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Jose C
Jose C

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React - useEffect hook - A Quick Guide

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.

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

hook explanation

Let's see some examples

On my app.js I’ve 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;
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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;
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example

Execute useEffect when the component gets mounted

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

import { useEffect } from "react";
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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;
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In this case I left my dependency array empty as I just want this code to be executed when it’s rendered for the first time.

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

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. Let’s check:

example

I’ve updated the numberA and numberB props but the function did not execute.

Execute useEffect each time the props gets updated

Now let’s 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 there’s 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;
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This time I did not leave my dependencies array empty and I’ve 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. Let’s check if it works:

example

Yes it does. Actually you will see that the first time the component gets mounted both useEffect hooks get executed.

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 I’ve 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;
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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:

mockup

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

Discussion (2)

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joeattardi profile image
Joe Attardi

useEffect is not quite equivalent to componentDidMount, it's a different mental model. See this article for a good explanation.

It's also worth noting that the cleanup from useEffect is not only run when the component is unmounted, but also every time the effect runs again when a dependency changes, the cleanup function returned by the previous effect will be run.

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josec profile image
Jose C Author

Hi Joe, thanks for sharing this react training post certainly my explanation about the useEffect and it's class methods equivalents it's not the best so I unpublished this post to fix it. 🙂