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Re: Alignment of three tables

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Eric Bednarz wrote:

> Stefan Weiss <krewecherl@gmail.com> writes:
>> On 2013-06-11 05:09, Thomas 'PointedEars' Lahn wrote:
>>> <http://www.ecma-international.org/ecma-262/5.1/#sec-11.6.1>
>>
>> There's one thing I don't understand about that algorithm:
>>
>>    {} + ""
> 
> That's a block statement followed by an unary + operator with the empty
> string as its operand.

Very good.  I had not thought of that.  To prove that ES 5.1, “11.6.1 The 
Addition operator” is properly implemented as concatenation operator:

  /* "[object Object]", as Stefan expected */
  var x = {} + "";

or

  /* "[object Object]" */
  ({}) + ""

or

  /* "[object Object]" */
  new Object() + ""

The curious – maybe not obvious – thing here is, though, that it is _not_ 
the "" that causes the "[object Object]" result, but the conversion of the 
Object instance to a primitive value.

The ToPrimitive() algorithm (§9.1) in step 5 –

| 5. Let lprim be ToPrimitive(lval).

– calls the [[DefaultValue]] internal method, passing the PreferredType on 
as argument for the hint.  As there is no PreferredType, there is no hint.

Object instances are native objects.  Their [[DefaultValue]] method is 
specified in ECMAScript (§8.12.8).  Calling it without hint works as if it 
was called with hint Number.  Thus, it attempts to converts its object 
argument to a primitive value by attempting to call its valueOf() method 
first.  Object instances inherit from Object.prototype a valueOf() method 
through the prototype chain; it returns a reference to the same object.  
That is *not* a primitive value.

If the valueOf() method of the object cannot be called or does not return a 
primitive value, [[DefaultValue]] attempts to call the object's toString() 
method.  Object instances inherit from Object.prototype a toString() method 
through the prototype chain which always returns a primitive string value 
that is the equivalent of "[object " + [[Class]] + "]" (we can use it to 
tell the [[Class]] of an object, for example for Array.isArray() emulation, 
as implemented by jsx.object.getClass()).  Because that return value 
satisfies the condition in §8.12.8, hint Number, step 4b, that is the return 
value of [[DefaultValue]] and of ToPrimitive().

That in turn satisfies the condition in §11.6.1, step 7, where it says:

| 7. If Type(lprim) is String or Type(rprim) is String, then […]

It follows from logical shortcut evaluation that it does not matter that 
Type("") is String, because Type(({}).toString()) already is String.  As a 
result, we observe in conforming implementations:

  /* "[object Object]23" */
  ({}) + 23

And because of

| 8. Return the result of applying the addition operation to
|    ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

  /* 42 */
  ({valueOf: function () { return 19; }}) + 23

[A possible application of the latter is:

  function Complex (re, im)
  {
    /* or throw exceptions */
    this.re = (re ? +re : 0)
    this.im = (im ? +im : 0);
  }

  /**
   * @overrides Object.prototype.valueOf()
   */
  Complex.prototype.valueOf = function () {
    if (!this.im)
    {
      return this.re;
    }

    return this;
  };

  /* 42 */
  new Complex(19) + 23

  /* 42 */
  new Complex(19) + new Complex(23)

Unfortunately, without the possibility of operator overloading the following 
does not work:

  /* _not_ new Complex(42, 42) */
  new Complex(19, 42) + new Complex(23)

But you can work around it:

  Complex.prototype.add = function (o) {
    /* or “Complex” if you do not care for polymorphism */
    var C = this.constructor;

    if (o instanceof C)
    {
      return new C(this.re + o.re, this.im + o.im);
    }

    return new C(this.re + (+o), this.im);
  };

  /* new Complex(42, 42) */
  (new Complex(19, 42)).add(new Complex(23));
]

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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Thread

Alignment of three tables "Mel Smith" <med_cutout_syntel@aol.com> - 2013-06-09 08:49 -0600
  Re: Alignment of three tables Scott Sauyet <scott.sauyet@gmail.com> - 2013-06-09 08:18 -0700
  Re: Alignment of three tables Denis McMahon <denismfmcmahon@gmail.com> - 2013-06-09 20:02 +0000
    Re: Alignment of three tables "Mel Smith" <med_cutout_syntel@aol.com> - 2013-06-09 15:01 -0600
      Re: Alignment of three tables Denis McMahon <denismfmcmahon@gmail.com> - 2013-06-09 22:50 +0000
    Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-10 00:46 +0200
      Re: Alignment of three tables John Harris <niam@jghnorth.org.uk.invalid> - 2013-06-10 14:41 +0100
        Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-10 17:35 +0200
      Re: Alignment of three tables "Mel Smith" <med_cutout_syntel@aol.com> - 2013-06-10 09:08 -0600
        Re: Alignment of three tables "Mel Smith" <med_cutout_syntel@aol.com> - 2013-06-10 11:16 -0600
          Re: Alignment of three tables Denis McMahon <denismfmcmahon@gmail.com> - 2013-06-11 02:05 +0000
            Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-11 05:09 +0200
              Re: Alignment of three tables "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2013-06-11 10:59 +0200
                Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-11 16:51 +0200
              Re: Alignment of three tables Stefan Weiss <krewecherl@gmail.com> - 2013-06-14 15:38 +0200
                Re: Alignment of three tables Eric Bednarz <bednarz@fahr-zur-hoelle.org> - 2013-06-14 20:26 +0200
                Re: Alignment of three tables Stefan Weiss <krewecherl@gmail.com> - 2013-06-14 21:22 +0200
                Re: Alignment of three tables Eric Bednarz <bednarz@fahr-zur-hoelle.org> - 2013-06-15 00:36 +0200
                Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-15 12:04 +0200
                Re: Alignment of three tables Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-15 11:41 +0200
                Re: Alignment of three tables Stefan Weiss <krewecherl@gmail.com> - 2013-06-15 13:53 +0200
                Re: Alignment of three tables Scott Sauyet <scott.sauyet@gmail.com> - 2013-06-17 04:28 -0700
  Re: Alignment of three tables "Andy" <andy@NOSPAMmanyplay.com> - 2013-06-10 09:19 +0100
  Re: Alignment of three tables Dr J R Stockton <reply1300@merlyn.demon.co.uk.invalid> - 2013-06-12 18:56 +0100

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