Draft of Function.prototype.bind.

David-Sarah Hopwood david.hopwood at industrial-designers.co.uk
Mon Nov 10 18:20:52 PST 2008


Waldemar Horwat wrote:
> Mark S. Miller wrote:
[Kona 15.3.4.5]
>>         11. If the [[Class]] property of /G/ is *"Function"*, then
>>                1. Get the *length* property of /G/.
>>                2. Let /L/ be Result(11a) minus the length of /A/.

[These substeps are actually labelled a. and b.]

> What does "minus" mean here?  Result(11a) could be anything.

The 'length' property of a built-in object with [[Class]] "Function"
is not Writable or Configurable [Kona 15.3.5.1], so it must be equal
to the number it was set to in either [Kona 13.2 step 7] or
[Kona 15.3.4.5 step 11c or 12], which are the only places where
built-in objects of [[Class]] "Function" are created (and [[Class]]
cannot be changed after creation). But that should be explained in a
NOTE section.

Caveat:
[section 15]
# The value of the [[Class]] property of a host object may be any value,
# even a value used by a built-in object for its [[Class]] property.

So any random host object might have [[Class]] "Function" (which is a
bug). If that is the case, all bets are off (and in that case steps 11
and 15 of the 'bind' algorithm are broken anyway -- for example, the
'length' or 'prototype' properties of G might not exist).


Incidentally, the following sentence in section 15:

# Unless specified otherwise, the [[Class]] property of a built-in
# object is "Function" if that built-in object has a [[Call]]
# property [...]

is misleading because:

# The value of the [[Class]] property is defined by this specification
# for every kind of built-in object.

i.e. there is no default for [[Class]] in the case of built-in objects.
That is what allowed me to make the above assertions for built-in
objects, otherwise I'd have had to read the entire spec rather than
just searching for "Function". The sentence that implies that there
are defaults for [[Class]] should be removed.

-- 
David-Sarah Hopwood


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