small-jsfuck

Generate small jsf code - RUN it HERE

Goal

The idea of converting JS code to only 6 characters []()!+ was implementend in JSFuck for theoretical reasons. This project was created to make jsfuck more practical by decrease size of generated code.

Algorithm

• Get input source-code string and convert each character code to four base4 (not confuse with base64!) numbers (each can have value 0,1,2 or 3) and write it as 4-digit string e.g

"c".charCodeAt(0).toString(4).padStart(4,0)  --> "1203"

• Convert each digit to value which have smallest JSfuck representation using following map

0 --> 0       jsf:   (+[])
1 --> 1       jsf: (+!![])
2 --> false   jsf:    +![]
3 --> true    jsf:   +!![]

e.g. "c" --> "1203" --> "1false0true"

• Encode such mapped values to jsf by add [] to beginning and join by + e.g.

"c" --> "1203" --> "1false0true" --> "[]+(+!![])+![]+(+[])+!![]". 

Steps 2 and 3 are done by following code which produce MIDDLE_STEP_CODE= []+(+!![])+![]+(+[])+!![]

"[]"+ "1203".replace(/0/g,"+(+[])").replace(/1/g,"+(+!![])").replace(/2/g,"+![]").replace(/3/g,"+!![]");

(whe add "[]" at the beginnig only on first character in whole code string)

• Include additional code (STUB) to make inversion of MIDDLE_STEP_CODE and run it:

Function((MIDDLE_STEP_CODE).replace(/true/g,3).replace(/false/g, 2).match(/..../g).map(x=>String.fromCharCode(parseInt(x,4))).join(""))()

after (jsf) optimalization it looks like follows

[]["flat"]["constructor"](MIDDLE_STEP_CODE)["split"](true)["join"](3)["split"](false)["join"](2)["match"]([]["flat"]["constructor"]("return/..../g")())["map"]([]["flat"]["constructor"]("return f=>String.fromCharCode(parseInt(f,4))")())["join"]([]))()

in above code finally we need to convert all strings (like "flat", "constructor",...) to jsf, true/fale and nubers, and paste MIDDLE_STEP_CODE to get final result. Above run and inverse conversion code gives ~23kB overhead.

If in above long line of code we remove code before (MIDDLE_STEP_CODE) and also remove last )() (at the end of line) - then after run this we get deconverted string (which will be not executed)

Benefits

In this way each character change to 4 digits, each digit change to 4-8 jsf characters. So in worst case each character is converted to 4*8=32 jsf characters, in best case it is converted to 4*4=16 characters. So 1kB of code will grow to 16-32kB after conversion to jsf. It is quite efficient if we compare this to direct jsf conversion.

Bonus - additional info about jsfuck

Get arbitrary character - at lower level

Belowe text came from this my pull request to jsfuck repo

You can use following technique (is based on that JS objects can have defined fields without quotes) to get A-Za-z$_ and many utf8 characters

Function("return Object.entries({\\u0043:false})")()[0][0]

after partial transformation it looks like

[]["flat"]["constructor"]("return"+" "+"Object"+"."+"entries"+"("+"{"+"\\"+"u0043"+":"+false+"}"+")")()[0][0]

This allows to get upper-case characters witout using buil-in methods like: escape, unescape, italics, fontcolor... but by using more 'low-level" language features - but we also need use following codes to get RegExp string, shlash, colon and finlly backslash

   // "RegExp" string: (""+"".matchAll()).split(" ")[1]
  ([]+("")["matchAll"]())["split"](" ")[1]

  // ":" - colon: (Function("return RegExp")()()+"")[3]
  ([]["flat"]["constructor"]("return "+([]+("")["matchAll"]())["split"](" ")[1])()()+[])[3]

  // "/" - slash: (Function("return RegExp")()()+"")[0]
  ([]["flat"]["constructor"]("return "+([]+("")["matchAll"]())["split"](" ")[1])()()+[])[0]
    
  // "\" - backslash: (Function("return RegExp(RegExp()+[])")()+[])[1]
  //       optimized: (Function(("return "+false+"("+false+"()+[])").split(false).join("RegExp"))()+[])[1]
  ([]["flat"]["constructor"](("return "+false+"("+false+"()+[])")["split"](false)["join"](([]+("")["matchAll"]())["split"](" ")[1]))()+[])[1]

Using this we can chave access to any character by String.fromCharCode - e.g for "!" (code 33):

String[Function("return Object.entries({from\\u0043har\\u0043ode:false})")()[0][0]](33)

// after optimization:
String[Function(("return Object.entries({from"+false+"har"+false+"ode"+":")["split"](false)["join"]("\\u0043")+false+"})")()[0][0]](33)

Credits

Thanks to aemkei for creating jsfuck, and for hazzik who kindly aswer my questions in issues in jsfuck repo (like this)

Big thanks for below StackOverflow comunity members who help me solve jsfuck problems during deconverter writing

• trincot for answers: 1, 2, 3 and 4
• Siguza for answer 1
• Jonas Wilms for answer 1