calling webservice inside js class

I have a javascript class built using prototype. Iside of one class methods i call a webservice like this:
Namespace.Class.prototype={
CallFunction:function(){
    Namespace.Service.ScriptMethod(params,this.SuccessMethod);
}
SuccessMethod:function(result){
}
}

SuccessMethod is also a member of that class. When i step into the SuccessMethod with debugger  it suddenly looks like a completely different object! I loose all members. I am not a javascript expert and i've been trying to deal with this issue for  several days now. Can anyone help me with this?

The webservice works fine, it returns results.

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