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Double Exclamation Marks

steva
steva asked
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Last Modified: 2013-01-04
I came across the line below is some code I was looking at.  Does anyone know what the double exclamation marks mean?

var w3cEventModel = !!document.addEventListener;


Thanks
Steve
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leakim971Multitechnician
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Distinguished Expert 2019

Commented:
same as :
var notW3CEventModel = !document.addEventListener;
var w3cEventModel = !notW3CEventModel;

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!null return return true
!!null return false

check this page : https://developer.mozilla.org/fr/docs/DOM/element.addEventListener
look for Internet Explorer section

Author

Commented:
But why?   How is

              (1)        var w3cEventModel = document.addEventListener;

different than

              (2)        var w3cEventModel = !!document.addEventListener;

In either case w3cEventModel will get an address if document.addEventListener exists and something that fails an if-clause (undefined or NULL or false) if it doesn't exist.

The code farther down is:

	// Use W3C method
	if (w3cEventModel) {
		addEvent(win, 'DOMContentLoaded', readyHandler);
	} else {
		// Use IE method
		addEvent(doc, "readystatechange", function() {
			if (doc.readyState === "complete") {
				removeEvent(doc, "readystatechange", arguments.callee);
				readyHandler();
			}
		});

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Wouldn't  w3cEventModel = document.addEventListener  work here?

Thanks
Steve
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Commented:
This one is on us!
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Author

Commented:
" More readable" - except for the !!  part.

Ok, I got it.

Thanks.
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