Turning off the lights

After creating my blog, I thought it would be good to be part of all the Dark Mode hype. And since I've never done it, I tried it out.

Since I'd never really worked with NextJS aside from building my blog, I had to do some research before getting my hands dirty.

For this tutorial, I'll continue using the code from this post, although I'll leave the changes for this tutorial in a branch called dark-mode.

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All right, let's get started

For this to happen we'll be using styled-components, if you're not familiar with it, I would suggest you take a quick look into their documentation.

In the root of your projects let us create a styles folder and a themes.js file:

mkdir styles && touch styles/themes.js

All right, we need a light and a dark theme, for that we can simply create 2 objects, one for each theme:

export const lightTheme = {
  body: '#FFFFFF',
  text: '#363537',
}

export const darkTheme = {
  body: '#101010',
  text: '#FAFAFA',
}

Don't worry about this, for now, we'll be using it later.

What I've done was creating a Layout wrapper that contains my global styles, theme styles and so forth. But before that let's define our global styles:

touch styles/global.js

Inside global.js we'll be using styled-components, so let's take care of installing it:

npm install styled-components --save

Let's start with something simple such as, background and text color:

import { createGlobalStyle } from 'styled-components';

// the `theme` object is comming from our ./themes.js file
export const GlobalStyles = createGlobalStyle`
  body {
    background-color: ${({ theme }) => theme.body};
    color: ${({ theme }) => theme.text}
  }
`

Cool, we should be done with this, let's look into implement the themes.

---

Layout and Theme Switch

I've mentioned that I used a Layout wrapper to take care of styles, themes, and so forth. Let's create that wrapper:

mkdir components && touch components/Layout.js

Paste the following into Layout.js:

import React from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

// Default dark mode to true for now
const darkMode = true

function Layout({ children }) {
  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

We should be ready to wrap our pages with Layout, if you look at the codebase I've provided at the beginning of this tutorial you should see a file /pages/index.js, on that file change the following lines:

import React from 'react'

import Layout from '../components/Layout'

export default function Index(props) {
  return (
    <Layout>
      ✍️ My blog about {props.blogCategory}
    </Layout>
  )
}

Index.getInitialProps = () => {
  return {
    blogCategory: 'ReactJS'
  }
}

Run the following commands and access your application at http://localhost:3000:

npm install && npm run dev

You should see the background turned dark and the text white 🥳

Let there be light

Let's change the Layout wrapper to allow us to switch between dark and light mode. Go to the components/Layout.js file and let's make some changes:

import React, { useState } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

// Default dark mode to true for now
const darkMode = true

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState(true)

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

If you refresh the page, you should see the same behaviour: dark background, white text.

Let's make a quick change to check if our Light theme is working as well, change our darkMode state from true to false:

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState(false)

  // ...
}

Refresh the page; you should see the original theme: white background, dark text 👌

The Switch

We'll be using the setDarkMode method from the previous code, we'll create a <button> with an onClick prop:

import React, { useState } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState(true)

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <div>
        <button onClick={() => setDarkMode(!darkMode)}>
          {darkMode ? 'Light it up!' : 'Turn the light off!'}
        </button>
      </div>
      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

So, what are we doing with onClick={() => setDarkMode(!darkMode)}, we're setting the darkMode state by using the ! (not) operator, which inverts the boolean expression:

let darkMode = true

darkMode = !darkMode // false
darkMode = !darkMode // true

---

Persistence

You should notice that every time you refresh your application and theme falls back to the light theme, well, let's deal with that.

For that we'll take advantage of localStorage and React Hooks useEffect method. Let's take a look:

import React, { useState, useEffect } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState(true)

  useEffect(() => {
    console.log(`Using dark mode? ${darkMode}`)
  }, [darkMode]) // Run every time `darkMode` changes

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <div>
        <button onClick={() => setDarkMode(!darkMode)}>
          {darkMode ? 'Light it up!' : 'Turn the light off!'}
        </button>
      </div>
      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

Open your developer console, every time you switch between light and dark mode, you should see logs such as:

Using dark mode? false
Using dark mode? true
Using dark mode? false

Great, that means useEffect is working as expected. Although, is you refresh the page it keeps falling back to the light theme.

Let's use localStorage to fix that:

import React, { useState, useEffect } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState(true)

  useEffect(() => {
    console.log(`Using dark mode? ${darkMode}`)
    localStorage.setItem('DARK_MODE', darkMode)
  }, [darkMode]) // Run every time `darkMode` changes

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <div>
        <button onClick={() => setDarkMode(!darkMode)}>
          {darkMode ? 'Light it up!' : 'Turn the light off!'}
        </button>
      </div>
      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

You can check if this is working as expected in the developer tools console, under Application → Local Storage → http://localhost:3000, you should see:

Key	Value
DARK_MODE	true or false

Once again, on refresh it's still falling back to the light theme, to fix that we need to make a small change, we need to get the value from localStorage and set as our default darkMode state:

import React, { useState, useEffect } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState()

  useEffect(() => {
    const darkModeValue = localStorage.getItem('DARK_MODE')
    // localStorage returns a string, not a boolean
    setDarkMode(darkModeValue === 'true')
  }, [])

  useEffect(() => {
    localStorage.setItem('DARK_MODE', darkMode)
  }, [darkMode]) // Run every time `darkMode` changes

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <div>
         <button onClick={() => setDarkMode(!darkMode)}>
          {darkMode ? 'Light it up!' : 'Turn the light off!'}
        </button>
      </div>

      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

We could have got the localStorage value before setting the state, but localStorage is still not available because it's not part of NextJS, so this is a nice work-around.

---

What the flash?

You might have noticed that when reloading your application while in dark mode, the page kind of flashes between light and dark mode. Well, we also can use useEffect as a good work-around to prevent that, let's set another state – mounted:

// ...

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState()
  const [mounted, setMounted] = useState(false)

  useEffect(() => {
    const darkModeValue = localStorage.getItem('DARK_MODE')
    // localStorage returns a string, not a boolean
    setDarkMode(darkModeValue === 'true')
    // Set mounted to `true` only after setting the `darkMode` state
    setMounted(true)
  }, [])

  // ...
}

Alright, now we can simply render an empty <div /> before the component has been mounted, this will prevent the component to render our styles, ThemeProvider etc before we set the darkMode state:

import React, { useState, useEffect } from 'react'
import { ThemeProvider } from 'styled-components'

import { GlobalStyles } from '../styles/global'
import { lightTheme, darkTheme } from '../styles/themes'

function Layout({ children }) {
  // Use the state to determine darkMode
  const [darkMode, setDarkMode] = useState()
  const [mounted, setMounted] = useState(false)

  useEffect(() => {
    const darkModeValue = localStorage.getItem('DARK_MODE')
    // localStorage returns a string, not a boolean
    setDarkMode(darkModeValue === 'true')
    // Set mounted to `true` only after setting the `darkMode` state
    setMounted(true)
  }, [])

  useEffect(() => {
    localStorage.setItem('DARK_MODE', darkMode)
  }, [darkMode]) // Run every time `darkMode` changes

  if (!mounted) return <div />

  return (
    // This will pass `theme` from `ThemeProvider` as a prop into `GlobalStyles`
    <ThemeProvider theme={darkMode ? darkTheme : lightTheme}>
      <div>
         <button onClick={() => setDarkMode(!darkMode)}>
          {darkMode ? 'Light it up!' : 'Turn the light off!'}
        </button>
      </div>

      <GlobalStyles />
      {children}
    </ThemeProvider>
  )
}

export default Layout

If you reload your application, the flash between light and dark mode should be gone.

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Source Code

You can find the source code of this tutorial here, don't forget it's under the branch dark-mode.

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