zk - Declaration of widget class in ZK

Question

I am getting cls undefined in register() function.

var cls = zk.$import(clsnm);
cls.prototype.className = clsnm;

The documentation says

Notice that the class must be declared before calling this method. In other words, zk.$import(clsnm) must return the class of the specified class name.

Where do I need to delcare class class name ?

Link of register function : Link