watch-document-title.js

// select the target node
var target = document.querySelector('title')

// create an observer instance
var observer = new MutationObserver(function(mutations) {
  // We need only first event and only new value of the title
  console.log('title changes', document.title)
})

// configuration of the observer:
var config = { subtree: true, characterData: true, childList: true }

// pass in the target node, as well as the observer options
observer.observe(target, config)

// credit https://stackoverflow.com/a/29540461/5765524