JavaScript needs to be interpreted and executed in order for it to do useful work. The functionality that does this in each web browser is the JavaScript engine: it understands the code written on the web page and executes it to provide the functionality that the developer expects.
The ServiceNow server-side platform uses the Rhino JavaScript engine, which is managed by the Mozilla Foundation, the maintainers of Firefox. Rhino itself is written in Java, which provides a hint of the backend platform code ServiceNow is written in. Rhino provides a full implementation of the language, with the exception of some objects that only make sense to the client. It's relatively complete and standard-compliant, though ServiceNow has made some changes to make scripting more forgiving (including not altering on missing if the function doesn't exist.) All versions of ServiceNow prior to Helsinki include a version of Rhino that supports up to version ECMAScript...