Variable names in JSON syntax (Javascript)

Why does the following Javascript display "defined/undefined" rather
than  
"undefined/defined"?

	prop='myString'
	var obj={prop:'defined'}
	alert(obj.prop+'/'+obj.myString)

According to any specs/examples I can find, JSON property names should be  
in quotes so wouldn't this mean the unquoted string 'prop' should be  
expanded to its value ('myString'), just like it would using array syntax:

	obj[prop]='value'

On Mon, 16 Jul 2007 15:54:08 +0100, Eik wrote:

> Why does the following Javascript display "defined/undefined"
rather than  
> "undefined/defined"?
> 
> 	prop='myString'
> 	var obj={prop:'defined'}
> 	alert(obj.prop+'/'+obj.myString)

ECMA-262, 11.2.1:

  "    The dot notation is explained by the following
       syntactic conversion:

   MemberExpression .  Identifier

        is identical in its behaviour to

   MemberExpression [ <identifier-string> ]"

In other words, (obj.prop) is identical to (obj['prop'])
which has the value 'defined'.

Try:

    alert (obj[prop]+'/'+obj.myString)

On Fri, 20 Jul 2007 10:30:52 +0100, Nisse Engström  
<news.NOSPAM.gokyoZPGx@luden.se> wrote:

> On Mon, 16 Jul 2007 15:54:08 +0100, Eik wrote:
>
>> Why does the following Javascript display "defined/undefined"
rather  
>> than
>> "undefined/defined"?
>>
>> 	prop='myString'
>> 	var obj={prop:'defined'}
>> 	alert(obj.prop+'/'+obj.myString)
>
> ECMA-262, 11.2.1:
>
>   "    The dot notation is explained by the following
>        syntactic conversion:
>
>    MemberExpression .  Identifier
>
>         is identical in its behaviour to
>
>    MemberExpression [ <identifier-string> ]"
>
> In other words, (obj.prop) is identical to (obj['prop'])
> which has the value 'defined'.
>
> Try:
>
>     alert (obj[prop]+'/'+obj.myString)

I'm talking about the assignment using object notation, not the bracket or  
dot notation:

   obj={prop:"defined"}

The examples I've seen (eg, http://www.json.org/js.html) usually quote the  
property names as well as the values
(obj={"prop":"defined"}), so why does  
using a variable name (prop) when defining a property name not result in  
that property being called 'myString'? Quotes matter in bracket syntax  
(obj[0] is not the same as obj['0']) so I'm just curious why object  
notation works differently. Creating associative arrays in PHP (the  
equivalent of using object notation in Javascript) requires key names to  
be in quotes so that you can distinguish between numbers, strings, and  
variable values:

   $str='myString';
   $arr=array($str => 'As expected!');
   echo $arr['myString'];

The equivalent of the above code using Javascript object notation would  
display 'undefined' because the property name would end up being 'str'  

On Sat, 21 Jul 2007 12:55:16 +0800, Andrew Gregory wrote:

>>> On Mon, 16 Jul 2007 15:54:08 +0100, Eik wrote:
>>>
>>>> Why does the following Javascript display
"defined/undefined" rather  
>>>> than
>>>> "undefined/defined"?
>>>>
>>>> 	prop='myString'
>>>> 	var obj={prop:'defined'}
>>>> 	alert(obj.prop+'/'+obj.myString)

On Fri, 20 Jul 2007 16:46:38 +0100, Eik wrote:
>> I'm talking about the assignment using object notation, not the bracket
or  
>> dot notation:
>> 
>>    obj={prop:"defined"}

Of course. I was in a hurry yesterday. :-(

> Probably because the prop in the first line is different from the prop in 

> the second line, [...
>                                    ...] Javascript is not going to  
> evaluate the value of the first prop and use that as the member name of  
> the object.

Exactly! The evaluation of prop in obj={prop:'defined'}
is detailed in 11.1.5:

  "The production PropertyName: Identifier is evaluated
   as follows:

   1. Form a string literal containing the same sequence
      of characters as the Identifier.
   2. Return Result(1)."

I.e., the identifier prop becomes string literal 'prop',
which is returned to be used as the property name.

On Mon, 16 Jul 2007 16:54:08 +0200, Eik <spam@hotmail.com> wrote:

> Why does the following Javascript display "defined/undefined"
rather  
> than "undefined/defined"?
>
> 	prop='myString'
> 	var obj={prop:'defined'}
> 	alert(obj.prop+'/'+obj.myString)

"defined/undefined" is correct.

Reason:

At the first line, a variable named "prop" is initialised. To this  
variable, you assign the string value of 'myString'. This variable is not  
used anywhere else in the script.

At the second lie, a new variable named "obj" is initialised. To this 

variable, you assign an object having one property named 'prop', which is  
assigned the string value of 'defined'. Watch: you are not reusing the  
prop variable you set in the first line. You are not setting a property  
named 'myString', as you expect. Instead, you create a new property named  
'prop'.

At the third line, you query the property created in the second line,  
which has the string value of 'defined'. Querying it will return its  
string value, 'defined'. You also query a property you didn't define  
anywhere, namely the myString property. Because that property isn't  
defined, querying it will return 'undefined' or undefined, depending on  
your javascript implementation.

To have the query return "undefined/defined", rewrite the code without
 
instantiation abbreviation, like so:

  	var prop='myString';
  	var obj = new Object();
	obj[prop] = 'defined';
  	alert(obj.prop+'/'+obj.myString);


>
> According to any specs/examples I can find, JSON property names should  
> be in quotes so wouldn't this mean the unquoted string 'prop' should be  
> expanded to its value ('myString'), just like it would using array  
> syntax:
>
> 	obj[prop]='value'
> 	alert(obj.myString) // Displays 'value'

This unfortunately is not true. JSON properties don't have to be strings.  
The non-string way will create properties just the same, and they will be  
able to be queried using array syntax either way. The reason to have the  
properties as strings as far as I can tell, is to make it easier to  
deserialise an JSON object.

-- 
Sincerely,
ΩJr

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