
The querySelectorAll, return the select items as Nodelist object. NodeList is not an Array, it is possible to iterate on it using forEach(). It can also be converted to an Array using Array.from(). (more)
Trigger the event

the e, is the mouse event, to get the actual 'source' element that clicked we use e.srcElement

to get the parent, we using the e.srcElement.parentNode

Put it all together
JavaScript:
//example of TR
[TR]
[TD] //first TD
[img]assets/cat_icon.png[/img]
[/TD]
[TD] //second TD
[url='list_topics.php?id=1']Health[/url]
[/TD]
[TD]2018-06-04[/TD]
[TD]
[B][/B]
[/TD]
[TD] //fourth TD
<a id="btn_edit" class="btn btn-primary btn-xs">Edit</a>
[/TD]
[/TR]

JavaScript:
var tbl_btns = document.querySelectorAll("#btn_edit");
tbl_btns.forEach(function(elem) {
elem.addEventListener("click", function(e) {
e.preventDefault();
var x = getSelected(e.srcElement);
console.log(x);
});
});
function getSelected(e){
//first TD
var id = e.parentNode.parentNode.cells[0].children[0].getAttribute('data-id');
//second TD
var forum_name = e.parentNode.parentNode.cells[1].children[0].text;
//fourth TD
var forum_private = e.parentNode.parentNode.cells[3].childElementCount;
return { id, forum_name, forum_private };
}
Optimize more!
JavaScript:
Array.prototype.forEach.call(document.querySelectorAll('#btn_edit'), function(el) {
el.addEventListener('click', function(e) {
e.preventDefault();
var x = getSelected(e.srcElement);
console.log(x);
})
})
function getSelected(e){
var tr = e.parentNode.parentNode;
var id = tr.cells[0].children[0].getAttribute('data-id');
var forum_name = tr.cells[1].children[0].text;
var forum_private = tr.cells[3].childElementCount;
return { id, forum_name, forum_private };
}
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