track-network-timings-ga.js

// send network request timings to Google Analytics
(function() {
  var nativeOpen = XMLHttpRequest.prototype.open
  var nativeSend = XMLHttpRequest.prototype.send
  var startTime = null
  var pendingCount = 0
  XMLHttpRequest.prototype.open = function(_, url) {
    this._url = url
    return nativeOpen.apply(this, arguments)
  }
  XMLHttpRequest.prototype.send = function() {
    if (this._url.indexOf('https://www.google-analytics.com') === -1) {
      if (startTime == null) startTime = Date.now()
      pendingCount += 1
      this.addEventListener('loadend', function() {
        pendingCount -= 1
        if (this.responseURL.indexOf('https://www.google-analytics.com') !== -1) return

        // group multiple network requests together,
        //   including those which are serially dependent
        setTimeout(function() {
          if (pendingCount == 0) {
            var elapsed = Date.now() - startTime
            var path = window.location.pathname + window.location.search
            startTime = null
            ga('send', 'timing', 'Network Chunk', 'load', elapsed, path)
          }
        })
      })
    }
    
    return nativeSend.apply(this, arguments)
  }
})()