NightwatchJS - Can't close browser window automatically after running test

I'm running my first ever Nightwatch test as shown in the code below, everything runs fine, selenium types 'wiki' into google and submits the form fine, test passes. However I want it to automatically close the browser window down after the test has finished.

I've tried adding the below:

driver.closeWindow();

However it fails as it cannot recognise the 'closeWindow()' function, I'm finding the documentation to be very lacking (http://ift.tt/2wcbKr9), so I'd be grateful if someone could point out how I need to implement this.

var webdriver = require('selenium-webdriver'),


By = webdriver.By,
  Until = webdriver.Until;

var driver = new webdriver.Builder()
  .forBrowser('chrome')
  .build();

driver.get('http://www.google.com');
driver.findElement(By.name('q')).sendKeys('wiki');
driver.findElement(By.name('btnK')).submit();
driver.wait(check_title, 1000);

function check_title(){
  var promise = driver.getTitle().then(function(title) {
    if (title === 'wiki - Google Search') {
      console.log('success');
      return true;
    } else {
      console.log('fail, title was ' + title);
    }
  });
  return promise;
}