sync.js

    $.fn.keepInSync = function($targets) {
        // join together all the elements you want to keep in sync
        var $els = $targets.add(this);
        $els.on("keyup change", function() {
            var $this = $(this);
            // exclude the current element since it already has the value
            $els.not($this).val($this.val());
        });
        return this;
    };

    var sync = document.getElementsByClassName("sync");
    for(var i = 0; i < sync.length; i++)
    {
        select = ('.' + Array.from(sync.item(i).classList).join('.'));
        $(select).keepInSync($(select));
    }