1170752 - Strict mode property uniqueness

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[kurienjoseph]

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0
Build ID: 20150525141253

Steps to reproduce:

Run the following code. 
"use strict";
var MYCHANGES = (function() {
  "use strict";
  var self = {};
  self.valueobj = { p: true, p: false };
    self.show = function () {
        alert(self.valueobj.p);
                       
    };
return self;
}());
MYCHANGES.show();


Actual results:

The program ran normally


Expected results:

As I had the use strict clause, I am expecting an error on duplicate attribute name.
Firefox 29 correctly throws the correct syntax error message
SyntaxError: property name p appears more than once in object literal

The new releases do not throw this error
config:option value
javascript.options.strict;true

Firefox website states the following:

Fourth, strict mode requires that all properties named in an object literal be unique. Normal code may duplicate property names, with the last one determining the property's value. But since only the last one does anything, the duplication is simply a vector for bugs, if the code is modified to change the property value other than by changing the last instance. Duplicate property names are a syntax error in strict mode:

"use strict";
var o = { p: 1, p: 2 }; // !!! syntax error

Comment 1

[Tooru Fujisawa [:arai]]

This is introduced by bug 1041128, which intentionally removed the error.  We need to fix the documentation.
  https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Strict_mode

Comment 2

[Tooru Fujisawa [:arai]]

Added note about the change.