Question

I was reading some source code of a firefox extension and i saw some expression like

0*this;

dose this expression means anything ?

ps: I’m chinese, if there is some thing wrong with my English, please tell me, Thanks!

Here is a function of the extension;

function inGetter(){
   0*this;
   var gin=  Components.lookupMethod(this,"innerHTML")();
   gin=String.newTainted(gin,"divElement.INNERHTML");

   if(__domIntruderObj.settings.enabled  )
      __domIntruderObj.log("Getter",this.tagName+".value",gin, __domIntruderObj.util.getCallStack(arguments));
   return gin;
}

Answer 1

The code is part of an XSS detection Firefox extension.

As mentioned in another answer, multiplication causes a call to the engine’s internal DefaultValue method. What happens there is: it tries valueOf and toString before throwing an error. For example,

2 * { valueOf: function () { return 3; }} // 6
2 * { toString: function () { return '4'; }} // 8
2 * { toString: function () { return this; }} // TypeError

So this side-effect can be used to gain some information on the object. Particularly, there is some custom toString implementation in the code, which I think this is used to generate some call stack log. I haven’t really gone into the details though.