chatOnPHP/index.js

var nextMessageId = 0;
var previousParagraphToInsertNext;


function jsonPost(url, data) {
    return new Promise((resolve, reject) => {
        var x = new XMLHttpRequest();
        x.onerror = () => reject(new Error('jsonPost failed'));
        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'))
            }
        }
    })
}

function getFullTime(timestamp) {
    var date = new Date(timestamp*1000);
    var hours = date.getHours();
    var minutes = "0" + date.getMinutes();
    var seconds = "0" + date.getSeconds();
    var formattedTime = hours + ':' + minutes.substr(-2) + ':' + seconds.substr(-2);
    return formattedTime;
}

async function getMessages(){
    var data = {func: 'getMessages', messageId: nextMessageId};
    var messages = await jsonPost("server.php", data);
    for (let msgIndex in messages.data){
        let par = chat.insertBefore(document.createElement('p'), previousParagraphToInsertNext);
        par.innerHTML = `
            (${getFullTime(messages.data[msgIndex].timestamp)}) 
            <strong>${messages.data[msgIndex].nick}:</strong> 
            ${messages.data[msgIndex].message}
        `;
        previousParagraphToInsertNext = par;
    }
    nextMessageId = messages.nextMessageId;
}



send.onclick = async event => {
    var a = await jsonPost("server.php", {func: 'addMessage', nick: nick.value, message: message.value});
    getMessages();
}

setInterval(() => getMessages(), 500);