search-on-imap.js

var Imap = require('imap'),
    inspect = require('util').inspect;

var imap = new Imap({
    user: 'USERNAME',
    password: 'PASSWORD',
    host: 'IMAP_HOST',
    port: 993, // Default port is 993
    tls: true,
    tlsOptions: {rejectUnauthorized: false}
});

function openInbox(cb) {
    // openReadOnly = false
    imap.openBox('Inbox', false, cb);
}

imap.once('ready', function () {
    openInbox(function (err, box) {
        if (err) throw err;

        // Search emails having "Some Subject" in their Subject headers
        imap.search([['HEADER', 'SUBJECT', 'Some Subject']], function (err, results) {
            if (err) throw err;

            try {
                var f = imap.fetch(results, {bodies: 'TEXT'});
                f.on('message', function (msg, seqno) {
                    msg.on('body', function (stream, info) {
                        var buffer = '';
                        stream.on('data', function (chunk) {
                            buffer += chunk.toString('utf8');
                        });
                        stream.once('end', function () {

                            // Mark the above mails as read
                            msg.once('attributes', function (attrs) {

                                let uid = attrs.uid;
                                imap.addFlags(uid, ['\\Seen'], function (err) {
                                    if (err) {
                                        console.log(err);
                                    } else {
                                        console.log("Marked as read!")
                                    }
                                });

                            });

                        });
                    });
                });
                f.once('end', function () {
                    imap.end();
                });
            } catch (errorWhileFetching) {
                console.log(errorWhileFetching.message);
                imap.end();
            }

        });
    });
});

imap.connect();