JSFiddle - React, Tailwind, and code Playground
HTML
<input id="filterconfphone" type="checkbox" onchange="filterconfphone_click()">
JavaScript
function filterconfphone_click() {
alert (document.getElementById('filterconfphone').checked);
//var checkbox = document.getElementById('filterconfphone');
//if (checkbox.checked != true)
//{
// alert(checkbox.checked);
//}
}