onVisibilityChange.js

function onVisibilityChange (callbackHidden, callbackVisible) {
  var browserPrefixes = ['moz', 'ms', 'o', 'webkit']
  var isVisible = true // internal flag, defaults to true

  // get the correct attribute name
  function getHiddenPropertyName (prefix) {
    return (prefix ? prefix + 'Hidden' : 'hidden')
  }

  // get the correct event name
  function getVisibilityEvent (prefix) {
    return (prefix || '') + 'visibilitychange'
  }

  // get current browser vendor prefix
  function getBrowserPrefix () {
    for (var i = 0; i < browserPrefixes.length; i++) {
      if (getHiddenPropertyName(browserPrefixes[i]) in document) {
        // return vendor prefix
        return browserPrefixes[i]
      }
    }

    // no vendor prefix needed
    return null
  }

  // bind and handle events
  var browserPrefix = getBrowserPrefix()
  var hiddenPropertyName = getHiddenPropertyName(browserPrefix)
  var visibilityEventName = getVisibilityEvent(browserPrefix)

  function onVisible () {
    // prevent double execution
    if (isVisible) {
      return
    }

    // change flag value
    isVisible = true
    
    if (callbackVisible)
      callbackVisible()
    }
  }

  function onHidden () {
    // prevent double execution
    if (!isVisible) {
      return
    }

    // change flag value
    isVisible = false
    
    if (callbackHidden) {
      callbackHidden()
    }
  }

  function handleVisibilityChange (forcedFlag) {
    // forcedFlag is a boolean when this event handler is triggered by a
    // focus or blur eventotherwise it's an Event object
    if (typeof forcedFlag === 'boolean') {
      if (forcedFlag) {
        return onVisible()
      }

      return onHidden()
    }

    if (document[hiddenPropertyName]) {
      return onHidden()
    }

    return onVisible()
  }

  document.addEventListener(visibilityEventName, handleVisibilityChange, false)

  // extra event listeners for better behaviour
  document.addEventListener('focus', function () {
    handleVisibilityChange(true)
  }, false)

  document.addEventListener('blur', function () {
    handleVisibilityChange(false)
  }, false)

  window.addEventListener('focus', function () {
    handleVisibilityChange(true)
  }, false)

  window.addEventListener('blur', function () {
    handleVisibilityChange(false)
  }, false)
}