Growing with the Web

How to make a bookmarklet


A bookmarklet is a section of JavaScript that extends the browsers currently loaded page by executing from a browser bookmark.

A bookmarklet is typically just a self-executing JavaScript function.

(function() {
  // Do something

After being minified it’s placed within an anchor’s href attribute with javascript: at the start. Typically the surrounding text indicates that it is a bookmarklet by giving instructions to dragging it to your bookmarks bar.

<p>Drag <a class="bookmarklet" href="javascript:(function(){alert('ZOMG');})()">ZOMG Alert</a> to your bookmarks bar.</p>

Injecting the script

Bookmarklets usually download a script from some external server and injects it into the page. This allows the script to be a large as necessary and also enables automatic updating. Here is an example bookmarklet that injects a script into the page:

(function () {
  var script = document.createElement('script');
  script.src = '//';

The usage of '//' in the URL ensures the same scheme (http/https) as the page is used for the request.

Keeping it fresh

The browser’s cache can work against us with bookmarklets by holding on to the old script when there is a newer version available. To get around this we can use the unique query string trick to force a new download.

(function () {
  var script = document.createElement('script');
  script.src = '//' + 
               Math.floor((new Date).getTime()/3600000);

The timestamp added to the query string above is the current timestamp divided by 3600000 (1000*60*60) to reduce precision to hourly, meaning the script will only be cached at most one time per hour.

Running multiple times

Special considerations may need to be made if the script is going to be run multiple times, such as resetting the state of the script or removing the old script from the page.

(function () {
  var uniqueId = 'gwtw-bookmarklet';
  var script = document.getElementById(uniqueId);
  if (script) {
  script = document.createElement('script');
  script.src = '//' + 
               Math.floor((new Date).getTime() / 3600000); = uniqueId;

Putting it all together

Once the script is all done, minify the Javascript and add javascript: to the front before adding it to the anchor.

<p>Drag <a class="bookmarklet" href="javascript:(function(){var a=document.getElementById("gwtw-bookmarklet");a&&a.parentNode.removeChild(s);a=document.createElement("script");a.src="//"+Math.floor((new Date).getTime()/3600000);"gwtw-bookmarklet";document.body.appendChild(a)})()">the bookmarklet</a> to your bookmarks bar.</p>


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