Simplified the usage by self invoking the function when it isrequired…

… or imported

CustomEvent.js

@@ -1,18 +1,15 @@
-function CustomEvent ( event, params ) {
-  params = params || {
-    bubbles: false,
-    cancelable: false,
-    detail: undefined,
-  };
-  var evt = document.createEvent( 'CustomEvent' );
-  evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
-  return evt;
-}
+(function() {
+  if (typeof window === "undefined") return false;
+  if (typeof window.CustomEvent === "function") return false;
+
+  function CustomEvent ( event, params ) {
+      params = params || { bubbles: false, cancelable: false, detail: undefined };
+      var evt = document.createEvent( 'CustomEvent' );
+      evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
+      return evt;
+  }
 
-module.exports = function ( undefined ) {
-  if ( typeof window.CustomEvent === 'function' ) {
-    return;
-  };
   CustomEvent.prototype = window.Event.prototype;
+
   window.CustomEvent = CustomEvent;
-};
+})();

README.md

@@ -13,11 +13,15 @@ npm install --save-dev jspolyfill-custom-event
 # Usage
 
 ```js
-require( 'jspolyfill-custom-event' )();
+require('jspolyfill-custom-event');
+```
+
+or
+
+```js
+import('jspolyfill-custom-event');
 ```
 
-**Note**: Make sure to add the extra set of parenthesis at the end to fire the `function` that
-registers the creation of the polyfill.
 
 # Syntax