JS script to send data to Hyper server

send_data.js

window.onload = function() {
    let button = document.getElementById("submit-button");
    console.log(button);

    button.addEventListener("click", function(event){
        event.preventDefault(); 
        send_data("http://localhost:3000")
        }, false);

    function send_data(url) {
        let username = document.getElementById("username").value;
        let password = document.getElementById("password").value;
        let json = JSON.stringify({"username": username, "password": password});
        let sendData = new Promise((resolve, reject) => {
            const xhr = new XMLHttpRequest();
            xhr.open("POST", url);
            xhr.setRequestHeader("Content-type", "application/json;charset=utf-8");
            xhr.onload = () => resolve(xhr.responseText);
            xhr.onerror = () => reject(xhr.statusText);
            xhr.send(json);
        });

        sendData.then((successMessage) => {
        console.log(successMessage);
    });
    }
}