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Looking at <http://software.hixie.ch/utilities/js/live-dom-viewer/saved/2202>, IE and Chrome allow calling all these without arguments; and Gecko allows calling prompt without arguments, while it throws for alert and confirm.
What's the point of calling these with no arguments? Is there a compat need here? I don't much care, but it does seem a bit wacked.
Presto also allows no argument (alerts ""). WebKit alerts "undefined" when omitting the argument. Looking at http://webdevdata.org/ data-dec2012, I see 10 instances of commented-out alert(); which I guess comes from alert-debugging. I didn't see any instances that were not commented-out.
Debugging alerts aren't a big deal since you're typically targeting a particular browser when debugging. But I guess if so many browsers do it... I've defaulted it to "". "undefined" seem unnecessarily ugly.
http://html5.org/tools/web-apps-tracker?from=7831&to=7832