Double colons in function declaration in Javascript?

Answer 1

Thanks for everyone who looked into this.

Anyway, just as one could expect, not after five minutes after submitting the question, I found How to handle an ActiveX event in Javascript question about ActiveX callback handlers in IE, so it answers the main part of the question. As well, as the question that @outis has linked.

But I can't find any documentation on MSDN and that's troubling.

Conclusion: this is IE-specific ECMAScript extension for event handler declaration.