Using J2V8 on Android, how should I get the function handle of JS

Question

I try to use optype.js in android, there is a function like this:

opentype.Font.prototype.stringToGlyphIndexes = function (s, options) {
    ....
    return bidi.getTextGlyphs(s);
};

i don't know how to call it in Java.

I try to call it using executeJSFunction function like this:

 v8.executeJSFunction("stringToGlyphIndexes", "test");

but Logcat gives an error message of TypeError: undefined is not a function. How should I call this JS function？