If you run currentAPI.js you’ll see that there are quite a few variations of MyAvatar.startAnimation(), each with differing number of parameters. The most comprehensive is MyAvatar.startAnimation(QString,float,float,bool,bool,float,float,QStringList).
Looking in the C++ the full set of parameters with defaults is …
startAnimation(url, fps = 30.0f, priority = 1.0f, loop = false,
hold = false, firstFrame = 0.0f, lastFrame = FLT_MAX, maskedJoints = QStringList());
That last parameter is presumably an array of masked joint names.