JSON is part of it, but it's really access to everything available in modern javascript. Looking at
http://en.wikipedia.org/wiki/JScript, I believe that TestComplete is using a version earlier than 5.8 (JSON was introduced in version 5.8). The version of JScript we'd really like to use is 9.0, which was introduced back in March of 2011 with IE9, and includes things like Array extras, array comprehensions, and expression closures. See
http://en.wikipedia.org/wiki/ECMAScript#Version_correspondence for javascript versions - upgrading from the old version of JScript to version 9.0 would gain us everything introduced in javascript versions 1.6 through 1.8.