As no-one seems to have said it yet:
The test is needed inside a JavaScript function so a conditional comment doesn't seem suitable.
You can easily put a conditional comment — a JScript conditional comment, not an HTML one — inside a function:
function something() {
var IE_WIN= false;
var IE_WIN_7PLUS= false;
/*@cc_on
@if (@_win32)
IE_WIN= true;
@if (@_jscript_version>=5.7)
IE_WIN_7PLUS = true;
@end
@end @*/
...
}
It's more typical to do the test once at global level though, and just check the stored flags thereafter.
CCs are more reliable than sifting through the mess that the User-Agent string has become these days. String matching methods on navigator.userAgent can misidentify spoofing browsers such as Opera.
Of course capability sniffing is much better for cross-browser code where it's possible, but for some cases — usually bug fix workarounds — you do need to identify IE specifically, and CCs are probably the best way to do that today.