JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="user-list" id="user_list">
<li class="user" select_u_id="1" onclick="show_message(1);"></li>
<li class="user" select_u_id="2" onclick="show_message(2);"></li>
<li class="user" select_u_id="3" onclick="show_message(3);"></li>
</ul>

JavaScript

var x=document.getElementsByClassName('user');
var b=new Array();
for(i=0;i<x.length;i++){
    
 var atribbute=x[i].getAttribute('select_u_id');
    
   b.push(atribbute);
    
    
    
}

alert(b);