JSFiddle - React, Tailwind, and code Playground

HTML

<ul><li><button class="testButton">Test button</button></li></ul>

JavaScript

function OnButtonDown (button) {
alert("The class work")
}

function Init () {
    var className = "testButton";
    var button = false;
    var buttons = document.getElementsByClassName(className);
    for (var i=0; i<buttons.length; i++) {
        button = buttons[i];
        
        if (button.addEventListener) {  // all browsers except IE before version 9
            button.addEventListener ("mousedown", function () {
                OnButtonDown (button)
                }, false);
            button.addEventListener ("mouseup", function () {
                OnButtonUp (button)
                }, false);
        }
        else {
            if (button.attachEvent) {   // IE before version 9
                button.attachEvent ("onmousedown", function () {
                    OnButtonDown (button)
                    });
                button.attachEvent ("onmouseup", function () {
                    OnButtonUp (button)
                    });
            }
        }
    }
}

Init();