Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.javascript > #33455

Re: x === x

Show all headers | View raw

Dr J R Stockton wrote:

> […] Thomas 'PointedEars' Lahn wrote:
>> This should be obvious even to you: The return values of consecutive
>> calls of Math.random() may be, but are not supposed to be, equal.  After
>> all, the method is supposed to generate *random* numbers.
> 
> Standard ECMA-262, 6th Edition / June 2015, HTML, includes a
> rather woolly "definition" :-

That may be so, however the current Edition is the 7th (ECMAScript 2016).
 
> […] 
>> > NaN === NaN
>> 
>> This evaluating to “false” is due to the implementation of the ECMAScript
>> Number type as a type of IEEE 754 “double-precision” floating-point
>> values.
> 
> I think not. IEEE 754 provides that a large number of 64-bit
> patterns represent NaN, and I think it provides that all of
> the patterns have the same arithmetic value. […]

IEEE 754-1985 (at least) and (probably) IEEE 754-2008 (too) basically 
specify that (the) NaN (values) is/are not equal to any other floating-point 
value.  That is, “In the syntax of C, the predicate x != y is True but all 
others, x < y , x <= y , x == y , x >= y and x > y, are False whenever x or 
y or both are NaN” [1].

That is one reason why ECMAScript 2015 (6th Ed.) introduced

  Object.is(NaN, NaN) === true

<http://www.ecma-international.org/ecma-262/6.0/#sec-object.is>
<http://www.ecma-international.org/ecma-262/7.0/#sec-object.is>

(I thought that would be common knowledge around here.)

> The indistinguishability and inequality of NaNs in JavaScript
> results from a choice taken by ECMA or by its relevant
> predecessors.

You are very likely mistaken there.

_________
[1] Kahan, William. "IEEE standard 754 for binary floating-point 
    arithmetic." Lecture Notes on the Status of IEEE 754.94720-1776 (1996):
    11.  
<http://li.mit.edu/Archive/Activities/Archive/CourseWork/Ju_Li/MITCourses/18.335/Doc/IEEE754/ieee754.pdf>

      [Et al.  I would rather have referred to the section of the standard 
       itself, but its text is not freely available from the original
       publisher:

       <http://ieeexplore.ieee.org/document/4610935/?part=1>

       I think it is a disgrace for organizations like the IEEE and ISO 
       to publish an industry standard and then hide it behind a paywall
       *despite* having a paying membership already.  Kudos to organizations
       like Ecma International, ISOC, and the W3C, that are not doing this.]
-- 
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.

Back to comp.lang.javascript | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: x === x Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-30 16:21 +0200
  Re: x === x John G Harris <niam@jghnorth.org.uk.invalid> - 2017-05-30 19:16 +0100
  Re: x === x Dr J R Stockton <J.R.Stockton@physics.org> - 2017-05-31 08:39 -0700
    Re: x === x Stefan Weiss <krewecherl@gmail.com> - 2017-05-31 18:08 +0200
      Re: x === x Dr J R Stockton <reply1700@merlyn.demon.co.uk.invalid> - 2017-06-02 22:27 +0100
    Re: x === x Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-31 21:03 +0200
      Re: x === x Dr J R Stockton <reply1700@merlyn.demon.co.uk.invalid> - 2017-06-02 22:12 +0100
    Re: x === x Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-05-31 21:09 +0200
      Re: x === x Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-01 01:58 +0100
        Re: x === x Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-06-01 05:01 +0200
          Re: x === x Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-01 14:44 +0100
          Re: x === x Dr J R Stockton <reply1700@merlyn.demon.co.uk.invalid> - 2017-06-02 21:15 +0100
        Re: x === x Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-01 14:17 +0100
    Re: x === x Julio Di Egidio <julio@diegidio.name> - 2017-06-01 00:10 -0700
      Re: x === x Dr J R Stockton <reply1700@merlyn.demon.co.uk.invalid> - 2017-06-02 22:16 +0100

csiph-web