callback function not working in node-scheduler - node.js

Question

I have successfully written a code to read unseen emails in my email box using node.js. it works fine when I run it in the normal javascript file. now I want to schedule the script to run every 30 seconds. so wrote a code snippet using node-scheduler as below. running this I just get reading unread emails....### as output, console.log(msg); not print anything. if I run this without nod-scheduler it works fine. what might be the problem? I tried await keyword with email.unreadEmail function, it didn't work too.
I believe that this is something happening because of asynchronous behavior. if so is there way to convert this code to promises? I am little bit confiuse because there are multiple nexted callback in imap interface.
schedule.scheduleJob('*/30 * * * * *', () => {
logger.info(`scheduler runs at ${new Date()} for check unread emails`);
let email = new mail.default();
email.unreadEmail((msg) => {
console.log(msg);
});
});
here is the code for email reading
unreadEmail(callback){
logger.info(`reading unread emails....###`);
imap.once('ready', () => {
openIncidents((err, box) => {
if (err) throw err;
imap.search([ 'UNSEEN'], (err, results) => {
if (err) throw err;
try{
// var f = imap.fetch(results, { bodies: '',markSeen: true });
var f = imap.fetch(results, { bodies: '' });
f.on('message', (msg, seqno) => {
console.log('Message #%d', seqno);
var prefix = '(#' + seqno + ') ';
msg.on('body', (stream, info) => {
simpleParser(stream, (err, mail) => {
callback(mail.subject);
});
// stream.pipe(fs.createWriteStream('msg-' + seqno + '-body.txt'));
});
msg.once('attributes', function(attrs) {
logger.info(prefix + 'Attributes: %s', inspect(attrs, false, 8));
});
msg.once('end', function() {
logger.info(prefix + 'Finished');
});
});
f.once('error', function(err) {
logger.error('Fetch error: ' + err);
});
f.once('end', function() {
logger.info('Done fetching all messages!');
imap.end();
});
}catch (e) {
logger.error('Error from fetching mails, probably there are no unseen emails in mailbox');
}
});
});
});
logger.info(`end of the email function....###`);
}
Updated
this behavior is same with setInterval function
setInterval(()=>{
email.unreadEmail((msg)=>{...})}, 30000)

Answer 1

Assuming you're using the imap NPM module (you didn't specify in the original question), the 'ready' event for the new imap connection only fires one time upon authenticating (see code for _login here: https://github.com/mscdex/node-imap/blob/master/lib/Connection.js#L1604).
Therefore, instead of putting imap.once('ready' ... within the schedule, you should put your schedule inside of the imap.once('ready' ... callback. I would expect something like the following to work making sure that you remove imap.once('ready', () => { ... from the unreadEmail function. Also, based on your use case setInterval is probably a better choice as opposed to node-schedule:
imap.once('ready', () => {
setInterval(() => {
logger.info(`scheduler runs at ${new Date()} for check unread emails`);
let email = new mail.default();
email.unreadEmail((msg) => {
console.log(msg);
});
}, 30000);
});