JSFiddle - React, Tailwind, and code Playground

HTML

<button type="button" onclick="changes = true">Add</button>

JavaScript

var changes = false;

window.onbeforeunload = function() {
    if (changes) {
        $.ajax({
            async: false,
            url: "/echo/json/",
            data: {
                undovideokey: 'foo'
            }
        });
    } else {
        return null;
    };
};