Manage Dark Mode in ReactJs, With Listener to Detect Dark-Mode in the Browser
Tags: react
Published: 2021-06-06 17:59:02


Nowadays most web/mobile applications have a dark mode presentation. That's why today I will explain to you how to do it in reactJs applications. There are several ways to achieve it, I will do it using CSS and HTML. 

In Addition, I'm going to add a listener to detect if the dark/light mode of your browser has changed, so the app will change styles automatically.

#### 1. Create a react app

Choose a location in your terminal and run this:

```
npx create-react-app my-dark-mode-app
```

#### 2. Add CSS

Let's modify some CSS. First, we are going to create some global css variables to use. Go to `index.css` and add this lines at the beginning of the file:

```css
:root {
  --background-color: #fefefe;
  --text-color: #3f3f3f;
}

.dark-mode {
  --background-color: #3f3f3f;
  --text-color: #ffffff
}
```

In the same, file add these properties to `body`:

```css
background-color: var(--background-color);
color: var(--text-color);
```

Those previous styles will help us to change dark mode styles: background color and text colors.

Now go to the file `App.css` and make sure you just have these styles:

```css
.App {
  text-align: center;
}

.App-header {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-size: calc(10px + 2vmin);
}
```

#### 3. Modify JavaScript

We are going to work with a simple view, so go to `App.js` and edit the `return`:

```javascript
return (
  <div className="App">
    <header className="App-header">
      <p>
        This is an example of dark/light mode using reactJs !!
      </p>
    </header>
  </div>
);
```

#### 4. Change Dark Mode

Inside `App.js` we should create a `useState` hook to save the mode of our application:

```javascript
const [mode, setMode] = useState('light')
```
Don't forget to import `useState`
```javascript
import {useState} from "react";
```

Also, with this new method we're going to save in our state the current mode, then ***add or remove*** the `dark-mode` classname of the `body` tag.

```javascript
const onSelectMode = (mode) => {
  setMode(mode)
  if (mode === 'dark')
    document.body.classList.add('dark-mode')
  else
    document.body.classList.remove('dark-mode')
}
```

Then, in this same file let's add some icons after the `p` tag:

```javascript
<div>
  {
    mode === 'dark' ?
      <svg className={'icons'} xmlns="http://www.w3.org/2000/svg" width={33} viewBox="0 0 24 24" fill="#f1e408"
           onClick={() => onSelectMode('light')}>
        <rect fill="none"/>
        <path
          d="M12,9c1.65,0,3,1.35,3,3s-1.35,3-3,3s-3-1.35-3-3S10.35,9,12,9 M12,7c-2.76,0-5,2.24-5,5s2.24,5,5,5s5-2.24,5-5 S14.76,7,12,7L12,7z M2,13l2,0c0.55,0,1-0.45,1-1s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S1.45,13,2,13z M20,13l2,0c0.55,0,1-0.45,1-1 s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S19.45,13,20,13z M11,2v2c0,0.55,0.45,1,1,1s1-0.45,1-1V2c0-0.55-0.45-1-1-1S11,1.45,11,2z M11,20v2c0,0.55,0.45,1,1,1s1-0.45,1-1v-2c0-0.55-0.45-1-1-1C11.45,19,11,19.45,11,20z M5.99,4.58c-0.39-0.39-1.03-0.39-1.41,0 c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0s0.39-1.03,0-1.41L5.99,4.58z M18.36,16.95 c-0.39-0.39-1.03-0.39-1.41,0c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0c0.39-0.39,0.39-1.03,0-1.41 L18.36,16.95z M19.42,5.99c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06c-0.39,0.39-0.39,1.03,0,1.41 s1.03,0.39,1.41,0L19.42,5.99z M7.05,18.36c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06 c-0.39,0.39-0.39,1.03,0,1.41s1.03,0.39,1.41,0L7.05,18.36z"/>
      </svg> :
      <svg className={'icons'} xmlns="http://www.w3.org/2000/svg" width={33} viewBox="0 0 24 24" fill="#000000"
           onClick={() => onSelectMode('dark')}>
        <rect fill="none" height="24" width="24"/>
        <path
          d="M9.37,5.51C9.19,6.15,9.1,6.82,9.1,7.5c0,4.08,3.32,7.4,7.4,7.4c0.68,0,1.35-0.09,1.99-0.27C17.45,17.19,14.93,19,12,19 c-3.86,0-7-3.14-7-7C5,9.07,6.81,6.55,9.37,5.51z M12,3c-4.97,0-9,4.03-9,9s4.03,9,9,9s9-4.03,9-9c0-0.46-0.04-0.92-0.1-1.36 c-0.98,1.37-2.58,2.26-4.4,2.26c-2.98,0-5.4-2.42-5.4-5.4c0-1.81,0.89-3.42,2.26-4.4C12.92,3.04,12.46,3,12,3L12,3z"/>
      </svg>
  }
</div>
```

As you can see, we are calling the `onSelectMode` method with the mode as parameter. I'm showing a `sun` icon if we are in dark mode, and a `moon` icon if light mode is active. Clicking in those icons we can change our mode.

Now let's add a missing style in `App.css`:

```css
.icons {
  cursor: pointer;
}
```
So far our `App.js` looks like this:
```javascript
import './App.css';
import {useState} from "react";

function App() {
  const [mode, setMode] = useState('light');

  const onSelectMode = (mode) => {
    setMode(mode)
    if (mode === 'dark')
      document.body.classList.add('dark-mode')
    else
      document.body.classList.remove('dark-mode')
  }

  return (
    <div className="App">
      <header className="App-header">
        <p>
          This is an example of dark/light mode using reactJs !!
        </p>
        <div>
          {
            mode === 'dark' ?
              <svg className={'icons'} xmlns="http://www.w3.org/2000/svg" width={33} viewBox="0 0 24 24" fill="#f1e408"
                   onClick={() => onSelectMode('light')}>
                <rect fill="none"/>
                <path
                  d="M12,9c1.65,0,3,1.35,3,3s-1.35,3-3,3s-3-1.35-3-3S10.35,9,12,9 M12,7c-2.76,0-5,2.24-5,5s2.24,5,5,5s5-2.24,5-5 S14.76,7,12,7L12,7z M2,13l2,0c0.55,0,1-0.45,1-1s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S1.45,13,2,13z M20,13l2,0c0.55,0,1-0.45,1-1 s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S19.45,13,20,13z M11,2v2c0,0.55,0.45,1,1,1s1-0.45,1-1V2c0-0.55-0.45-1-1-1S11,1.45,11,2z M11,20v2c0,0.55,0.45,1,1,1s1-0.45,1-1v-2c0-0.55-0.45-1-1-1C11.45,19,11,19.45,11,20z M5.99,4.58c-0.39-0.39-1.03-0.39-1.41,0 c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0s0.39-1.03,0-1.41L5.99,4.58z M18.36,16.95 c-0.39-0.39-1.03-0.39-1.41,0c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0c0.39-0.39,0.39-1.03,0-1.41 L18.36,16.95z M19.42,5.99c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06c-0.39,0.39-0.39,1.03,0,1.41 s1.03,0.39,1.41,0L19.42,5.99z M7.05,18.36c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06 c-0.39,0.39-0.39,1.03,0,1.41s1.03,0.39,1.41,0L7.05,18.36z"/>
              </svg> :
              <svg className={'icons'} xmlns="http://www.w3.org/2000/svg" width={33} viewBox="0 0 24 24" fill="#000000"
                   onClick={() => onSelectMode('dark')}>
                <rect fill="none" height="24" width="24"/>
                <path
                  d="M9.37,5.51C9.19,6.15,9.1,6.82,9.1,7.5c0,4.08,3.32,7.4,7.4,7.4c0.68,0,1.35-0.09,1.99-0.27C17.45,17.19,14.93,19,12,19 c-3.86,0-7-3.14-7-7C5,9.07,6.81,6.55,9.37,5.51z M12,3c-4.97,0-9,4.03-9,9s4.03,9,9,9s9-4.03,9-9c0-0.46-0.04-0.92-0.1-1.36 c-0.98,1.37-2.58,2.26-4.4,2.26c-2.98,0-5.4-2.42-5.4-5.4c0-1.81,0.89-3.42,2.26-4.4C12.92,3.04,12.46,3,12,3L12,3z"/>
              </svg>
          }
        </div>
      </header>
    </div>
  );
}

export default App;
```

Until this point, we are able to change manually our dark/light mode. What if we want that our app automatically detects if dark/light mode of our devices has changed?.

*Let's do it!*

#### 5. Adding a Listener

First, we need to know how we can detect if dark/light mode has changed in our browser(don't copy yet):

```javascript
window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => onSelectMode(e.matches ? 'dark' : 'light'));
```
The `e.matches` will return true if __dark mode__ has been enabled.

Let's create a `useEffect` hook, to create our listener the first time our app loads.

```javascript
useEffect(() => {
  // Add listener to update styles
  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => onSelectMode(e.matches ? 'dark' : 'light'));

  // Setup dark/light mode for the first time
  onSelectMode(window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light')

  // Remove listener
  return () => {
    window.matchMedia('(prefers-color-scheme: dark)').removeEventListener('change', () => {
    });
  }
}, []);
```

We must know that the `Listener` is going to detect when dark mode changes. However, the first time our app loads we need to know the current mode of our browser, that's why I added this line in our `useEffect`:

```javascript
onSelectMode(window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light')
```

![image1](https://gist.githubusercontent.com/gustavo-gomez/85bfe6f24c74dbc883bee598b0d5d3cd/raw/07ca892d45354058f2829e20987bac7c07856345/dark_mode_manual.GIF)
![image1](https://gist.githubusercontent.com/gustavo-gomez/85bfe6f24c74dbc883bee598b0d5d3cd/raw/07ca892d45354058f2829e20987bac7c07856345/dark_mode_system.GIF)


##Conclusion

We created a reactJs application, in which we were able to implement dark mode using CSS to change the background color and text color.
Every text inside the `body` tag will be changed in dark/light mode. Then we recognize the browser mode to show our page in dark or light mode automatically.


You can see the demo: [dark-mode-reactjs-demo](https://dark-mode-reactjs-sx2ua.ondigitalocean.app)

Github repo:
{% github gustavo-gomez/dark-mode-reactJs no-readme %}