Add Check Media Queries From JavaScript as a JavaScript TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1462 TILs and counting..._
+_1463 TILs and counting..._
 
 ---
 
@@ -449,6 +449,7 @@ _1462 TILs and counting..._
 - [Check If A Number Is Positive Or Negative](javascript/check-if-a-number-is-positive-or-negative.md)
 - [Check If File Exists Before Reading It](javascript/check-if-file-exists-before-reading-it.md)
 - [Check If Something Is An Array](javascript/check-if-something-is-an-array.md)
+- [Check Media Queries From JavaScript](javascript/check-media-queries-from-javascript.md)
 - [Check The Password Confirmation With Yup](javascript/check-the-password-confirmation-with-yup.md)
 - [Compare The Equality Of Two Date Objects](javascript/compare-the-equality-of-two-date-objects.md)
 - [Computed Property Names In ES6](javascript/computed-property-names-in-es6.md)

javascript/check-media-queries-from-javascript.md

@@ -0,0 +1,28 @@
+# Check Media Queries From JavaScript
+
+I'm usually thinking about and [using media
+queries](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries)
+from a CSS context. I use them to control what styles are displayed for a
+variety of scenarios, such as at different screen widths, when a user prefers
+reduced motion, or when the user prefers a dark color scheme.
+
+The current value of various media queries can be checked from a JavaScript
+context as well.
+
+For instance, if we want to see if the user prefers a _dark_ color schema, we
+can look for a _match_ on that media query with
+[`matchMedia`](https://developer.mozilla.org/en-US/docs/Web/API/Window/matchMedia).
+
+```javascript
+> window.matchMedia('(prefers-color-scheme: dark)')
+MediaQueryList {media: '(prefers-color-scheme: dark)', matches: true, onchange: null}
+> window.matchMedia('(prefers-color-scheme: dark)')['matches']
+true
+```
+
+This queries for the [`prefers-color-scheme` media
+feature](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme).
+
+The [Astro.build Blog
+Tutorial](https://docs.astro.build/en/tutorial/6-islands/2/#add-client-side-interactivity)
+shows an example of using this to wire up a Light/Dark mode toggle.