How can I obfuscate (protect) JavaScript?

Question

I want to make a JavaScript application that's not open source, and thus I wish to learn how to can obfuscate my JS code? Is this possible?

Answer 1

Obfuscation:

Try YUI Compressor. It's a very popular tool, built, enhanced and maintained by the Yahoo UI team.

You may also use:

• Google Closure Compiler
• UglifyJS

UPDATE: This question was originally asked on 2008, and The mentioned technologies are deprecated. you can use:

• terser - more information in web.dev.

Private String Data:

Keeping string values private is a different concern, and obfuscation won't really be of much benefit. Of course, by packaging up your source into a garbled, minified mess, you have a light version of security through obscurity. Most of the time, it's your user who is viewing the source, and the string values on the client are intended for their use, so that sort of private string value isn't often necessary.

If you really had a value that you never wanted a user to see, you would have a couple of options. First, you could do some kind of encryption, which is decrypted at page load. That would probably be one of the most secure options, but also a lot of work which may be unnecessary. You could probably base64 encode some string values, and that would be easier.. but someone who really wanted those string values could easily decode them. Encryption is the only way to truly prevent anyone from accessing your data, and most people find that to be more security than they need.

Sidenote:

Obfuscation in Javascript has been known to cause some bugs. The obfuscators are getting a little better about it, but many outfits decide that they see enough benefit from minifying and gzipping, and the added savings of obfuscation isn't always worth the trouble. If you're trying to protect your source, maybe you'll decide that it's worth your while, just to make your code harder to read. JSMin is a good alternative.

Answer 2

I'm surprised no one has mentioned Google's Closure Compiler. It doesn't just minify/compress, it analyzes to find and remove unused code, and rewrites for maximum minification. It can also do type checking and will warn about syntax errors.

JQuery recently switched from YUI Compresser to Closure Compiler, and saw a "solid improvement"

Answer 3

Obfuscation can never really work. For anyone who really wants to get at your code, it's just a speed bump. Worse, it keeps your users from fixing bugs (and shipping the fixes back to you), and makes it harder for you to diagnose problems in the field. Its a waste of your time and money.

Talk to a lawyer about intellectual property law and what your legal options are. "Open Source" does not mean "people can read the source". Instead, Open Source is a particular licensing model granting permission to freely use and modify your code. If you don't grant such a license then people copying your code are in violation and (in most of the world) you have legal options to stop them.

The only way you can really protect your code is to not ship it. Move the important code server-side and have your public Javascript code do Ajax calls to it.

See my full answer about obfuscators here.

Answer 4

You can obfuscate the javascript source all you want, but it will always be reverse-engineerable just by virtue of requiring all the source code to actually run on the client machine... the best option I can think of is having all your processing done with server-side code, and all the client code javascript does is send requests for processing to the server itself. Otherwise, anyone will always be able to keep track of all operations that the code is doing.

Someone mentioned base64 to keep strings safe. This is a terrible idea. Base64 is immediately recognizable by the types of people who would want to reverse engineer your code. The first thing they'll do is unencode it and see what it is.

Answer 5

There are a number of JavaScript obfuscation tools that are freely available; however, I think it's important to note that it is difficult to obfuscate JavaScript to the point where it cannot be reverse-engineered.

To that end, there are several options that I've used to some degree overtime:

• YUI Compressor. Yahoo!'s JavaScript compressor does a good job of condensing the code that will improve its load time. There is a small level of obfuscation that works relatively well. Essentially, Compressor will change function names, remove white space, and modify local variables. This is what I use most often. This is an open-source Java-based tool.

• JSMin is a tool written by Douglas Crockford that seeks to minify your JavaScript source. In Crockford's own words, "JSMin does not obfuscate, but it does uglify." It's primary goal is to minify the size of your source for faster loading in browsers.

• Free JavaScript Obfuscator. This is a web-based tool that attempts to obfuscate your code by actually encoding it. I think that the trade-offs of its form of encoding (or obfuscation) could come at the cost of filesize; however, that's a matter of personal preference.

Answer 6

What i would do:

A. Troll the hacker!

This is will be in the second part my fake/obfuscated secret javascript code LAUNCHER. The one you see in the source code.

What does this code?

1. loads the real code
2. sets a custom header
3. posts a custom variable

---

var ajax=function(a,b,d,c,e,f){
 e=new FormData();
 for(f in d){e.append(f,d[f]);};
 c=new XMLHttpRequest();
 c.open('POST',a);
 c.setRequestHeader("Troll1","lol");
 c.onload=b;
 c.send(e);
};
window.onload=function(){
 ajax('Troll.php',function(){
  (new Function(atob(this.response)))()
 },{'Troll2':'lol'});
}

B. Obfuscate the code a little

What is that?

1. thats the same code as above in base64
2. this is not the SECRET javascript code

---

(new Function(atob('dmFyIGFqYXg9ZnVuY3Rpb24oYSxiLGQsYyxlLGYpe2U9bmV3IEZvcm1EYXRhKCk7Zm9yKGYgaW4gZCl7ZS5hcHBlbmQoZixkW2ZdKTt9O2M9bmV3IFhNTEh0dHBSZXF1ZXN0KCk7Yy5vcGVuKCdQT1NUJyxhKTtjLnNldFJlcXVlc3RIZWFkZXIoIlRyb2xsMSIsImxvbCIpO2Mub25sb2FkPWI7Yy5zZW5kKGUpO307d2luZG93Lm9ubG9hZD1mdW5jdGlvbigpe2FqYXgoJ1Ryb2xsLnBocCcsZnVuY3Rpb24oKXsgKG5ldyBGdW5jdGlvbihhdG9iKHRoaXMucmVzcG9uc2UpKSkoKX0seydUcm9sbDInOidsb2wnfSk7fQ==')))()

C Create a hard to display php file with the real code inside

What does this php code?

1. Checks for the right referrer (domain/dir/code of your launcher)
2. Checks for the custom HEADER
3. Checks for the custom POST variable

If everything is ok it will show you the right code else a fake code or ban ip, close page.. whatever.

<?php
$t1=apache_request_headers();
if(base64_encode($_SERVER['HTTP_REFERER'])=='aHR0cDovL2hlcmUuaXMvbXkvbGF1bmNoZXIuaHRtbA=='&&$_POST['Troll2']=='lol'&&$t1['Troll1']='lol'){
 echo 'ZG9jdW1lbnQuYm9keS5hcHBlbmRDaGlsZChkb2N1bWVudC5jcmVhdGVFbGVtZW50KCdkaXYnKSkuaW5uZXJUZXh0PSdBd2Vzb21lJzsNCg==';//here is the SECRET javascript code
}else{
 echo 'd2luZG93Lm9wZW4oJycsICdfc2VsZicsICcnKTt3aW5kb3cuY2xvc2UoKTs=';
};
?>

base64 referrer = http://here.is/my/launcher.html

SECRET javascript = document.body.appendChild(document.createElement('div')).innerText='Awesome';

FAKE = window.open('', '_self', '');window.close();

Now .. if you define event handlers in the SECRET javascript it's probably accessible.. you need to define them outside with the launchcode and pointing to a nested SECRET function.

SO... is there a easy wayto get the code? document.body.appendChild(document.createElement('div')).innerText='Awesome';

I'm not sure if this works but i'm using chrome and checked Elements,Resources,Network,Sources,Timeline,Profiles,Audits but i didn't find the line above.

note1: if u open the Troll.php url from Inspect element->network in chrome you get the fake code.

note2: the whole code is written for modern browsers. polyfill needs alot more code.

EDIT

launcher.html

<!doctype html><html><head><meta charset="utf-8"><title></title><script src="data:application/javascript;base64,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"></script></head><body></body></html>

Troll.php

<?php $t1=apache_request_headers();if(/*base64_encode($_SERVER['HTTP_REFERER'])=='PUT THE LAUNCHER REFERER HERE'&&*/$_POST['Troll2']=='lol'&&$t1['Troll1']='lol'){echo 'ZG9jdW1lbnQuYm9keS5hcHBlbmRDaGlsZChkb2N1bWVudC5jcmVhdGVFbGVtZW50KCdkaXYnKSkuaW5uZXJUZXh0PSdBd2Vzb21lJzsNCg==';}else{echo 'd2luZG93Lm9wZW4oJycsICdfc2VsZicsICcnKTt3aW5kb3cuY2xvc2UoKTs=';}; ?>

Answer 7

The problem with interpreted languages, is that you send the source to get them working (unless you have a compiler to bytecode, but then again, it is quite trivial to decompile).

So, if you don't want to sacrifice performance, you can only act on variable and function names, eg. replacing them with a, b... aa, ab... or a101, a102, etc. And, of course, remove as much space/newlines as you can (that's what so called JS compressors do).
Obfuscating strings will have a performance hit, if you have to encrypt them and decrypt them in real time. Plus a JS debugger can show the final values...

Answer 8

Try JScrambler. I gave it a spin recently and was impressed by it. It provides a set of templates for obfuscation with predefined settings for those who don't care much about the details and just want to get it done quickly. You can also create custom obfuscation by choosing whatever transformations/techniques you want.

Answer 9

Contrary to most of the other answers I suggest against YUI Compressor; you should use Google Closure.

Not much because it compresses more, but mostly because it will catch javascript errors such as a = [1,2,3,]; which make IE go haywire.

Answer 10

I can recommend JavaScript Utility by Patrick J. O'Neil. It can obfuscate/compact and compress and it seems to be pretty good at these. That said, I never tried integrating it in a build script of any kind.

As for obfuscating vs. minifying - I am not a big fan of the former. It makes debugging impossible (Error at line 1... "wait, there is only one line") and they always take time to unpack. But if you need to... well.

Answer 11

A non-open-source Javascript-based application is fairly silly. Javascript is a client-side interpreted language.. Obfuscation isn't much protection..

JS obfuscation is usually done to reduce the size of the script, rather than "protect" it. If you are in a situation where you don't want your code to be public, Javascript isn't the right language..

There are plenty of tools around, but most have the word "compressor" (or "minifier") in its name for a reason..

Answer 12

You can't secure client side code: just press F12 on Google Chrome, pause javascript execution and you will get all strings, even those encrypted. Beautify it and rename variables and you will get almost the original code.

If you're writing server side javascript (i.e. NodeJS) is afraid of someone hacking into your server and want to make the hacker work more difficult, giving you more time to get your access back, then use javacript compilers:

You need to use Closure Compiler on Advanced Compilation, as it's the only tool that renames all your variables, even if those are used in multiple files/modules. But it just have a problem: it only work if you write in it's coding style.

Answer 13

I would suggest first minify with something like YUI Compressor, and then convert all string and numbers to HEX Values using something like http://www.javascriptobfuscator.com/

With this, the code would be rendered near impossible to understand and I think at this Stage it will take more time for a Hacker to re-enact your code than actually if he re-wrote from scratch. Rewriting and Cloning is what you cant actually stop. After all we are free-people !

Answer 14

Dean Edward's Packer is an excellent obfuscator, though it primarily obfuscates the code, not any string elements you may have within your code.

See: Online Javascript Compression Tool and select Packer (Dean Edwards) from the dropdown

Answer 15

Try this tool Javascript Obfuscator

I used it on my HTML5 game not only it reduced it size from 950KB to 150 but also made the source code unreadable closure compilers and minifiers are reversable I personally dont know how to reverse this obfuscation.

Answer 16

Have you tried Bananascript? It produces highly compressed and completely unreadable code.

Answer 17

I'm under the impression that some enterprises (e.g.: JackBe) put encrypted JavaScript code inside *.gif files, rather than JS files, as an additional measure of obfuscation.

Answer 18

I am using Closure-Compiler utility for the java-script obfuscation. It minifies the code and has more options for obfuscation. This utility is available at Google code at below URL:
Closure Tools

But now a days I am hearing much of UglifyJS. You can find various comparison between Closure Compiler and UglifyJS in which Uglify seems to be a winner.
UglifyJS: A Fast New JavaScript Compressor For Node.js That’s On Par With Closure

Soon I would give chance to UglifyJS.

Answer 19

I've been using Jasob for years and it is hands down the best obfuscator out there.
It has an advanced UI but is still intuitive and easy to use.
It will also handle HTML and CSS files.

The best way to use it is to prefix all of your private variables with something like an underscore, then use the sort feature to group them all together and check them off as targets for obfuscation.

Users can still view your source, but it's much more difficult to decipher when your private variables are converted from something like _sUserPreferredNickName to a.

The engine will automatically tally up the number of targeted variables and prioritize them to get the maximum compression.

I don't work for Jasob and I get nothing out of promoting them, just offering some friendly advice.
The downside is that it's not free and is a little pricey, but still worth it when stacked against alternatives - the 'free' options don't even come close.

Answer 20

As a JavaScript/HTML/CSS obfuscator/compressor you can also try Patu Digua.

Answer 21

I've used this in the past, and it does a good job. It's not free, but you should definitely take a look.
JavaScript Obfuscator & Encoder

Answer 22

You definitely should consider taking a look at Obfuscriptor.

I goes beyond the typical Javascript minifying tricks we've seen from other tools such as YUI Compressor or Google Closure.

The obfuscated code looks more like encrypted. Unlike anything I've seen before.