In javascript, I would like to declare the variable name that contains the index of the repeat statement.

Currently, the function I implemented is implemented as shown in the image below.

Here's the function.

To implement this functionality

I used jequery+js code

(function(){
        $(document).ready(function(){
        // Gets the index of the last book item.
        //(If there are 3 books, the value of 3 is stored in $book_list_num).
        $book_list_num = $("div").data("listNum");

        for(i=1; i<= parseInt($book_list_num); i++){
           //What do you want to know?
            var btni = $("#btn"+i)
            console.log(btni)
        }
    });

})();

I wanted to make a button variable as many as the number of buttons generated in html to hold the button and use the onclick method to apply the effect, so I used the repetition statement in the repetition sentence. I'm not sure how to hold it with button+index.

Please give me some advice.