correct.js

function createUser(username, callback) {
    DatabaseClient.connect()
        .then(function(connection) {
            return connection.collection('users')
        })
        .then(function(users) {
            return users.query({username: username})
        })
        .then(function(exising){
            if(existing) {
                throw new Error('User already exists');
            }
            
            return users.create({username: username})
        })
        .then(function(newUser) {
            callback(null, newUser);
        })
        .catch(callback)
}

es7.js

async function createUser(username, callback) {
    
    var connection = await DatabaseClient.connect();
    try{ 
        var users = await connection.collection('users');
        var query = await users.query({username: username});
        if(query){
            throw new Error("User already exists: " + username);
        } else {
            return await  users.create({username: username});
        }
    } finally {
        connection.close();
    }
}

original.js

function createUser(username, callback) {
    return DatabaseClient.connect().then(function(connection) {
        return connection.collection('users').then(function(users) {
            return users.query({username: username}).then(function(existing) {
                if (existing) {
                    callback("User already exists: " + username);
                } else {
                    return users.create({username: username}).then(function (user) {
                        callback(null, user);
                    });
                }
            });
        }).fin(connection.close);
    }).catch(callback);
}