changed function implementation

AJAX.js

/**
 * @namespace AJAXHelper
 */
var AJAXHelper = (() => {
	const HTTP_STATUS_OK = 200; // status 200 is a successful return.
	const XHR_READY_STATE_DONE = 4; // readyState 4 means the request is done.
	
	/**
	 * Makes a POST request to the server and returns any result to the caller.
	 * @param {!string} url POST URL
	 * @param {!object} data map of values like <br><code>{"name": "value", "foo": "bar"}</code>
	 * @param {Function} callback callback function that returns true or false plus an eventual message from the server
	 * @memberof AJAXHelper
	 */
	function postData(url, data, callback, async = true) {
		let xhr = new XMLHttpRequest();
		xhr.open('POST', url, async);
		xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

		let dataStr = getAJAXString(data);

		xhr.onload = () => {
			if (xhr.readyState === XHR_READY_STATE_DONE) {
				if (xhr.status === HTTP_STATUS_OK)
					callback(true, xhr.responseText);
				else
					callback(false, xhr.responseText);
			}
			else
				callback(false, null);
		};
		xhr.send(dataStr);
	}

	/**
	 * Makes a GET request to the server and returns the result to the caller.
	 * @param {string} url GET URL
	 * @param {object} data map of values like <br><code>{"name": "value", "foo": "bar"}</code>
	 * @param {Function} callback function that returns an object value from the JSON result of the GET function
	 * @memberof AJAXHelper
	 */
	function getData(url, data, callback, async = true) {
		const GET_PREFIX = "?";

		let dataStr = getAJAXString(data);

		let getURL = url + GET_PREFIX + dataStr;

		let xhr = new XMLHttpRequest();
		xhr.open('GET', getURL, async);
		xhr.send(null);

		xhr.onload = () => {
			if (xhr.readyState === XHR_READY_STATE_DONE) {
				if (xhr.status === HTTP_STATUS_OK) {
					let jsonData = parseJSON(xhr.responseText);
					callback(jsonData);
				}
				else
					console.error("Generic error: " + xhr.status);
			}
		};
	}
	
	/**
	 * Makes a GET request to the server and returns the result to the caller.
	 * @param {string} url GET URL
	 * @param {object} data map of values like <br><code>{"name": "value", "foo": "bar"}</code>
	 * @returns {Promise} returns the Promise object with the result of the asyncronous operation
	 * @memberof AJAXHelper
	 */
	function getDataPromise(url, data) {
		const GET_PREFIX = "?";
		
		let dataStr = getAJAXString(data);

		let getURL = url + GET_PREFIX + dataStr;

		var promiseObj = new Promise((resolve, reject) => {
			let xhr = new XMLHttpRequest();
			xhr.open('GET', getURL, true);
			xhr.send(null);

			xhr.onreadystatechange = () => {
				if (xhr.readyState === XHR_READY_STATE_DONE) {
					if (xhr.status === HTTP_STATUS_OK) {
						let jsonData = parseJSON(xhr.responseText);
						resolve(jsonData);
					}
					else
						reject(xhr.status);
				}
			};
		});
		return promiseObj;
	}

	/**
	 * Creates a string with parameters from the passed object to append to the URL for the REST calls.
	 * @private
	 * @param {!object} data map of values like <br><code>{"name": "value", "foo": "bar"}</code>
	 * @returns {string} a string like <code>name=value&foo=bar</code>
	 * @memberof AJAXHelper
	 */
	function getAJAXString(data) {
		let dataArr = [];
		for (let [name, value] of Object.entries(data))
			dataArr.push(`${name}=${value}`);

		let dataStr = "";
		if (dataArr.length > 0)
			dataStr = dataArr.join("&");

		return dataStr;
	}
	
	/**
	 * Returns a JavaScript obect from a "JSON-formatted" string.
	 * @param {!string} JSONStr
	 * @returns {object}
	 */
	function parseJSON(JSONStr) {
		try {
			let data = JSON.parse(JSONStr);
			return data;
		}
		catch (e) {
			console.error("JSON parse error: " + e);
			return null;
		}
	}

	// public functions
	return {
		getData: getData,
		postData: postData,
		getDataPromise: getDataPromise
	};
}) ();

PromiseSample.js

const URL = "my-website-url";

const userIDs = [123, 456, 789];

let data = {};
data.cmd = "getUserDetails";

for (userID of userIDs) {
	// when server response is ready, the processDetails function is called
	AJAXHelper.getDataPromise(URL, userID).then(processDetails, error => {
		console.error("Error: " + error);
	});
}

function processDetails(result) {
	console.log(`User name is ${result.name} and surname is ${result.surname}`);
}

Sample.js

const URL = "my-website-url";

// posting data
let data = {};
data.cmd = "setUserData";
data.name = "John";
data.surname = "Doe";
data.userID = 123;
AJAXHelper.postData(URL, data, (res, msg) => {
    if (res)
        console.log(msg);
});

// posting data syncronously
AJAXHelper.postData(URL, data, (res, msg) => {
    if (res)
        console.log(msg);
}, false);

// getting data
let data = {};
data.cmd = "getUserData";
data.userID = 123;
AJAXHelper.getData(URL, data, (res) => {
    if (res)
       console.log(res);
});