Few days back I was wondering how to know if the system theme is dark or light in web using JavaScript and I found a way to detect that.

In this article, I will share the implementation that I used to check the system theme.

• We will use the method matchMedia() provided by the browser's window interface.

• matchMedia method expects a mediaQueryString as a parameter and it will return a MediaQueryList object.

• MediaQueryList object will have a property called matches of Boolean data type.

Code

if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
    // Dark Mode
}

prefers-color-scheme is a CSS media feature used to detect the system color theme.

We can also have a listener, if user changes the theme in-between.

window
   .matchMedia("(prefers-color-scheme: dark)")
   .addEventListener("change", (event) => {
       const theme = event.matches ? "dark" : "light";
   });

Let me know in the comments if you are aware of any other way to detect the system theme in web.

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