JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" value="1" />Checkbox
<br/>
<br/>
<span id="result"></span>

JavaScript

window.onload = function () {
    var input = document.querySelector('input[type=checkbox]');
    
    function check() {
        if (input.checked) {
            var a = "1";
        } else {
            var a = "0";
        }
        document.getElementById('result').innerHTML = 'result ' + a;
    }
    input.onchange = check;
    check();
}