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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Identifying special values
Date: Wed, 20 Sep 2017 00:16:19 +0200
Organization: PointedEars Software (PES)
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Stefan Ram wrote:

> |0 Test:
> |
> |if(variable === 0 && typeof variable === "number")
> 
>   Is there any value in keeping these notes?

Regarding *this* note, I do not think so.  “===” explicitly does _not_ do 
type conversion:

  var variable = {valueOf: function () { return  0; }};

  /* true */
  variable == 0;

  /* false */
  variable === 0;

It is possible that you had changed “==” to “===” and kept the previously 
useful “typeof” test.

You should insert a space after “if”, though, to avoid the appearance of a 
function call.
 
>   The tests seem to be too complicated!

ACK.
 
>   (Assuming no one has redefined the value of »undefined«,
>   otherwise we could use »void 0«.)

ACK.  It should be noted that, while in newer implementations assignment to 
the “undefined” property of the global object has no effect in non-strict 
mode and throws a TypeError in strict mode (because it is read-only)¹, it is 
still possible to have a variable (in both modes) or user-defined property 
(in non-strict mode, using the “with” statement) in the scope chain –

  var undefined = 23;

− because “undefined” is _not_ a reserved word².  (I think this should be 
fixed at least for strict mode.)

________
¹  <http://www.ecma-international.org/ecma-262/5.1/#sec-15.1.1>
   <http://www.ecma-international.org/ecma-262/5.1/#sec-11.13.1>
   <http://www.ecma-international.org/ecma-262/8.0/#sec-undefined>
   <http://www.ecma-international.org/ecma-262/8.0/#sec-assignment-operators-runtime-semantics-evaluation>
   (tested in Chromium 57.0.2987.98 on Debian/Devuan GNU/Linux)
²  <http://www.ecma-international.org/ecma-262/5.1/#sec-7.6.1>
   <http://www.ecma-international.org/ecma-262/8.0/#sec-reserved-words>
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