How to convert anything to a String safely in JavaScript

Question

If I have:

var test = {toString: function(){alert("evil code"); return "test";}};

how can I convert test to a string? without calling test.toString() and without using a typeof x == "string" check since I want to allow non strings.

Note: this is for a FF extension dealing with objects from a content page's js scope.

Answer 1

JavaScript allows you to modify the properties of pretty much any object that is accessible to your script, including Object.prototype itself, meaning any object is vulnerable to "evil code" in the manner that you explained.

Only primitives are guaranteed to be safe, so the only way to ensure that "evil code" is never executed is to do something like this:

function safeToString(x) {
  switch (typeof x) {
    case 'object':
      return 'object';
    case 'function':
      return 'function';
    default:
      return x + '';
  }
}

Answer 2

One option is:

Object.prototype.toString.call(test)

This gives:

"[object Object]"

in the case of test. Basically, it just gives type information. However, I wonder what the exact scenario is here. How is the evil object getting loaded into the page? If they can execute arbitrary code on the page, you're basically out of luck. Among other things, it is then possible to redefine Object.prototype.toString.