Incorrect syntax in "Code-Behind Implementation Resource" documentation

On http://doc.qt.io/qt-5/qtqml-javascript-resources.html#code-behind-implementation-resource, there is a snippet that has the following code:

property var clickCount = 0;   // this state is separate for each instance of MyButton

I've never seen this syntax before; if it's actually some kind of extension to JS syntax, it should be documented.