GNU LibreJS 6.0.13

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This manual is for GNU LibreJS (version 6.0.13, 04 May 2016).

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1 Overview

GNU LibreJS —an add-on for GNU IceCat and Mozilla Firefox— detects and blocks nonfree nontrivial JavaScript while allowing its execution on pages containing code that is either trivial and/or free.

Many websites run nontrivial JavaScript on your computer. Some use it for complex tasks; many use it gratuitously for minor jobs that could be done easily with plain HTML. Sometimes this JavaScript code is malicious. Either way, the JavaScript code is often nonfree. For explanation of the issue, see "The JavaScript Trap"(

If you care about freedom in your computing, and don’t wish to let all and sundry make you run nonfree programs, now you can prevent it by using LibreJS.

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2 Disclaimer

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3 Installation

You can install LibreJS directly using a generated librejs.xpi file, or by building it from source.

3.1 Building the Package

After installing jpm, you should be able to use the make command to build LibreJS from source.

After running make, a new file, librejs.xpi should be generated. This is the file that can be installed in a Mozilla browser.

3.2 Installing LibreJS

To install the add-on for all users, run:

sudo make install

or as root:

make install

Next time you open a Mozilla-browser as a user of your system, you should be notified that a new add-on (in this case, LibreJS) as been installed and whether to allow it to run or not.

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4 How to Use

4.1 LibreJS in action

After installing the add-on, you will see the LibreJS widget in the add-on bar at the top right of the browser window. After loading a page, left-click on the widget to view the deactivated JavaScript code from the page (both on page and external) and, if applicable, the scripts that were accepted.

4.2 Complaint Feature

It is very important to complain when a site has nonfree JavaScript code, especially if it won’t work without that code. LibreJS makes it easy to complain by heuristically finding where to send the complaint.

When nonfree/nontrivial code is detected in a page, LibreJS attempts to find a relevant contact link or email for the website you are visiting. In order to do so, it will attempt to visit a few links from the current page (for instance, a link labeled “contact” on the same domain as the current page, …)

LibreJS detects contact pages, email addresses that are likely to be owned by the maintainer of the site, Twitter and links, and phone numbers.

After LibreJS detects any of the above, a “Complain” tab will appear on the right of your web browser. When you click on this tab, a large panel will appear with contact information. Ideally, at the top you will find the email address of the maintainer, labeled as the “Email you should use”.

When you complain to the website for their nonfree nontrivial JavaScript, provide them with the link to the JavaScript Trap essay so that they can get more information on what the issue is and how they can solve it on their own site.

LibreJS includes a default subject line and body for the complaint email, with a link to the JavaScript Trap essay. This can be configured in the LibreJS add-on preferences in your web browser.

4.3 Options


LibreJS lets you whitelist domain names and subdomains to bypass the regular JavaScript check. This might be useful, for example, if you are running your own code in a local web server. In order to add a whitelisted domain or url, go to Tools >> Add-ons, or press Control + Shift + A. Inside the add-on window, click on Extensions, and in the list, where you see LibreJS, click on the Preferences button. You will see an input field labeled Whitelist. In the field, enter comma-separated domain names. Do not enter the protocol. For instance to whitelist all the pages of and, enter ‘’. To allow all subdomains from, enter: ‘*’. This will match such sites as and

Complaint tab

This specifies whether the complaint tab appears when a site is running nonfree JavaScript.

Display notifications of JavaScript analysis

This option enables an info bar of realtime JavaScript analysis.

Complaint email subject

Configure the default subject used in complaint emails.

Complaint email body

Configure the default body used in complaint emails.

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5 JavaScript Detection

In practice, the JavaScript code in your page may be found trivial by LibreJS if, as a whole:

However, in some instances, you may be required by LibreJS to add a stylized comment to JavaScript code that may be otherwise trivial.

When an external file defines a function, it becomes available to all other external scripts. That is the case if another script defines a function that makes AJAX calls, when an external script loads other scripts dynamically (which in turn could also make AJAX calls, …), or when a script is written with constructs that may do any of these.

For instance, if your page contains the following:

<script src="jquery.js"></script>

While $.doSomething(); may seem trivial, you will nevertheless have to add a stylized license comment on your main HTML page because the external script (in this case jQuery) has been found to define methods that make AJAX calls. $.doSomething() might make an AJAX call, and LibreJS does not check for that. The rule of thumb is that when you use a library or code that handles AJAX, JSON, JSONP, the loading of scripts dynamically, you should have license mentions for all your JavaScript files and for your main page regardless. In practice this is a case that happens very often with code that uses libraries.

In practice also, the JavaScript code in an external file (an external .js file loaded on your page) may be found trivial if it does not define functions/methods.

And in the same manner it will be considered nontrivial if AJAX calls, dynamic script loading, or non-obvious dynamic JavaScript constructs are used in another script.

If your JavaScript code makes AJAX requests, it’s important to get an accurate Content-Type in the response from the server. For example, if you’re using JSON, set it to application/json. This is because LibreJS alters the content of text/html responses.

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6 Free Licenses Detection

6.1 Detected Free Licenses

In order for a file to be detected as free, the license notice should appear in a JavaScript file above all code, at the very top of the file.

For inline JavaScript code inside <script> tags in HTML pages, the license notice should appear once per page as a comment inside a <script> tag, before all the code in that script. When the only inline JavaScript code is within element attributes (onload, onclick), place the license notice in an otherwise empty <script> at the top of the page. This is sometimes needed when an external script performs AJAX calls or embeds scripts dynamically, and the only inline JavaScript is an event attribute making a method call, e.g.: <body onload="methodCall('remote-data.xml');">

When people speak of the “MIT license” they mean either the X11 license or the Expat license. Please see which license the code uses, and label it accordingly.

Currently LibreJS checks for the following licenses:

6.2 Undetected Free Licenses

If you are using a free license that isn’t detected by LibreJS and isn’t listed in the previous section, please send a message to regarding this license, where code released under this license can be found, and where to find the license text and information.

Many free licenses are listed in this page:

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7 Setting Your JavaScript Free

The first step is releasing your JavaScript under a free license. If you are already using a free library, or you’re not using any third-party libraries, it might only take a few minutes.

All JavaScript code on a web page (inline, on-page, and external) shares a common scope. Thus, code is generally either rejected or accepted as a whole by LibreJS. If some JavaScript code is found to be nontrivial and nonfree, then most of the time, all the the rest is discarded as well.

On your website, take a look at your HTML source. You can identify distinct pieces of JavaScript that might be free and some other that are nonfree.

Tip: By running LibreJS on your page, you will get a list of all the JavaScript that was blocked. This gives you an overview of the JavaScript in your page.

Imagine a page that contains several pieces of JavaScript from various sources:

JavaScript that is already free

First, you must ensure that the library is free. If the file contains a copyright and a license notice, you won’t need to look any further. But if there’s no mention of the license, or if it’s too brief, you’ll have to look for a COPYING or LICENSE file within the original library’s source package, or on the library’s official website.

Your own JavaScript

The free license given to your code should be compatible with the rest of the JavaScript on a page. A good way to check is to read up on them:

Nonfree JavaScript

This might be the case with an analytics tracker, social media widgets, and code that runs ads. Removing these pieces of code from your site is required to have the rest accepted as free. There are often alternatives to nonfree libraries or to third-party services:

7.1 JavaScript Web Labels

One way to make your website work with LibreJS is by defining a JavaScript Web Labels table.

A JavaScript Web Labels table is informative to both site visitors and the LibreJS program. You make a Web Labels table on a new HTML page that’s linked to from your main page. The table lists each of your site’s JavaScript files, that file’s corresponding human-readable source file, and the canonical url of its free license.

When using a JavaScript Web Labels table for your own files, it’s important to put a copying permission statement at the top of each source file listed in right-most column of the Web Labels table. For info on how properly release your code as free software, see Future versions of LibreJS will require a copying permission statement or other license notice for source files listed in a Web Labels table.

More information on JavaScript Web Labels is detailed here: and here:

7.1.1 Specifying multiple licenses for a single JavaScript file

If you compile or concatenate your JavaScript into a single file, the source files you’re combining may be released under different licenses. You can specify multiple licenses for the file in a JavaScript Web Labels table, like this:

<table id="jslicense-labels1">
        <td><a href="all.min.js">all.min.js</a></td>
            <a href=""
            <br />
            <a href=""
            <a href="gpl-script.js">gpl-script.js</a>
            <br />
            <a href="apache-script.js">apache-script.js</a>

The <br /> tags just make the table more understandable when looking at the rendered version of it on the license page. They aren’t required by LibreJS.

If all the licenses contained in the second column are recognized by LibreJS to be free licenses, then LibreJS will allow the file in the first column to be run.

7.2 Adding a stylized comment in your JavaScript files and on your page

See a “Convention for releasing free JavaScript programs” in the JavaScript Trap

Adding this notice will ensure LibreJS will find the JavaScript file to be free. The @licstart and @licend lines at the beginning and end of the stylized comment are necessary to make a clear statement that the _entire code_ in the file is free. This means that you must ensure that no nonfree code was carelessly appended at the end of the file.

In the main HTML page, the license notice covers JavaScript contained in all <script> tags with on-page code and the inline JavaScript (in event attributes such as onload, onclick, etc, …). Since external files have their own stylized comment, they are not covered by the notice in the main HTML page. Make sure to identify all the licenses available. LibreJS will only ensure it matches a notice of an allowed license once, so the order does not matter, but the responsibility is on you to make sure all code is under the free licenses mentioned between @licstart and @licend.

You should make only one @licstart @licend comment in your page, since it pertains to the entire code on page across all <script> tags and inline html attributes.

When you use the JavaScript Web Labels method, you should still include a license notice at the top of each of your source files. This ensures that if someone copies the file and uses it for something else, the license remains intact.

For more info on making your JavaScript LibreJS-compliant, see this web page:

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8 LibreJS Development Notes

8.1 Running LibreJS from the source directory

Download jpm, then do something like this in the LibreJS directory:

jpm run -b `which abrowser`

8.2 Debugging

Uncomment lines 22 and 23 in lib/main.js to enable printing of console.debug() statements.

8.3 Adding new whitelisted libraries

Run the data/script_libraries/ script, using the URL to a JavaScript file as the argument, then add the output of that command to data/script_libraries/script-libraries.json.

8.4 Releasing a new version

Update the version number in:

Then run ‘make info’ to build the docs with

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Appendix A Installation Requirements

A.1 Mozilla Browser

You will need one of the many flavors of the Mozilla browser to use LibreJS. It can be installed on the following:

GNU IceCat, Mozilla Firefox, Trisquel Abrowser, Debian Iceweasel.

LibreJS works on these browsers starting from version 29. We recommend that you use the latest version of your Mozilla browser. LibreJS has been tested extensively on multiple GNU/Linux distributions, but it is compatible any operating system as long as you’re using a compatible Mozilla browser.

A.2 Mozilla’s Jetpack Manager for Node.js

LibreJS uses the Mozilla jpm tool, a set of utilities for creating, testing, running and packaging Mozilla Jetpack Addons.

You do not need jpm to use the LibreJS xpi file or to install it using the packaged version, but it is required in order to package the LibreJS source code into an xpi file. If you would like to run the tests for LibreJS or make changes to the source files, you will need jpm as well. For the “make” command to work properly, you must have the jpm command available on your system from the command line.

You can find information on getting jpm at these links:

JPM is released under the Mozilla Public License 2.0.

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Appendix B LibreJS Internals

LibreJS intercepts HTTP responses and rewrites their contents after analyzing JavaScript within them. It does not remove script nodes and attributes from the page, but instead “deactivates” them by modifying the type and src attributes on script elements and by moving the contents of inline JavaScript attributes such as onClick into harmless attributes.

LibreJS detects the most common cases using the HTTP response method described above, but in extremely rare cases, or when running code locally, LibreJS cannot detect JavaScript during the response stage.

To remedy this issue, and as a final safeguard, LibreJS takes a look at the scripts that are about to be executed while the browser engine is parsing the page. If the script is not found in a list of accepted scripts populated earlier, the execution will be prevented. This is to ensure content types that are not regular HTML (binhex with HTML in it, …) and JavaScript do not fall through the cracks and get executed.

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Appendix C Tests

In order to better understand how LibreJS works, you can try to visit these pages with LibreJS installed and enabled and see how they are being processed:

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Appendix D GNU Free Documentation License

Version 1.3, 3 November 2008
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


    This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

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    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
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    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
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    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
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    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

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    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”


    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.


    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


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    Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

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    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See

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    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.