How to get the attribute value of the clicked element in VUE JS?

Question

Vue.js

Hello. Here is the code https://jsfiddle.net/prL6axfL/11/
There is such a piece
methods: {
goIndex: function(){
alert( "hello index" );
}
}
I want to get the value of the data-index attribute on the div with the button class, these are actually switches. But it didn’t work out for me, I know there are specifics in vue. Please tell me how to get the value of the clicked attribute in my case...