Example demonstrating how to Promisify a Ajax/XHR request.

// Promisified XHR (ES6)
// Source: https://github.com/taniarascia/laconia/blob/master/public/js/scripts.js
const makeRequest = (method, url, data) => {
    return new Promise((resolve, reject) => {
        var request = new XMLHttpRequest();

        request.open(method, url);

        request.onload = function() {
            if (this.status >= 200 && this.status < 300) {
                resolve(request.response);
            } else {
                reject({
                    status: this.status,
                    statusText: request.statusText
                });
            }
        };

        request.onerror = function() {
            reject({
                status: this.status,
                statusText: request.statusText
            });
        };

        request.send(data);
    });
};

// ES5
// Promisified XHR
// Source: https://github.com/taniarascia/laconia/blob/master/public/js/scripts.js
// "use strict";
var makeRequest = function(method, url, data) {
    return new Promise(function(resolve, reject) {
        var request = new XMLHttpRequest();

        request.open(method, url);

        request.onload = function() {
            if (this.status >= 200 && this.status < 300) {
                resolve(request.response);
            } else {
                reject({
                    status: this.status,
                    statusText: request.statusText
                });
            }
        };

        request.onerror = function() {
            reject({
                status: this.status,
                statusText: request.statusText
            });
        };

        request.send(data);
    });
};

//
// Example Usage:

makeRequest("POST", url, formData)
    .then(data => {
        switch (formId) {
            case "form-register":
            case "form-login":
                if (data !== "Proceed") {
                    showMessage(data);
                } else {
                    window.location.href = "/dashboard";
                }
                break;
            case "form-forgot-password":
            case "form-create-password":
            case "form-settings":
            case "form-create-list":
            case "form-edit-list":
            case "form-delete-user":
                showMessage(data);
                // Remove password form on create-password
                if (data === "Your password has been updated") {
                    thisForm.remove();
                }
                // Clear inputs on create form
                if (data === "List successfully created") {
                    thisForm.reset();
                    title.focus();
                }
                // Clear inputs on create form
                if (data === "User deleted") {
                    window.location.href = "/";
                }
                break;
            case "form-comments":
                if (
                    data === "Nice try" ||
                    data === "This is a duplicate comment"
                ) {
                    showMessage(data);
                } else {
                    while (thisForm.firstChild)
                        thisForm.removeChild(thisForm.firstChild);
                    const message = document.createElement("div");
                    message.classList.add("message");
                    message.textContent = "Your comment has been posted!";
                    message.style.display = "block";
                    thisForm.appendChild(message);

                    // Display comment
                    const arr = JSON.parse(data);

                    const uname = document.createElement("a");
                    uname.href = `${window.location.href + arr[0]}`;
                    uname.textContent = arr[0];

                    const ucomment = document.createElement("p");
                    ucomment.textContent = arr[1];

                    const commentContainer = document.createElement("div");
                    commentContainer.classList.add("comments");
                    commentContainer.appendChild(uname);
                    commentContainer.appendChild(ucomment);

                    comments.appendChild(commentContainer);
                }
                break;
            default:
                showMessage(data);
                // Remove list item on front end view for list item deletion
                if (data === "List deleted") {
                    const thisList = thisForm.closest(".items");
                    thisList.remove();
                }
        }
    })
    .catch(error => {
        console.error("There was an error!", error.statusText);
    });