function jsonPost(url, data) {
	return new Promise((resolve, reject) => {
		const x = new XMLHttpRequest();
		x.onerror = () => reject(new Error('jsonPost failed'))
		//x.setRequestHeader('Content-Type', 'application/json');
		x.open("POST", url, true);
		x.send(JSON.stringify(data))

		x.onreadystatechange = () => {
			if (x.readyState === XMLHttpRequest.DONE && x.status === 200){
				resolve(JSON.parse(x.responseText))
			} else if (x.status !== 200){
				reject(new Error('status is not 200'))
			}
		}
	});
}

const url = "http://students.a-level.com.ua:10012";
let nextId = 0;

sendButton.addEventListener('click', () => {
	let nickValue = nickInput.value;
	let messageValue = messageInput.value;

	const addZero = data => data < 10 ? `0${data}` : data;

	const buildDate = date => `${date.getDate()}/${date.getMonth()+1}/${date.getFullYear()} ${addZero(date.getHours())}:${addZero(date.getMinutes())}:${addZero(date.getSeconds())}`;

	const buildMessage = (nick, message, time) => {
		return `<p>nick - ${nick}, message - ${message}, date - ${buildDate(new Date(time))}</p>`;
	}

	jsonPost(url, {func: 'addMessage', nick: nickValue, message: messageValue});

	let messagesArray = jsonPost(url, {func: 'getMessages', messageId: nextId});
	messagesArray.then(messages => {
		for (const message of messages.data) {
			chatMessages.insertAdjacentHTML('afterbegin', buildMessage(message.nick, message.message, message.timestamp));
		}
		nextId = messages.nextMessageId;
	});
});