Project owner:Alex Russell

ShrinkSafe is a standalone Java-based JavaScript compressor which utilizes Rhino to parse code and safely shorten the results. ShrinkSafe renames local references to short names prefixed with an underscore. This saves bytes on the wire and also provides some obfuscation of the code. It also eliminates whitespace and comments when generating the new code. Global references and property names remain unchanged such that external references to the compressed code should be safe.

Getting the Code

While ShrinkSafe is available as an online service and part of the tool chain for the Dojo Build system, it is also possible to use ShrinkSafe independent of Dojo.

ShrinkSafe comes bundled with the -src releases of the Dojo Toolkit in the folder util/shrinksafe/. Alternately, you can download ShrinkSafe standalone from by selecting the -shrinksafe archive.


Shrinksafe can operate on a list of file or urls, or if none are specified it will use stdin. Output goes to stdout, which may be redirected to a file or process.

java -jar shrinksafe.jar bigcode.js > smallercode.js   Compresses bigcode.js to smallercode.js

java -jar shrinksafe.jar file1.js file2.js file3.js > combined.js   Compresses three files into one

mygeneratorscript | java -jar shrinksafe.jar

java -jar shrinksafe.jar http://myhost/mypath/foo.js > bar.js

java -jar shrinksafe.jar -?   Get the usage message with information on optional arguments.

ShrinkSafe uses the encoding settings for the JVM, which may vary depending on your JDK installation. To override the encoding, use the file.encoding system property:

java -jar -Dfile.encoding=UTF8 shrinksafe.jar

Developer information

ShrinkSafe has its own jar file, shrinksafe.jar, which utilizes an unmodified Rhino jar, js.jar. Note that the linkage requires the same version of Rhino used to build the shrinksafe.jar. In versions prior to Dojo 1.3, ShrinkSafe was bundled into Rhino by way of patch, and shipped as custom_rhino.jar.

Known bugs

  • Shrinksafe removes comments containing IE's proprietary conditional compilation code ticket #2298
  • Forward references are not always resolved properly. See ticket #3241
  • ShrinkSafe appears to not handle '$super' in this code. See ticket #5436