如何实现Javascript CustomEvent IE Polyfill

Question

我在理解如何实现IE CustomEvent Polyfill时遇到了问题。我在其他浏览器中使用CustomEvent但似乎无法使Polyfill工作。

(function () {

    // CustomEvent polyfill
    // https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#Polyfill
    if ( typeof window.CustomEvent === "function" ) return false; //If CustomEvent supported return

    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;
    }

    CustomEvent.prototype = window.Event.prototype;

    window.CustomEvent = CustomEvent;


})();

// CustomEvent supported
// From example: https://www.kirupa.com/html5/custom_events_js.htm

document.body.addEventListener("fontsLoaded", doSomething, false);

function doSomething(e) {
    alert("Event is called: " + e.type);
}

var myFontsLoaded = new CustomEvent("fontsLoaded");
document.body.dispatchEvent(myFontsLoaded);