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Re: Unexpected value when using "<"

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RobG wrote:

> Given the comparison:
> 
>   'foo' < 2
>   
> I was expectingt the result to be *undefined*, based on the abstract
> relational comparison algorithm[1].

A relational expression never evaluates to the “undefined” value; its 
evaluation result is (as it must be) of type Boolean.
 
> In step 3a, the left hand side is converted to a number using
> ToNumber('foo'), which returns NaN.

Correct.

> Step 3c of the algorithm says:
> 
>   If nx is NaN, return undefined

Also correct.
 
> So I was expecting the result to be *undefined*. The introduction to the
> section also says that the comparison:
> 
>     "...produces true, false, or undefined (which indicates that at least
>     one operand is NaN)".
>   
> However, in the couple of browsers I tested, the result was Boolean false.
> Can someone please explain, or point to a suitable explanation elsewhere,
> why the result is *false* and not *undefined*?

You have overlooked that the Abstract Relational Comparison Algorithm does 
not terminate the algorithm of the operator:

| 11.8.1 The Less-than Operator ( < )
| 
| […]
| 5. Let r be the result of performing abstract relational comparison
|    lval < rval. (see 11.8.5)
| 6. If r is undefined, return false. Otherwise, return r.

IOW, *that* algorithm always returns either “true” or “false”. ∎

Apropos: The ECMAScript Support Matrix includes – and will probably use in 
the future – a draft reference implementation of ECMAScript Ed. 5.1 (and 
probably later of Ed. 6) in ECMAScript which supports a trace mode that is 
intended to make it easier to see such behavior.  It does not include an 
implementation of the less-than operator or the Abstract Relational 
Comparison Algorithm yet.

You can enable trace mode with the assignment “es.enableTrace = true”.  
Specification-related symbols are properties of the object referred to by 
“es”, so you can see what is implemented by typing “es.” in most script 
consoles already (but at the point not necessarily fully).  Selecting an 
item from the usual auto-complete list would display the source code of the 
algorithm implementation (without comments).  The entire source code can be 
found in [2] (which I will switch to Git along with the rest of my public 
repositories).  Feedback and contributions are appreciated.

> 1. http://ecma-international.org/ecma-262/5.1/#sec-11.8.5

[2] <http://PointedEars.de/wsvn/es-matrix/trunk/application/scripts/es.js>
-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
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Thread

Unexpected value when using "<" RobG <rgqld@iinet.net.au> - 2014-11-23 17:45 -0800
  Re: Unexpected value when using "<" Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-11-24 03:37 +0100
    Re: Unexpected value when using "<" RobG <rgqld@iinet.net.au> - 2014-11-23 19:53 -0800
      Re: Unexpected value when using "<" Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-11-24 19:43 +0100

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