Promises work like that

promises.js

function change()
{
    if (window.promise_test) {
        window.promise_test.then(() => console.log('Second message'));
    } else {
        window.promise_test = new Promise(
            function (resolve, reject) {
                console.log('Show first message');
                setTimeout(
                    () => {
                        console.log('Resolved');
                        window.promise_test = null;
                        resolve();
                    },
                    2000
                );
            }
        );
    }
}