(Node.js) How to Download Messages in MessageSet One at a Time

If a message set contains a huge number of emails, it's NOT a good idea to try to download all at once into an email bundle using a method such as FetchBundle. It's better to iterate over the messages in the set to download one by one.

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    var imap = new chilkat.Imap();

    // Connect using TLS.
    imap.Ssl = true;
    imap.Port = 993;
    var success = imap.Connect("imap.example.com");
    if (success !== true) {
        console.log(imap.LastErrorText);
        return;
    }

    // Authenticate
    success = imap.Login("email_account_login","email_account_password");
    if (success !== true) {
        console.log(imap.LastErrorText);
        return;
    }

    // Select an IMAP mailbox
    success = imap.SelectMailbox("Inbox");
    if (success !== true) {
        console.log(imap.LastErrorText);
        return;
    }

    // Search for messages and return a set of matching messages.
    // (This example will simply search for ALL messages.)
    var fetchUids = true;
    // messageSet: MessageSet
    var messageSet = imap.Search("ALL",fetchUids);
    if (imap.LastMethodSuccess !== true) {
        console.log(imap.LastErrorText);
        return;
    }

    console.log("Number of messages = " + messageSet.Count);

    var i = 0;
    while (i < messageSet.Count) {
        // email: Email
        var email = imap.FetchSingle(messageSet.GetId(i),fetchUids);
        if (imap.LastMethodSuccess !== true) {
            console.log(imap.LastErrorText);

            return;
        }

        console.log(email.From + "; " + email.Subject);

        i = i+1;
    }

    console.log("OK");

}

chilkatExample();