JavaScript學習：JSFuck代碼閱讀筆記
來自： http://my.oschina.net/Tsybius2014/blog/614234
JSFuck源碼地址（GitHub）：https://github.com/aemkei/jsfuck
JSFuck在OSC上的介紹頁面：http://www.oschina.net/p/jsfuck
JSFuck可以將JavaScript代碼進行轉換，轉換后的代碼只使用6個字符（[，]，(，)，!，+），實現的功能和轉換前代碼是一樣的。出于好奇和學習的目的，我研究了一下JSFuck的源碼。
在網站 http://www.jsfuck.com/ 中有一個例子，將JavaScript語句 alert(1) 轉換為只由六種字符的版本：
將這段代碼放到HTML文件的script標簽下，就可以運行了：
<html> <head> <title>happy new year</title> </head> <body> <script> [][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]][([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]((![]+[])[+!+[]]+(![]+[])[!+[]+!+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]+(!![]+[])[+[]]+(![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]]+[+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]])() </script> </body> </html>
不過，我想了解的是，為什么這段代碼會起作用。研究后才發現，這段代碼等價于另一段代碼：
[]["filter"]["constructor"]("return eval")()("alert(1)") 在這段代碼中，alert(1)是我們要轉換的JavaScript源碼，這段代碼的意思是執行JavaScript代碼alert(1)。這段代碼是由四個字符串和字符[、]、(、)構成的。也就是說，我們只要能把字符串中的每一個字符，都用[，]，(，)，!，+這六個字符表示出來，那么我們就完全可以將任何一段JavaScript代碼，找到僅用這六個字符表示的等價形式。
那么，我們來看一下各個字符的等價形式：（如果用console.log輸出右邊的部分，則會返回左邊的字符，0-9這10個字符外面又套了一層[]，這樣做是為了保證在之后用+運算符進行拼接時不被系統識別為加法運算）
'0':'[+[]]'
'1':'[+!+[]]'
'2':'[!+[]+!+[]]'
'3':'[!+[]+!+[]+!+[]]'
'4':'[!+[]+!+[]+!+[]+!+[]]'
'5':'[!+[]+!+[]+!+[]+!+[]+!+[]]'
'6':'[!+[]+!+[]+!+[]+!+[]+!+[]+!+[]]'
'7':'[!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]]'
'8':'[!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]]'
'9':'[!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]+!+[]]'
'a':'(false+"")[1]'
'b':'(Function("return{}")()+"")[2]'
'c':'([]["filter"]+"")[3]'
'd':'(undefined+"")[2]'
'e':'(true+"")[3]'
'f':'(false+"")[0]'
'g':'(false+[0]+String)[20]'
'h':'(+(101))["toString"](21)[1]'
'i':'([false]+undefined)[10]'
'j':'(Function("return{}")()+"")[10]'
'k':'(+(20))["toString"](21)'
'l':'(false+"")[2]'
'm':'(Number+"")[11]'
'n':'(undefined+"")[1]'
'o':'(true+[]["filter"])[10]'
'p':'(+(211))["toString"](31)[1]'
'q':'(+(212))["toString"](31)[1]'
'r':'(true+"")[1]'
's':'(false+"")[3]'
't':'(true+"")[0]'
'u':'(undefined+"")[0]'
'v':'(+(31))["toString"](32)'
'w':'(+(32))["toString"](33)'
'x':'(+(101))["toString"](34)[1]'
'y':'(NaN+[Infinity])[10]'
'z':'(+(35))["toString"](36)'
'A':'(+[]+Array)[10]'
'B':'(+[]+Boolean)[10]'
'C':'Function("return escape")()(("")["italics"]())[2]'
'D':'Function("return escape")()([]["filter"])["slice"]("-1")'
'E':'(RegExp+"")[12]'
'F':'(+[]+Function)[10]'
'G':'(false+Function("return Date")()())[30]'
'H':'Function("return unescape")()("%"+(48)+"")'
'I':'(Infinity+"")[0]'
'J':'Function("return unescape")()("%"+(4)+"a")'
'K':'Function("return unescape")()("%"+(4)+"b")'
'L':'Function("return unescape")()("%"+(4)+"c")'
'M':'(true+Function("return Date")()())[30]'
'N':'(NaN+"")[0]'
'O':'(NaN+Function("return{}")())[11]'
'P':'Function("return unescape")()("%"+(50)+"")'
'Q':'Function("return unescape")()("%"+(51)+"")'
'R':'(+[]+RegExp)[10]'
'S':'(+[]+String)[10]'
'T':'(NaN+Function("return Date")()())[30]'
'U':'(NaN+Function("return{}")()["toString"]["call"]())[11]'
'V':'Function("return unescape")()("%"+(56)+"")'
'W':'Function("return unescape")()("%"+(57)+"")'
'X':'Function("return unescape")()("%"+(58)+"")'
'Y':'Function("return unescape")()("%"+(59)+"")'
'Z':'Function("return unescape")()("%"+(5)+"a")'
' ':'(NaN+[]["filter"])[11]'
'!':'Function("return unescape")()("%"+(21)+"")'
'"':'("")["fontcolor"]()[12]'
'#':'Function("return unescape")()("%"+(23)+"")'
'$':'Function("return unescape")()("%"+(24)+"")'
'%':'Function("return escape")()([]["filter"])[20]'
'&':'("")["link"](0+")[10]'
''':'Function("return unescape")()("%"+(27)+"")'
'(':'(false+[]["filter"])[20]'
')':'(true+[]["filter"])[20]'
'*':'Function("return unescape")()("%"+(2)+"a")'
'+':'(+(+!+[]+(!+[]+[])[!+[]+!+[]+!+[]]+[+!+[]]+[+[]]+[+[]])+[])[2]'
',':'([]["slice"]["call"](false+"")+"")[1]'
'-':'(+(.+[0000000001])+"")[2]'
'.':'(+(+!+[]+[+!+[]]+(!![]+[])[!+[]+!+[]+!+[]]+[!+[]+!+[]]+[+[]])+[])[+!+[]]'
'/':'(false+[0])["italics"]()[10]'
':':'(RegExp()+"")[3]'
';':'("")["link"](")[14]'
'<':'("")["italics"]()[0]'
'=':'("")["fontcolor"]()[11]'
'>':'("")["italics"]()[2]'
'?':'(RegExp()+"")[2]'
'@':'Function("return unescape")()("%"+(40)+"")'
'[':'(Function("return{}")()+"")[0]'
'\':'Function("return unescape")()("%"+(5)+"c")'
']':'(Function("return{}")()+"")["slice"]("-1")'
'^':'Function("return unescape")()("%"+(5)+"e")'
'_':'Function("return unescape")()("%"+(5)+"f")'
'`':'Function("return unescape")()("%"+(60)+"")'
'{':'(NaN+[]["filter"])[21]'
'|':'Function("return unescape")()("%"+(7)+"c")'
'}':'([]["filter"]+"")["slice"]("-1")'
'~':'Function("return unescape")()("%"+(7)+"e")' 要將上面的字符轉換成六種字符的等價形式，還需要用到下面的幾個等價形式帶入解決：
var SIMPLE = {
'false': '![]',
'true': '!![]',
'undefined': '[][[]]',
'NaN': '+[![]]',
'Infinity': '+(+!+[]+(!+[]+[])[!+[]+!+[]+!+[]]+[+!+[]]+[+[]]+[+[]]+[+[]])' // +"1e1000"
};
var CONSTRUCTORS = {
'Array': '[]',
'Number': '(+[])',
'String': '([]+[])',
'Boolean': '(![])',
'Function': '[]["filter"]',
'RegExp': 'Function("return/"+false+"/")()'
}; 使用SIMPLE和CONSTRUCTORS內的等價形式，即可推算出上面那些字符的內容。比如a的等價形式是(false+"")[1]，將false替換為![]，將""替換為[]，將1替換為+!+[]，就可以推算出a使用六個字符表示的等價形式為(![]+[])[+!+[]]。（PS：從中不難發現作者aemkei真是用心良苦）
這樣看來，以alert(1)為例，每個字符的等價形式如下：
'a':'(![]+[])[+!+[]]'
'l':'(![]+[])[!+[]+!+[]]'
'e':'(!![]+[])[!+[]+!+[]+!+[]]'
'r':'(!![]+[])[+!+[]]'
't':'(!![]+[])[+[]]'
'(':'(![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]]'
'1':'[+!+[]]'
')':'(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]])[!+[]+!+[]+[+[]]]' 將它們用符號“+”連接起來，就可以獲取"alert(1)"的等價形式了！同理我們獲得了"filter"、"constuctor"、"return eval"的等價形式，再回頭看看這段代碼：
[]["filter"]["constructor"]("return eval")()("alert(1)") 將字符串替換后，放入HTML中的script標簽里面，或是放到一個js文件里面，就可以用瀏覽器打開HTML文件查看效果啦！
（圖中瀏覽器版本為：Google Chrome 46.0.2490.80 m）
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