Qt Script is a separate module, designed with scripting applications in mind. It has a mature and rich API that permits a really deep integration of scripting functionality. It allows evaluating and debugging of scripts, and advanced use of objects and functions. It also gives access to a really low level ECMAScript engine API. Full documentation about using Qt Script to make an application scriptable can be found here.
This is a simple API, very similar to Qt Script, but limited to basic functionality. The main classes are QJSEngine and QJSValue, which can be used to embed pure ECMAScript functionality like evaluating scripts and calling functions.
All of these three scripting solutions seamlessly inter-operate with the Meta-Object System, which means that all signals and slots and properties of a QObject instance are accessible in an ECMAScript environment.