Add SSR support

resolved #6

Author: evenchange4

index.js

@@ -13,10 +13,15 @@ try {
   window.removeEventListener('testPassive', null, opts)
 } catch (e) {}
 
-let getPosition = () => ({
-  x: window.pageXOffset,
-  y: window.pageYOffset,
-})
+let getPosition = () => {
+  if (typeof window === 'undefined') {
+    return { x: 0, y: 0 };
+  }
+  return {
+    x: window.pageXOffset,
+    y: window.pageYOffset,
+  };
+};
 
 let defaultOptions = {
   throttle: 100,