Google Closure Compiler - How to create an Extern for a variable (variable name can't change as it is in an Eval)

Question

I am using Google Closure Compiler in "SIMPLE_OPTIMIZATIONS" mode. The JavaScript uses an "Eval" statement with the variable "_u" embedded in the string. When Google Closure Compiler obfuscates the code, the variable name is changed to "a" and I get an error that "_u" is not defined in the console. My understanding is that an Extern will solve this problem, but I'm not sure how to write it. Thoughts?

Code Snippet:

var FuncName = (function(){

  var ht=escape(_w.location.href)

  function _fC(_u){
    _aT=_sp+',\\/,\\.,-,_,'+_rp+',%2F,%2E,%2D,%5F';
    _aA=_aT.split(',');
    for(i=0;i<5;i++){
      eval('_u=_u.replace(/'+_aA[i]+'/g,_aA[i+5])')
    }
    return _u
  };

  return {
  O_LC:function(){ 
    _w.open('https://someurl?referer='+_fC(_ht))
  }
};
})();

After Google Closure Compiler modifies the code:

var FuncName = function() {
  function a(a) {
    _aT = _sp + ",\\/,\\.,-,_," + _rp + ",%2F,%2E,%2D,%5F";
    _aA = _aT.split(",");
    for (i = 0;5 > i;i++) {
      eval("_u=_u.replace(/" + _aA[i] + "/g,_aA[i+5])");
    }
    return a;
  }
  escape(_w.location.href);
  return {O_LC:function() {
    _w.open("https://someurl?referer=" + a(_ht));
  }};
}();

Answer 1

You can only prevent renaming of global variables or properties with externs. Locals will always be renamed. You can modify the code in a number of ways to fix this, including using a function constructor:

new Function("_u", "... function body ...")

But I would rewrite the code to avoid using eval to construct the regex. This would work:

_u=_u.replace(new RegExp(_aA[i], "g"), _aA[i+5]);

You also may be interested in:

Escape string for use in Javascript regex

General documentation regarding the Closure Compiler limitations are here:

https://developers.google.com/closure/compiler/docs/limitations