Dark mode toggle with JavaScript in Tailwind - Performance & Optimization

Performance: Dark mode toggle with JavaScript

MEDIUM IMPACT

This affects page interactivity and visual stability when toggling themes.

Implementing a dark mode toggle that switches the page theme

Tailwind

const toggleDarkMode = () => {
  document.documentElement.classList.toggle('dark');
};

button.addEventListener('click', toggleDarkMode);

Toggling a single class leverages CSS for style changes, causing only one reflow and repaint, improving responsiveness and visual stability.

📈 Performance GainSingle reflow and repaint, reduces layout thrashing, improves CLS and INP metrics.

Implementing a dark mode toggle that switches the page theme

Tailwind

const toggleDarkMode = () => {
  document.body.style.backgroundColor = '#000';
  document.body.style.color = '#fff';
};

button.addEventListener('click', toggleDarkMode);

Directly changing many inline styles triggers multiple reflows and repaints, causing layout thrashing and slower interaction.

📉 Performance CostTriggers multiple reflows and repaints per style change, blocking rendering for 50-100ms on complex pages.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Inline style changes per property	Multiple style updates on body	Multiple reflows triggered	High paint cost due to many changes	[X] Bad
Toggle single CSS class on root	One class toggle on <html>	Single reflow triggered	Minimal paint cost	[OK] Good

Rendering Pipeline

When toggling dark mode by adding/removing a class, the browser recalculates styles, performs layout if needed, repaints affected areas, and composites the final pixels.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout and Paint stages are most expensive if many inline styles change individually.

Core Web Vital Affected

CLS

This affects page interactivity and visual stability when toggling themes.

Optimization Tips

1Toggle a single CSS class on the root element to switch themes.

2Avoid changing multiple inline styles individually to prevent layout thrashing.

3Use DevTools Performance tab to monitor style recalculations and layout events.

Performance Quiz - 3 Questions

What is the main performance benefit of toggling a CSS class for dark mode instead of changing inline styles directly?

A It triggers fewer reflows and repaints

B It increases the bundle size

C It blocks the main thread longer

D It causes more layout thrashing

DevTools: Performance

How to check: Open DevTools > Performance tab. Record while toggling dark mode. Look for style recalculation, layout, and paint events.

What to look for: Check if style recalculation and layout happen once per toggle. Multiple layout events indicate inefficient style changes.

Practice

1. What does adding the dark class to the html element do in Tailwind CSS?

easy

A. It disables all CSS styles on the page.

B. It enables dark mode styles throughout the website.

C. It changes the font size to larger values.

D. It hides all images on the page.

Solution

1. Step 1: Understand Tailwind dark mode mechanism

   Tailwind applies dark mode styles when the dark class is present on the html element.

2. Step 2: Effect of the dark class

   Adding dark triggers alternate color schemes and styles designed for dark backgrounds.

3. Final Answer:

   It enables dark mode styles throughout the website. -> Option B

4. Quick Check:

   dark class = enables dark mode [OK]

Hint: Remember: dark class on html toggles dark mode styles [OK]

Common Mistakes:

• Thinking dark class disables styles
• Confusing dark class with font size changes
• Assuming dark class hides images

2. Which JavaScript code correctly toggles the dark mode class on the html element when a button is clicked?

easy

A. document.documentElement.classList.toggle('dark');

B. document.body.classList.add('dark');

C. document.getElementById('dark').classList.remove('dark');

D. document.querySelector('html').style.dark = true;

Solution

1. Step 1: Identify the root element for dark mode

   Dark mode toggles the dark class on the html element, accessible as document.documentElement.

2. Step 2: Use correct method to toggle class

   The classList.toggle('dark') method adds or removes the class on each click.

3. Final Answer:

   document.documentElement.classList.toggle('dark'); -> Option A

4. Quick Check:

   Toggle dark class on html element = document.documentElement.classList.toggle('dark'); [OK]

Hint: Use document.documentElement.classList.toggle('dark') to toggle dark mode [OK]

Common Mistakes:

• Using document.body instead of document.documentElement
• Using add or remove instead of toggle
• Trying to set style.dark property which doesn't exist

3. What will be the visible background color of this page after clicking the toggle button once?

<html class="dark">
  <body class="bg-white dark:bg-gray-900">
    <button id="toggle">Toggle Dark Mode</button>
  </body>
</html>

<script>
  document.getElementById('toggle').addEventListener('click', () => {
    document.documentElement.classList.toggle('dark');
  });
</script>

medium

A. Gray background (#1a202c)

B. No background color

C. Black background

D. White background

Solution

1. Step 1: Initial state has dark class on html

   The page starts with dark class, so background is gray (#1a202c) from dark:bg-gray-900.

2. Step 2: Clicking toggle removes dark class

   Toggle removes dark class, so bg-white applies, making background white.

3. Final Answer:

   White background -> Option D

4. Quick Check:

   Toggle removes dark class, background = white [OK]

Hint: Toggle flips dark class, switching background colors [OK]

Common Mistakes:

• Assuming background stays gray after toggle
• Confusing initial class presence
• Ignoring that toggle removes dark class

4. This JavaScript code is meant to toggle dark mode but does not work. What is the error?

const btn = document.getElementById('toggle');
btn.onclick = function() {
  document.querySelector('html').classList.add('dark');
  document.querySelector('html').classList.remove('dark');
};

medium

A. The class name should be 'dark-mode', not 'dark'.

B. The selector 'html' is incorrect; should be 'body'.

C. It adds and removes 'dark' class immediately, so no visible change.

D. The event listener should be 'addEventListener', not 'onclick'.

Solution

1. Step 1: Analyze the toggle logic

   The code adds then immediately removes the dark class on the same element in one function call.

2. Step 2: Effect on the page

   Because add and remove happen instantly, the class never stays applied, so no dark mode effect is visible.

3. Final Answer:

   It adds and removes 'dark' class immediately, so no visible change. -> Option C

4. Quick Check:

   Immediate add and remove cancels out [OK]

Hint: Toggle class once; don't add and remove immediately [OK]

Common Mistakes:

• Thinking 'onclick' is invalid
• Using wrong selector for dark mode
• Confusing class name with 'dark-mode'

5. You want to save the user's dark mode preference so it stays after page reload. Which JavaScript approach correctly implements this with Tailwind CSS?

1. On toggle, add/remove 'dark' class on <html>.
2. Save preference in localStorage.
3. On page load, read preference and set 'dark' class accordingly.

Which code snippet correctly sets dark mode on page load?

hard

A. if(localStorage.getItem('theme') === 'dark') { document.documentElement.classList.add('dark'); } else { document.documentElement.classList.remove('dark'); }

B. document.documentElement.classList.toggle(localStorage.getItem('theme'));

C. localStorage.setItem('theme', document.documentElement.classList.contains('dark'));

D. document.body.classList.add(localStorage.getItem('theme'));

Solution

1. Step 1: Retrieve saved theme from localStorage

   Use localStorage.getItem('theme') to get saved preference.

2. Step 2: Apply or remove 'dark' class based on preference

   If preference is 'dark', add dark class to html, else remove it.

3. Final Answer:

   if(localStorage.getItem('theme') === 'dark') { document.documentElement.classList.add('dark'); } else { document.documentElement.classList.remove('dark'); } -> Option A

4. Quick Check:

   Check saved theme and add/remove class accordingly [OK]

Hint: Check localStorage theme and add/remove 'dark' class on html [OK]

Common Mistakes:

• Using toggle with localStorage value directly
• Saving boolean instead of string in localStorage
• Adding class to body instead of html