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Re: Overriding function and adding a bit

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Thomas 'PointedEars' Lahn wrote:

>   ,-------------------------.  :
>   | global : object[Object] |  :

This should be

>   | : object[global] |

at least for V8 in Node.js.

However, in Chromium the global object appears to have "Window" as the value 
of its @@toStringTag internal property.

Since ECMAScript 2015, the concept of the @@toStringTag property of an 
object replaces the concept of the internal [[Class]] property for object 
types that are not explicitly recognized by the changed 
Object.prototype.toString() algorithm.

For objects referred by “obj” that have properties and those whose type is 
explictly recognized by the algorithm, obj[Symbol.toStringTag] is the 
“undefined” value.  So obj[Symbol.toStringTag] could provide a more 
efficient to retrieve the equivalent of the former internal [[Class]] 
property than parsing the return value of ({}).toString.call(obj), provided 
that the @@toStringTag property value has not been modified.

However, the Specification explicitly mentions the possibility that programs 
can modify the @@toStringTag property value which makes “legacy type tests” 
unreliable.

Example:

  var o = {
    label: "crook",
    exec: function () {
      /* there’s no there there */
      return null;
    }
  };

  /* I’m not a crook^W^W^W a RegExp, _believe me_[tm]! */
  o[Symbol.toStringTag] = "RegExp";

  /* [object RegExp] */
  ({}).toString.call(o);

Such lies can be thwarted:

  function dontLieToMe (obj)
  {
    var desc = Object.getOwnPropertyDescriptor(o, Symbol.toStringTag);

    return Object.defineProperty(o, Symbol.toStringTag, {
      /* “writable: false, configurable: false” is implicit */
      value: desc ? desc.value : void 0
    });
  }

  var o = {
    label: "crook",
    exec: function () {
      /* there’s no there there */
      return null;
    }
  };

  dontLieToMe(o);

  /* I’m not a crook^W^W^W a RegExp, _believe me_[tm]! */
  o[Symbol.toStringTag] = "RegExp";

  /* [object Object] */
  ({}).toString.call(o);

  /* “Uncaught TypeError: Cannot redefine property:
     Symbol(Symbol.toStringTag)” :-b */
  Object.defineProperty(o, Symbol.toStringTag, {
    value: desc ? desc.value : void 0,
    writable: true
  });

But in the end, duck typing may always be the best approach ;-)
 
<http://www.ecma-international.org/ecma-262/6.0/#sec-object.prototype.tostring>
<http://www.ecma-international.org/ecma-262/7.0/#sec-object.prototype.tostring>

-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | <http://PointedEars.de/es-matrix>
<https://github.com/PointedEars> | <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | Please do not cc me./Bitte keine Kopien per E-Mail.

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Thread

Overriding function and adding a bit Andrew Poulos <ap_prog@hotmail.com> - 2017-05-10 11:59 +1000
  Re: Overriding function and adding a bit Doc O'Leary  <droleary@2017usenet1.subsume.com> - 2017-05-10 15:33 +0000
    Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-10 19:14 +0200
      Re: Overriding function and adding a bit Doc O'Leary  <droleary@2017usenet1.subsume.com> - 2017-05-10 22:57 +0000
        Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-11 01:49 +0200
          Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-11 17:03 +0200
            Re: Overriding function and adding a bit Andrew Poulos <ap_prog@hotmail.com> - 2017-05-12 08:32 +1000
              Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-12 22:12 +0200
            Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-12 03:13 +0200
          Re: Overriding function and adding a bit John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-11 16:33 +0100
            Re: Overriding function and adding a bit Tim Streater <timstreater@greenbee.net> - 2017-05-11 18:54 +0100
          Re: Overriding function and adding a bit Doc O'Leary  <droleary@2017usenet1.subsume.com> - 2017-05-11 22:02 +0000
            Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-12 01:53 +0200
              Re: Overriding function and adding a bit John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-12 10:05 +0100
              Re: Overriding function and adding a bit Doc O'Leary  <droleary@2017usenet1.subsume.com> - 2017-05-12 21:43 +0000
                Re: Overriding function and adding a bit Tim Streater <timstreater@greenbee.net> - 2017-05-12 23:03 +0100
                Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-12 17:28 -0700
                Re: Overriding function and adding a bit John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-13 14:39 +0100
                Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-13 07:17 -0700
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-13 16:25 +0100
                Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-13 08:30 -0700
                Re: Overriding function and adding a bit Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2017-05-13 17:56 +0200
                Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-13 09:04 -0700
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-13 20:55 +0100
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-14 00:00 +0100
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-14 03:07 +0100
                Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-13 18:39 -0700
                Re: Overriding function and adding a bit John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-14 11:23 +0100
                Re: Overriding function and adding a bit John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-14 11:36 +0100
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-14 12:34 +0100
                Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-14 14:11 +0200
                Re: Overriding function and adding a bit Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-14 15:37 +0100
                Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-14 17:16 +0200
                Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-14 15:40 +0200
          Re: Overriding function and adding a bit Gene Wirchenko <genew@telus.net> - 2017-05-11 15:39 -0700
        Re: Overriding function and adding a bit Tim Streater <timstreater@greenbee.net> - 2017-05-11 10:39 +0100
    Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-11 10:05 -0700
  Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-10 19:13 +0200
    Re: Overriding function and adding a bit Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-10 19:22 +0200
    Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-11 10:08 -0700
  Re: Overriding function and adding a bit Julio Di Egidio <julio@diegidio.name> - 2017-05-11 09:56 -0700

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