What is useEffect?

useEffect is a react built-in hook that allows us to run code on the mount, update and unmount stages of our components lifecycle. Its the equivalent of the class based components to the componentDidMount, componenDidUpdate and componentWillUnmount methods.

The useEffect hook receives two parameters: one function to be executed and an array of dependencies.

Let's see some examples

On my app.js Ive set two input numbers and one button alongside the Calculator component. We set the numbers on the inputs and when we click the button the states get updated and the component gets the number via props. Nothing fancy.

import { useRef, useState } from "react";import "./App.css";import Calculator from "./Calculator";const App = () => {  const inputA = useRef(0);  const inputB = useRef(0);  const [numbers, setNumbers] = useState({    numberA: inputA.current.value,    numberB: inputB.current.value,  });  const getTotalHandler = () => {    setNumbers({    numberA: +inputA.current.value,    numberB: +inputB.current.value,    });  };  return (    <>    <div className="container">        <div>        <label>Input A</label>        <input type="number" ref={inputA} />        </div>        <div>        <label>Input B</label>        <input type="number" ref={inputB} />        </div>        <button onClick={getTotalHandler}>Calculate</button>        <Calculator numberA={numbers.numberA} numberB={numbers.numberB} />    </div>    </>  );};export default App;

I have this component called Calculator that receives via props numberA and numberB and returns the sum.

const Calculator = ({ numberA, numberB }) => {  return <h1>The total is {numberA + numberB}</h1>;};export default Calculator;

Execute useEffect when the component gets mounted

Now I will use useEffect to execute a function that logs me when the component its mounted. First I import the useEffect hook from react.

import { useEffect } from "react";

To implement this hook I have to set a function to be executed and an array of dependencies and then my component looks like this:

import { useEffect } from "react";const Calculator = ({ numberA, numberB }) => {  useEffect(() => {    console.log(`First render`);  }, []);  return <h1>The total is {numberA + numberB}</h1>;};export default Calculator;

In this case I left my dependency array empty as I just want this code to be executed when its rendered for the first time.

Now when I first load my component I see it on my console:

As the dependency array is empty this log will be executed only when the component gets mounted. If I update the App component state and update the Calculator props this log function will not execute again. This is the equivalent to the componentDidMount on class-based components. Lets check:

Ive updated the numberA and numberB props but the function did not execute.

Execute useEffect each time the props gets updated

Now lets execute a function each time the props get updated. This is useful to perform side effects based on new props for example to retrieve details from an api on the basis of a value received from props. At the end of the post theres an example with this.

First I add a second useEffect to my previous code.

import { useEffect } from "react";const Calculator = ({ numberA, numberB }) => {  useEffect(() => {    console.log(`First render`);  }, []);  useEffect(() => {    console.log(`This gets executed each time the props are updated`);  }, [numberA, numberB]);  return <h1>The total is {numberA + numberB}</h1>;};export default Calculator;

This time I did not leave my dependencies array empty and Ive added the numberA and numberB props on it so the useEffect hook knows that each time one of the props gets updated it has to run the function. Lets check if it works:

Yes it does. Actually you will see that the first time the component gets mounted both useEffect hooks get executed. This is the equivalent to the componentDidMount method on class-based components.

Running code when the component gets unmounted

Until now Ive shown how to run code each time the component gets mounted or updated but what if I have some code that I just wanna run when the component gets unmounted? Luckily for us the useEffect hook provides a clean up function. Inside the useEffect code we have to return a function and this function will execute before unmounting:

useEffect(() => {...return () => {    console.log   }})

The useEffect hook combined with the clean up function its the equivalent to the componentWillUnmont method in class-based components.

Using useEffect to get data from an API each time the props get updated

Now I will show you how to take advantage of the useEffect hook to get data from an API each time the props of our component is updated.

For this example Ive created a new component called Rick that uses an id received via props to get data from the Rick and Morty public api.

import { useState, useEffect } from "react";const Rick = ({ id }) => {  const [character, setCharacter] = useState(null);  const [request, setRequest] = useState(`pendent`);  useEffect(() => {    setRequest(`pendent`);    const getApiResults = (characterId) => {    fetch(`https://rickandmortyapi.com/api/character/${characterId}`)        .then((response) => response.json())        .then((data) => {        setCharacter(data);        setRequest(`completed`);        });    };    getApiResults(id);  }, [id]);  let content = ``;  if (request === `completed`) {    content = (    <>        <img src={character.image} alt={character.name} />        <h1>{character.name}</h1>    </>    );  }  return content;};export default Rick;

From the app component I have a button that generates a random number and passes it via props to the Rick component. On the first render we just have the button:

But each time we click a random number its generated and passed via props. This triggers the useEffect function that gets the belonging Rick and Morty character based on that id: