more JSON spec questions

Hallvord R. M. Steen hallvord at
Wed Aug 26 15:03:56 PDT 2009

the feedback below refers to the "Initial ECMA formated ES5 draft" and  
consists of a few things I've noticed while working on a JSON test suite.

First, I think considerable work has gone into the JSON part of the spec  
since I first looked at it - it's beginning to look really good. My test  
suite is a work in progress, and of course both the tests and my questions  
might just show that there's something I haven't entirely understood  
rather than problems with the specification. Feedback is welcome.

Now, the questions..

1) Apparently the grammar still disallows literal tab inside JSONString.
Implementations that pass this test allow it:
- from earlier discussions I had the impression that the spec would  

2) The grammar and an existing test in the test suite disallows  
final commas in array input (JSON.parse('[1,]') should apparently throw) -  
IE8, Safari 4 and Firefox 3.5 all seem to happily accept it though, as  
does the yet-to-go-public Opera implementation. I'd like some reassurance  
 from other browser vendors that you consider this a bug and intend to fix  
it before I push for a fix here because this is the kind of thing that  
might cause compat problems :-o (Guess this is more a question for the  
implementers on the list than for the spec authors.)

3) Should JSON.parse() without arguments throw? It's not obvious from the  
spec as far as I can tell, but it's what most browser implementations do.

4) (Editorial) - spec says "The abstract operation Str(key, holder) has  
access  to
PropertyList and ReplacerFunction" - the algorithm does not actually use
PropertyList (very trivial issue, of course)

5) (Editorial) It's not apparent from the prose description of  
JSON.stringify() that if the replacer argument is an array, it will only  
be applied to objects. I expected to be able to do something like  
JSON.stringify( [0,1,2], [1] ) => '[1]'. Suggest a small clarification,  
perhaps just saying "selecting object keys that will be stringified"  
instead of "selecting the keys that will be stringified"

6) (Editorial) Per the algorithm, passing in a "replacer" array to  
JSON.stringify() will cause it to also stringify dontEnum properties. An  
example would be JSON.stringify( Number, ['MAX_VALUE'] ) including the  
MAX_VALUE property where JSON.stringify(Number) would not. Perhaps clarify  
this in the prose too, not just in the algorithm? Calling it a "whitelist"  
seems it will only narrow down the number of elements that are included,  
not expand it..

7) I also didn't understand why the "reviver" function was called once  
with "key" being the empty string and "value" being the entire output  
object until I read the algorithm carefully.

Quick word about the test suite: it's current home is where you can have a look at the README,  
load runner.htm to run all tests (Warning: if you do that in IE8 something  
makes it eat all your memory and make the PC entirely unusable!) or browse  
through folders to individual tests. Hope it's useful to all you  
implementers out there. Feedback welcome :)

Hallvord R. M. Steen, Core Tester, Opera Software

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