Qt has three main ways to help make an application scriptable. All of them allow easy integration of the ECMAScript (more widely known as JavaScript) language into the application. Depending on how deep the integration should be, one of these APIs can be used:

Qt Script Module

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.

JS API

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.

QML

QML exposes a modified ECMAScript environment. It is designed to mix well with the JavaScript API mentioned earlier. QML may be used without Qt Quick components, which may be useful for server side scripting. With QML it is possible to mix pure ECMAScript and declarative components (Use Case - Integrating JavaScript in QML).

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.