[jQuery] how to add a callback to my plugin?

hi there,
it may seems trivial, but i would like to know the proper way to add a
callback to a javascript function.
Is this correct?
function dummy(arg1,arg2,callback){
// do this, do that...
if (typeof callback == 'function'){
callback();
}
}
or a simple eval(callback) ? what's the best practice ?
Thanks for your input !
alex