() => setCounter2(counter2 + 1)}>Click me</button>
    </div>
  );
};

export default A;

Here useEffect will only run when the value inside dependency array will update/change.

This is why

• when the value of “counter” changes , useEffect will run.
• when the value of “counter2” changes , useEffect did not run.

Dependency array prevent from infinite loop

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

const B = () => {
  var [data1, setData] = useState();

  async function getUserData() {
    const data = await fetch("https://api.github.com/users");
    const response = await data.json();
    // console.log(response);
    setData(response);
  }
  //infinite loop , wrong way
  useEffect(() => {
    console.log("use effect called");
    getUserData();
  });
 

  //correct way using dependecy array
  useEffect(() => {
    console.log("use effect called");
    getUserData();
  });


  return (
    <div>
      <h1>fds</h1>
      {data1?.map((each, index) => (
        <p key={index}>{each.login}</p>
      ))}
    </div>
  );
};

export default B;

1. useEffect(() => {});  
it will run after every render. 

2. useEffect(() => {}, []);
it will run once after the component has rendered the first time. 

3. useEffect(() => {}, [value1 , value2]);
it will only run when the value inside array will update

Component unmounted using useEffect – Cleanup function

Replacement for “componentWillUnmount“

Sometimes we need to stop the subscription , or stop the side effect intertionally, that time we need a cleanup function.

This is an optional function.

Syntax –

 useEffect(() => {
    console.log("use effect called");    

    return () => {
      console.log("component unmounted");
    };
  }, [counter]);

Important thing to keep in head about useEffect()

• useEffect() take 2 argument : a function and a dependency array(optional)
• it helps to run a function which need to be run on first render / every page load / conditional render , etc
• function inside useEffect() will be called after component has been rendered.

Hope you have understood and found it helpful.