the opposite of

deobfuscator.rb

require 'erb'

src = File.read(ARGV[0])

data_segment = src.split(/(;)/).first
code_segment = src.split(/(;)/)[1..-1].join
data_segment.match(/var (_0x\w*{4})=/)
holder_var = $1

$strings_arr = eval(data_segment.split('=').last)

regex = Regexp.new("#{holder_var}\\[(\\d{1,2})\\]")

prepared_erb = code_segment.gsub(regex, "\"<%=convert(\\1)%>\"")

def convert(i)
  $strings_arr[i]
end

replaced_src =  ERB.new(prepared_erb).result.sub(';','')

puts replaced_src

example.js

// original source
//
// var a="Hello World!";
// function MsgBox(msg)
// {
//     alert(msg+"\n"+a);
// }
// MsgBox("OK");

//obfuscated with http://www.javascriptobfuscator.com/default.aspx
var _0x70aa=["\x48\x65\x6C\x6C\x6F\x20\x57\x6F\x72\x6C\x64\x21","\x0A","\x4F\x4B"];var a=_0x70aa[0];function MsgBox(_0xd4fcx3){alert(_0xd4fcx3+_0x70aa[1]+a);} ;MsgBox(_0x70aa[2]);

output.js

//output of ruby deobfuscator.rb example.js > output.js
var a="Hello World!";function MsgBox(_0xd4fcx3){alert(_0xd4fcx3+"
"+a);} ;MsgBox("OK");