'useEffect' in depth

What is useEffect for?

useEffect was introduced in react 16.8, before this was released react developers would use componentDidMount and componentWillUnmount.
The useEffect hook is a built-in React hook that allows you to run side effects in functional components. This side effects are actions that do not directly affect the UI but it can include things like fetching data, adding event listeners, etc.

Why useEffect accept two arguments?

It takes two arguments, the first argument being a function that contains the code for the side effect you want to perform and the second argument which is optional being how many times we want it to be perform.

First Example

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

function Timer() {
  const [time, setTime] = useState(0);

  useEffect(() => {
    const intervalId = setInterval(() => {
      setTime(prevTime => prevTime + 1);
    }, 1000);
  }, []);

  return <div>{time} seconds have passed.</div>;
}

In the example above it will run just once after the component in mounted.

Second Example

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

function Timer() {
  const [time, setTime] = useState(0);

  useEffect(() => {
    const intervalId = setInterval(() => {
      setTime(prevTime => prevTime + 1);
    }, 1000);
  }, [time]); 

  return <div>{time} seconds have passed.</div>;
}

In the example above we can see how i passed the second argument so it just renders every time 'time' changes.

Cleanup in useEffect?

This is something not always is being used or even taught, but is indeed really important for time efficient and memory leaks.
We should run cleanup in useEffect to we avoid having memory leaks in our application which can cause our app to be slower.

Example with cleanup

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

function Timer() {
  const [time, setTime] = useState(0);

  useEffect(() => {
    const intervalId = setInterval(() => {
      setTime(prevTime => prevTime + 1);
    }, 1000);

    return () => {
      clearInterval(intervalId);
    };
  }, [time]);

  return <div>{time} seconds have passed.</div>;
}

In the example above we can see that i added a return which will return a clearInterval that will do a cleanup with the intervalId and we will avoid memory leaks in our application.

Hope someone found this useFul and if you have any questions and would be happy to answer them.

Lautaro.

Comments

[Florian Rappl]

For an article titled "in depth" its rather shallow and is also not fully accurate.

1. The second argument is optional - if no dependency array is passed in then it just runs on every render.
2. The dependency array is actually crucial. You should add information that it makes a shallow comparison. So, as an example, instead of passing in an object or so you should try to make to only pass in the used properties of an object.
3. There is also useLayoutEffect - while this article is about useEffect it would still be really good to highlight why there are two different hooks and what is the difference.

Personally, I loved the useFul joke. Not sure if it was intentional though or just a typo.

[Lautaro Suarez]

I appreciate your answer Florian.
I missed some really importante points that i took from granted, on next posts i will explain more in depth as the title says what would happen depending on the situation.

And yeah, i always put the same joke on every post and you are the first one who understand it was a joke.

Again, thank you for the comment.

[Blake Creasser]

Love it, short and concise but good information!

[Lautaro Suarez]

Thank you man, i usually try to do short posts but with good info

[React Hunter]

It described 3 functionalities.
Looks Good!