0.3.0 • Public • Published


Code Climate

Saveba.js is a JavaScript library that, relying on the Network Information API, tries to save bandwidth to users having a slow connection by removing unnecessary* resources (at the moment images only).

*An explanation of what's considered an unnecessary resource can be found in the "How does Saveba.js work" section.


To see Saveba.js in action you can take a look at the live demo.

Notes on Saveba.js

This library is intended more as a proof-of-concept of what is possible to do using the Network Information API than something you should use on your website, at least at the moment. In the demo provided in this repository, I show a possible use case for Saveba.js. Using the library, the demo removes unnecessary resources but also offers a way to load them in case the user wants to.

Notes on the Network Information API

The current specifications of the Network Information API integrates the previous version (more details in section "Previous versions") and try to address all the criticisms directed to the previous proposal. This version adds a downlinkMax property to the already defined type property, exposed through the window.navigator object.

The downlinkMax property represents the maximum downlink speed, in megabits per second (Mbps). Some examples of values are: 2 for an UMTS connection, 10 for an Ethernet connection, and 100 for an LTE connection. You can find more values in the table of maximum downlink speeds reported in the specifications.

The type property returns the user agent's connection type and can assume one of the following values:

  • bluetooth
  • cellular
  • ethernet
  • mixed
  • none
  • other
  • unknown
  • wifi
  • wimax

Previous versions

The previous version of the specifications defined an interface that exposed only the type property described in section "Notes on the Network Information API". This version of the specifications is what most modern browsers (Chrome 38, Opera 25, Firefox 31+) support, so it's the one developers can use today.

The previous version superseded the older specifications that exposed two properties: bandwidth and metered. The first was a double representing an estimation of the current bandwidth in megabytes per second (MB/s), while the second was a Boolean that specified if the network connection of the device was subject to limitations. This even older version of the API is implemented in versions of Firefox prior to 31.

To learn more on this API, you can read my article HTML5: Network Information API.


The previous version of the specifications has been widely criticized by many developers. The main critique is that knowing only the type of the connection (Wi-Fi, Ethernet, and so on) isn't a good indication of the actual speed. For example, a user visiting a website on a 4G connection (exposed under the cellular value) may have a faster connection than another user on a Wi-Fi connection (exposed under the wifi value). This very concern has been expressed also by Sindre Sorhus that in a tweet has ironically written:

Future: "Best viewed on Wifi", "Sorry, ur connection [fast 4g] isnt fast enough for this experience, try wifi [slow]"

The debate on this API is really open as demonstrated by the continue evolution of the specifications and by Paul Irish that in a post on Google+ asked:

Question: is it worth exposing more information about the type of cellular (and other) connections? E.g. HSPA vs LTE, 2G vs 3G vs 4G. If yes, how would you use it? Concrete use cases would go a long way here! And if not, why not?

Whether you think it's worth or not, the current specifications are a mere W3C editor's draft, so there is still time to contribute to the discussion and provide your feedback.

How does Saveba.js work

Saveba.js relies on the information exposed by the Network Information API (all versions but the newest because it isn't supported by any browser) to retrieve the information of the user's connection and replace zero or more resources (currently images only) with a placeholder, based on the connection in use. Resources that are already in the browser's cache are shown regardless of the type of connection.

Specifically, Saveba.js performs one of the following operations:

  • Replaces all non-cached images for slow connections
  • Replaces non-content images (images having an empty alt attribute) that aren't in the browser's cache for average connections*
  • Do nothing for fast connections

*Available in oldest specifications only. More on this in "How connections are classified".

How connections are classified

A slow connection is a connection whose type is cellular or bluetooth. In case the oldest specifications are supported, a slow connection is a metered one (metered property set to true) or a connection whose speed ranges from 0 to slowMax (more on this in "Configuration" section).

An average connection is a connection that isn't metered and whose speed ranges from two values passed to the library (from slowMax to fastMin). Because this level depends on the metered property and the speed of the connection, it's only available in browsers that support the oldest specifications.

A fast connection is a connection whose type is wifi or ethernet.

The library does nothing also in case of an absent (none), unknown (unknown), or unspecified (other) connection.

How to use

To use Saveba.js, download the JavaScript file contained in the "src" folder and include it in your web page.

<script src="path/to/saveba.js"></script>

That's it!

The library will do its work and expose a global object called saveba, available as a property of the window object.


For the reasons described in the Criticisms section, Saveba.js is far from being perfect and 100% reliable. Therefore, you may want to remove the effect of this library allowing a user on a detected slow or average connection to see all the resources of the web page. To achieve this goal, you can invoke the destroy() method of the saveba object.

Let's say that the page has a button whose ID is restore-resources. You can add an event listener to the click event that restores all the resources as shown below:

document.getElementById('restore-resources').addEventListener('click', function(event) {

Alternatively, if you want to remove the changes of the library on a specific element or a collection of elements you can pass either the element or the collection to destroy() as shown below:

document.getElementById('restore-resources').addEventListener('click', function(event) {

or even:

document.getElementById('restore-resources').addEventListener('click', function(event) {


Saveba.js has few options you can set after you've included the library. The options are exposed through the defaults property of saveba and are:

  • ignoredElements (Array or NodeList. Default: []): A set of elements the library must ignore;
  • slowMax (Number. Default: 0.5): The maximum speed in MB/s after which a connection isn't considered slow anymore;
  • fastMin (Number. Default: 2): The minimum speed in MB/s a connection must have to be considered fast.

Let's say that you want to avoid the optimizations for all the elements having class ignore, you can write in your pages:

<script src="path/to/saveba.js"></script>
   saveba.defaults.ignoredElements = document.getElementsByClassName('ignore');

Browsers supported

Saveba.js uses the Network Information API, so it works in the same browsers that support this API, and specifically:

  • Firefox 30+. Prior to Firefox 31, the browser supports the oldest version of the API. In Firefox 31 the API has been disabled on desktop
  • Chrome 38+, but it's only available in Chrome for Android, Chrome for iOS, and ChromeOS
  • Opera 25+
  • Browser for Android 2.2+

Special thanks

A special thanks goes to Ilya Grigorik and Steve Souders (in alphabetical order) for their invaluable suggestions and comments that helped me in creating this library.


Saveba.js is dual licensed under MIT and GPL-3.0


Aurelio De Rosa (@AurelioDeRosa)


npm i saveba-js

DownloadsWeekly Downloads





(MIT OR GPL-3.0)

Last publish


  • aurelioderosa