refactor: extract some utility code

Code splitting for better testing and mantainability

rollup.config.js

@@ -2,14 +2,11 @@ import { eslint } from 'rollup-plugin-eslint'
 import { terser } from 'rollup-plugin-terser'
 import pkg from './package.json'
 
-const isDev = process.env.BUILD === 'development'
-
 let external = Object.keys(pkg.dependencies)
 let plugins = [eslint()]
 
-if (!isDev) {
-	plugins.push(terser())
-}
+const isDev = process.env.BUILD === 'development'
+if (!isDev) plugins.push(terser())
 
 export default {
 	input: 'src/focus-within.js',

src/utils/supportsFocusWithin.js

@@ -0,0 +1,16 @@
+/**
+ * Check for :focus-within pseudo class support
+ * @returns {Boolean}
+ */
+function supportsFocusWithin () {
+	try {
+		document.querySelector(':focus-within')
+		console.info('focus-within-polyfill: focus-within supported')
+		return true
+	} catch (error) {
+		console.info('focus-within-polyfill: focus-within not supported')
+		return false
+	}
+}
+
+export default supportsFocusWithin