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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: custom event in IE
Date: Tue, 10 Jan 2017 12:53:37 +0100
Organization: PointedEars Software (PES)
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Andrew Poulos wrote:

> I used a try/catch and when it fails for IE the "catch" part uses
> initCustomEvent
>    <https://msdn.microsoft.com/en-us/library/ff975987(v=vs.85).aspx>
> to create the event:
> 
> function dispatch(elem, theEvent) {
>    try {
>      elem.dispatchEvent(new CustomEvent(theEvent));
>    } catch(err) {
>      var evt = document.createEvent("CustomEvent");
>      evt.initCustomEvent(theEvent, true, false, {});
>      elem.dispatchEvent(evt);
>    }
> }

Because not only “new CustomEvent(theEvent)” may throw an exception, you 
should not simply handle all exceptions caused by this statement with the 
catch code, but only the specific one.  Since there is no compatible way to 
catch only a specific type of exception (AFAIK only Mozilla JavaScript has a 
way, see the ES Matrix), you should rethrow the exception if it is not the 
one that you want to handle.  For example:

  try {
    elem.dispatchEvent(new CustomEvent(theEvent));
  } catch (err) {
    if (!(err instanceof TypeError)) throw err;

    var evt = document.createEvent("CustomEvent");
    evt.initCustomEvent(theEvent, true, false, {});
    elem.dispatchEvent(evt);
  }

In JScript the “number” property of the Error instance referred by “err” 
could also help to tell apart this exception from other TypeError 
exceptions:

<https://msdn.microsoft.com/en-us/library/hc53e755(v=vs.94).aspx>
<https://msdn.microsoft.com/en-us/library/1dk3k160(v=vs.94).aspx>

But note that a TypeError exception for “new CustomEvent(theEvent)” is not 
thrown only in IE/MSHTML, but also in all user agents that do not implement 
CustomEvent which was introduced in W3C DOM 4 and the WHATWG DOM:

<https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/initCustomEvent> f.
<http://www.w3.org/TR/2015/REC-dom-20151119/#interface-customevent>


Since the first argument to the CustomEvent constructor and the 
initCustomEvent() method is specified to be a DOMString designating the type 
of the event, you should name your formal parameter something like 
“eventType”; not “theEvent” which conveys the misconception that it would be 
a reference to an event object.

-- 
PointedEars
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