JavaScript custom plug-in to implement tab switching function

<script src="jquery-3.1.0.js"></script>
    <script src="plugs/demo01.js"></script>
    <style>
        #tabs>div{
            height: 200px;
            width: 200px;
            background-color: pink;
            display: none;
        }
        #tabs div.div-active{
            display: block;
        }
        .btn-active{
            background-color: orange;
        }
</style>

(function ($) {
 //tabs plugin $.fn.tabs = function (options) {
        let defaults = {
            activeIndex:1,
            titleActive:"btn-active",
            contentActive:"div-active",
            attr:"rel"
        }
        /*Merge parameters*/
        $.extend(defaults,options);
        /*Get all buttons*/
        let btns = this.find("["+defaults.attr+"]");
        /*Get the value in rel*/
        let rels = [];
        btns.each(function (index,element) {
            rels.push($(element).attr(defaults.attr));
        });
        /*Get all divs*/
        let divs = this.find(rels.toString());
        /* Check if the specified subscript is out of bounds*/
        if (defaults.activeIndex > btns.length-1) {
            defaults.activeIndex = 0;
        }
        /*Set the default display content*/
        btns.eq(defaults.activeIndex).addClass(defaults.titleActive);
        divs.eq(defaults.activeIndex).addClass(defaults.contentActive);
        /*Bind click event to the button*/
        btns.click(function () {
            $(this).addClass(defaults.titleActive)
                .siblings().removeClass(defaults.titleActive);
            divs.eq($(this).index()).addClass(defaults.contentActive)
                .siblings().removeClass(defaults.contentActive);
        });
    }
})(jQuery);