Your license to use React.js is revoked if you compete with Facebook
React is a popular JavaScript technology for creating rich user interfaces. It originates from Facebook (Instagram originally) and is widely used. The library is open sourced under BSD, but it comes with an added patent clause that you should be aware of.
If you are using or considering using React in a project you might want to consult a lawyer. Because of the patent clause you are not allowed to do anything that constitutes as competing with Facebook. If you do take legal actions or in other ways challenge Facebook, your license to use React is immediately revoked.
Your license is also revoked if you have any legal disputes if you have legal disputes with any other company using React. So if you're looking to create a Facebook killer with React, you might want to call up your lawyer. Technology wise there are other viable alternatives like Vue, Angular and Aurelia, so your venture won't fall because of this.
This is the reason why both Google and Microsoft employees are not allowed to use React.js in their work. While this may be a theoretical impact for most implementation projects, it's certainly interesting and can limit some other projects like WordPress Calypso which have built a deep coupling to the library. They maybe unable to leverage their userbase in the future in ways that could be considered as competition by Facebook.
Here is the whole clause any developer using React.js in their projects agree to:
TweetAdditional Grant of Patent Rights Version 2
"Software" means the React software distributed by Facebook, Inc.
Facebook, Inc. ("Facebook") hereby grants to each recipient of the Software
("you") a perpetual, worldwide, royalty-free, non-exclusive, irrevocable
(subject to the termination provision below) license under any Necessary
Claims, to make, have made, use, sell, offer to sell, import, and otherwise
transfer the Software. For avoidance of doubt, no license is granted under
Facebook's rights in any patent claims that are infringed by (i) modifications
to the Software made by you or any third party or (ii) the Software in
combination with any software or other technology.The license granted hereunder will terminate, automatically and without notice,
if you (or any of your subsidiaries, corporate affiliates or agents) initiate
directly or indirectly, or take a direct financial interest in, any Patent
Assertion: (i) against Facebook or any of its subsidiaries or corporate
affiliates, (ii) against any party if such Patent Assertion arises in whole or
in part from any software, technology, product or service of Facebook or any of
its subsidiaries or corporate affiliates, or (iii) against any party relating
to the Software. Notwithstanding the foregoing, if Facebook or any of its
subsidiaries or corporate affiliates files a lawsuit alleging patent
infringement against you in the first instance, and you respond by filing a
patent infringement counterclaim in that lawsuit against that party that is
unrelated to the Software, the license granted hereunder will not terminate
under section (i) of this paragraph due to such counterclaim.A "Necessary Claim" is a claim of a patent owned by Facebook that is
necessarily infringed by the Software standing alone.A "Patent Assertion" is any lawsuit or other action alleging direct, indirect,
or contributory infringement or inducement to infringe any patent, including a
cross-claim or counterclaim.
-- Additional Grant of Patent Rights Version 2 for React