How do I get base CSS class for custom component?

When working with creating your own custom component how do you access the base CSS Class that is supplied to the component?

This is what I want to do:

this.masterUL = document.createElement("UL");

$(this.masterUL).addClass(this.CSSClass);

However the base CSS class, that is part of 90% of all components, it's the value circled in the attached image.

Thanks,

Pete