You're probably right in that it could be hard to cover the spec 100%
with a polyfill. But then, most vendor implementations don't, so the aim
might be to have a "good enough" implementation, no?!
So what is your argument here?! Not to provide polyfills at all? Not
polyfil Function.bind()? Or just caution against making "100%" the
threshold for implementing them?
T.
On 07/24/2012 02:49 PM, Derrell Lipman wrote:
On Tue, Jul 24, 2012 at 7:11 AM, Martin Wittemann
<[email protected] <mailto:[email protected]>> wrote:
2. We supply a buggy polyfill and another lib will fail
Thats nothing we have planed. We should take care of implementing
the spec 100%. If thats not possible, we don't add a polyfill
because that can cause problems.
Polyfills are not necessarily able to implement the spec 100%. A good
case in point is Function.bind() for which a pretty good polyfill is
available, but there are cases that some 3rd party libraries (or the
user!) may depend on the native functionality and not get it.
Here is an excerpt from
https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind which
discusses some issues of the polyfill for bind(). In particular, the
last two bullet points are relevant:
Some of the many differences (there may well be others, as this
list does not seriously attempt to be exhaustive) between this
algorithm and the specified algorithm are:
* The partial implementation relies |Array.prototype.slice|,
|Array.prototype.concat|, |Function.prototype.call| and
|Function.prototype.apply|, built-in methods to have their
original values.
* The partial implementation creates functions that do not have
immutable "poison pill" |caller| and |arguments| properties
that throw a |TypeError| upon get, set, or deletion. (This
could be added if the implementation supports
|Object.defineProperty|
<https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/defineProperty>,
or partially implemented [without throw-on-delete behavior] if
the implementation supports the|__defineGetter__|
<https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/defineGetter>
and
|__defineSetter__|
<https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/defineSetter>
extensions.)
* The partial implementation creates functions that have a
|prototype| property. (Proper bound functions have none.)
* The partial implementation creates bound functions whose
|length| property does not agree with that mandated by
ECMA-262: it creates functions with length 0, while a full
implementation, depending on the length of the target function
and the number of pre-specified arguments, may return a
non-zero length.
If you choose to use this partial implementation, *you must not
rely on those cases where behavior deviates from ECMA-262, 5th
edition!* With some care, however (and perhaps with additional
modification to suit specific needs), this partial implementation
may be a reasonable bridge to the time when |bind()| is widely
implemented according to the specification.
Cheers,
Derrell
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