test.js

var auth = require('./auth.js')

function testAuthentication(username, password) {
  auth.authenticateUser(username, password, function onResult(isAuthenticated){
    console.log(username + ' can authenticate with '+ password 
                + ' --> ' + isAuthenticated);
  });
}

function runTest(){
  auth.addUser('test-user', 'my password', function testUser() {
    testAuthentication('test-user', 'my password');
    testAuthentication('test-user', 'bad password');
  });

  testAuthentication('invalid-user', 'something');
}

auth.purgeDatabase(runTest);