UserDocument.remove({}, function(err:any){
    if(err) debug("There was an error removing the existing user collection: "+JSON.stringify(err));
    else debug("User collection cleared.");
});

USERS.forEach( (user) => {
    let userDocument = new UserDocument(user);
    userDocument.save((err:any, user:IUserDocument) =>{
        if (err){
            debug("There was a problem creating the user: "+JSON.stringify(err));
        } else {
            debug("User created with email: "+user.emailAddress);
            UserDocument.find({}, (err, users: IUserDocument[]) => {
                debug("Users in DB right now: "+JSON.stringify(users));
            } );
        }
    });
});

[1]   mydb:init User created with email: someone@gmail.com +125ms
[1]   mydb:init User created with email: someoneelse@gmail.com +5ms
[1]   mydb:init Users in DB right now: [{"_id":"5877fe142381a7080b6b970b","emailAddress":"someone@gmail.com","username":"Someone","__v":0,"admin":true},{"_id":"5877fe142381a7080b6b970c","emailAddress":"someoneelse@gmail.com","username":"SomeoneElse","__v":0,"admin":false}] +69ms
[1]   mydb:init User collection cleared. +89ms
[1]   mydb:init Users in DB right now: [] +0ms

USERS.forEach( (user) => {//USERS is just an array of fake user data
    let userDocument = new UserDocument(user);
    userDocument.save((err:any, user:IUserDocument) =>{
        if (err){
            debug("There was a problem creating the user: "+JSON.stringify(err));
        } else {
            debug("User created with email: "+user.emailAddress);
        }
    });
    userIds.push(userDocument._id.toString());//This works - the IDs are all there.
});