How to detect page visibility in web applications

You are building a web application and need the application to pause whenever the user stops interacting with the page; for example, the user opens up another browser tab or minimizes the browser itself. Example scenarios include games where you want to automatically pause the action or video/chat applications where you’d like to raise a notification.

The main advantage of such an API is to prevent resource wastage (battery life on mobile, internet bandwidth or unnecessary computing tasks). Definitely, something to have in mind especially for developers targeting mobile devices. So how would you this?

Can I use event listeners?

Technically, you could use a global event listener on the window object to listen for focus/blur events however, this can not detect browser minification. Also, the blur/focus event would be fired whenever the page loses focus; however, it is possible that a webpage is still visible despite losing focus – think about users having multiple monitors.

The good news is that this is possible with the PageVisibilityAPI which comes with the browsers and this post shows how to use this.

Deep dive into details

The Document interface has been extended with two more attributes – visibilityState and hidden.

Hidden

This is true whenever the page is not visible. What counts as being not visible includes lock screens, minimization, being in a background tab etc.

VisibilityState

This can be one of 4 possible enums explaining the visibility state of the page.

• hidden: page is hidden, hidden is true
• visible: page is visible, hidden is false
• prerender: page is being pre-rendered and not visible. Support for this is optional across browsers and not enforced
• unloaded: page is being unloaded; hidden would also be false too. Support for this is also optional across browsers

Show me some code!

document.addEventListener('visibilitychange',function(){
  if(document.hidden) {
    console.log('hidden');
  } else {
    console.log('visible');
  }
}, false);

Browser support for PageVisibility APIs

You can have it in nearly all modern browsers except Opera mini. Also, you might need to specify vendor prefixes for some of the other browsers. See this.

Conclusion

There it is; you now know a way to effectively manage resource consumption – be it battery, internet data or computing power.

You can use this to determine how long users spend on your page, automatically pause streaming video/audio (with some nice fadeout effects for audio especially) or even raise notifications.

Did you enjoy this post? Here are a few more related posts:

• How to track errors in JavaScript Web applications
• Tips for printing from web applications
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