The toString method for a Number object is returning the string 'undefined' (a string literal, not the undefined object) instead of the string representation of the number in question. This can be tested with a literal: (5).toString(); Or with a variable: var test = 5; test.toString(); And occurs in both in code running on a page, in the built-in JS console, and in Firebug, with Firefox versions 40.0.3 and 41.0. Expected result would be the string literal "5". (Which is exactly what I get with other browsers.) This does not conform to the spec located here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toString Is this a known defect? If so, is there a recommended workaround? UPDATE: Looks like this is specific to a particular site, so I presume the client has implemented something that overrides the toString method. Feel free to close and ignore this request.