@vince_57
The jscript/interface api files exist only to provide auto-complete functionality when typing in the configuration window. You should ignore them.

As for javascript language support, yes we are stuck several generations behind what current javascript developers are working with. We are stuck with JScript 5.8/Javascript 1.5. This is basically IE8 levels of support. While the JScript9 option in the configuration window can provide slightly faster performance, there are no language improvements or changes.
While we don't have functions like alert or console.log, we do have alternatives. We can output message to the foobar2000 Console using fb.trace(message);. If you really wanted to, you can use this little snippet to have console.log available...
var console = {
log: function (msg) {
fb.trace(msg);
}
};
console.log("Hello world!");
console.log(2 * 5);
You can display popup windows using this bit of code...
fb.ShowPopupMessage("hello");
//code here will execute immediately even with the popup window still open
If you want the behaviour to more like alert, you can do this:
var WshShell = new ActiveXObject("WScript.Shell");
WshShell.popup("hello", 0); //0 is the code for displaying just an OK button
//code here won't execute until the window is dismissed.
There is no problem with making web requests and working with JSON. I do this a fair bit myself such as using the musicbrainz API like this...
http://i.imgur.com/pu2wL2W.png
Of course when it comes to displaying text in the panel itself, this is only possible using the gdiDrawText or DrawString functions which are C++ but we can pass text, fonts, colours, etc via our script.
Do you plan to bring that function back?
No.