When I checked my work I see undefined in my work. It’s not supposed to be there. Can anyone help?

jQuery
$(document).ready(function () {
var listItems = $("#productList li");
listItems.each(function (idx, li) {
var product = $(li);
product.append(" - <b> ("+product.attr('data-type')+") </b>");
});
});
HTML
<ul id="productList">
<li data-activity-type="family">Snow Tubbing</li>
<li data-activity-type="family">Snowcat Mountain Tour</li>
<li data-activity-type="adventure">SnowSlam 11 Racing Course</li>
<li data-activity-type="adventure">Snowboarding</li>
<li data-activity-type="adventure">Edge 11 Performance Ski Racing</li>
<li data-activity-type="family">Dog Sled Tour</li>
<li data-activity-type="family">Snowshoeing</li>
<li data-activity-type="kids">Kid's Night Out</li>
<li data-activity-type="kids">Bear Cub Kamp at the Nursery</li>
</ul>
>Solution :
I’m assuming the activity-type value is going to append to innerHTML.
the data-type should be data-activity-type.
$(document).ready(function () {
var listItems = $("#productList li");
listItems.each((index, item) => {
item = $(item);
item.html( item.html() + `- <b>${ item.attr('data-activity-type') }</b>` );
});
});