React Hooks Explained

Why hooks?

Functional components passing props are amazing because they are simple, perform fast, and require little code- but they can lead to the much dreaded "wrapper hell" in the quest for maintaining encapsulated components. On the other hand, class components are often confusing to work with for both humans and machines- they often lack a positive developer experience, which makes it more difficult creating a more positive user experience as well. Hooks provide a way for us to use state and lifecycle methods with minimal code- like giving your components super powers!

My components need super powers?

In general, using props is enough to create an amazing component, but sometimes you need more. Here are some really useful examples to know when to use hooks:

1. If you want to open or close a modal or dialog- use the useState hook.
2. If you want to fetch some data only at the beginning of the program execution, use the useEffect hook.
3. If you want to share information within all components, use the useContext hook. We will talk more about the useContext hook on a separate lesson.

Through experience, you will learn when to use a hook. If you don't think you need them, DON'T use them! The less the better.

Most applications need at least one useState and one useEffect. To make use of hooks, we MUST FIRST IMPORT them at the beginning of our file. For example, if we need useState, we would do:

import React, { useState } from 'react';

And if we want to use useEffect as well, we can include like this:

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

Now let's learn how to use them :)

The useState hook:

The most important hook, almost unavoidable! The useState helps you initialize a variable and change its value over time without the need of parent components. This is how you can use it:

//       variable name      setter name               initial value (any value)
const [ mySuperVariable, mySuperFunction ] = useState(          null        );

Basically mySuperVariable will be initialized with null, and then you will be able to re-set its value by calling mySuperFunction like this:

// here we are re-setting the value of mySuperVariable = 'hello' when the user clicks on a button:
<button onClick={() => mySuperFunction('hello')}></button>

Possible uses for the useState hook

1. Counting: Displaying the number of likes on the screen and being able to increase or decrease when the user clicks, click here for demo
2. Timer/Clock: You can use the system time to show the current time on the screen, but since time changes all the time, we store it with a state variable, click here for demo
3. Showing an input on the screen: The best practice to get the content from any input is by storing it on a state variable- this is called "Controlled Input", click here for a controlled input example
4. Opening/Closing (show/hide): A typical use case is having a dialog that asks a question or allows a user to sign up for a newsletter, click here for the example..
5. Thousands of other possible applications.

Let's explain this hook with a small Modal window example. Here is the live code:

To implement a "Modal Window" we decided to create a hooked variable called opened that is true if the modal window has to be shown to the user.

If the user clicks on "close", we simply use the hook function setOpened to change the value of opened to false.

The useEffect hook:

useEffect is another amazing hook that you will use if you want to execute some code after the component renders, for example:

1) After the component renders for the first time (like the good old componentDidMount).

const MyComponent = () => {
    useEffect(() =>

        // whatever you code here will execute only after the first time the component renders

 , []);// <------ PLEASE NOTICE THE EMPTY ARRAY


    return <Some HTML>;
}

2) Every time (or some times) after the component re-renders.

const MyComponent = () => {
    useEffect(() =>
        // this will run every time the component re-renders
        if(some_condition){
            //this will run only if some_condition is true
        }
    );// <------ PLEASE NOTICE THE EMPTY ARRAY IS GONE!

    return <Some HTML>;
}

3) When the component will unmount or stop being rendered (like the good old componentWillUnmount function used by class components).

const MyComponent = () => {
    useEffect(() =>
        // this will run only the first time the component renders.
        return () => {
            // this will run only right before the component unmounts
        }
    ,[]);// <------ PLEASE NOTICE THE EMPTY ARRAY

    return <Some HTML>;
}

Building a Todo List Using just useState and useEffect Hooks

For example, let's say I'm building a todo list, and I have to load the list of tasks from an API. I will have to fetch the information right after the component renders for the first time:

const Todos = (props) => {
    // initialize the tasks variable to an empty array and hook it to setTasks function
    const [ tasks, setTasks] = useState([]);

    // this function useEffect will run only one time, when the component is finally loaded the first time.
    useEffect(() =>
        // here i fetch my todos from the API
        fetch('https://assets.breatheco.de/apis/fake/todos/user/alesanchezr')
            .then(r => r.json())
            .then(data => setTasks(data)) //here it re-set the variable tasks with the incoming data
    , []);

    return <ul>{tasks.map(t => <li>{t.label}</li>)}</ul>;
}

Further Reading

For more information, including how to build your own hooks, check out: Official React Documentation