Get writer index from callback

[fxpar]

Hi, I have multiple writers (looping on all characters in an expression). How to get the index of the current writer when there is a mistake?

writer[i].quiz({
    onMistake: function(strokeData) {
            console.log('currentWriterIndex:' + ???);
    },

I did get something with writer.findIndex(({_char}) => _char === strokeData.character); but it won't work with expressions with repetitive characters like 哥哥 or 弟弟.

Thanks for your help.