added skip for ssr and non-browser environments

src/index.ts

@@ -1,10 +1,12 @@
 import { polyfill } from './polyfill.js';
 
 // apply polyfill
-if (document.readyState !== 'complete') {
-  window.addEventListener('load', () => {
+if (typeof window !== 'undefined') {
+  if (document.readyState !== 'complete') {
+    window.addEventListener('load', () => {
+      polyfill();
+    });
+  } else {
     polyfill();
-  });
-} else {
-  polyfill();
+  }
 }