JavaScript Error while calling function by JSNI

Question

Exception caught: (TypeError):((intermediate value)(intermediate value),c).B is not a function

the JavaScript function like this:

A.prototype = {
    B: function() {
    },
    C: function() {
    }
};

the java function like this:

public final native void test()/*-{
    return this.B();
}-*/;

In my case, I can't edit the JavaScript file. So why this issue happend?