JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="todo_list"></ul>
<input type="button" value="Add Todo" id="add_todo"/>

JavaScript

document.querySelector("#add_todo").onclick = function(){
   var count = document.querySelector("#todo_list").children.length;
   var li = document.createElement("li");
    li.innerHTML = "Todo " + count;
   document.querySelector("#todo_list").appendChild(li);
}