javascript 43 lines · 8 steps

Building a dark-mode toggle that respects the OS

A theme switcher that honors the system preference until the user explicitly overrides it, then remembers that choice.

Explained by highlit
Intermediate dark-mode localstorage matchmedia accessibility event-handling
1const STORAGE_KEY = "theme-preference";
2 
3function getSystemTheme() {
4 return window.matchMedia("(prefers-color-scheme: dark)").matches
5 ? "dark"
6 : "light";
7}
8 
9function getStoredTheme() {
10 return localStorage.getItem(STORAGE_KEY);
11}
12 
13function applyTheme(theme) {
14 document.documentElement.dataset.theme = theme;
15 const toggle = document.querySelector("[data-theme-toggle]");
16 if (toggle) {
17 toggle.setAttribute("aria-pressed", String(theme === "dark"));
18 }
19}
20 
21function resolveTheme() {
22 return getStoredTheme() ?? getSystemTheme();
23}
24 
25function initThemeToggle() {
26 applyTheme(resolveTheme());
27 
28 const systemQuery = window.matchMedia("(prefers-color-scheme: dark)");
29 systemQuery.addEventListener("change", (event) => {
30 if (getStoredTheme() === null) {
31 applyTheme(event.matches ? "dark" : "light");
32 }
33 });
34 
35 const toggle = document.querySelector("[data-theme-toggle]");
36 toggle?.addEventListener("click", () => {
37 const next = resolveTheme() === "dark" ? "light" : "dark";
38 localStorage.setItem(STORAGE_KEY, next);
39 applyTheme(next);
40 });
41}
42 
43document.addEventListener("DOMContentLoaded", initThemeToggle);
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Fall back to the OS preference only until the user makes an explicit choice, then persist it.
  2. 2Nullish coalescing cleanly expresses 'stored value wins, otherwise derive a default'.
  3. 3Keeping DOM state and ARIA attributes in one apply function keeps visuals and accessibility in sync.

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