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up vote 1 down vote favorite

This code fails with an exception indicating invalid JSON:

var example = '{ "AKEY": undefined }';
jQuery.parseJSON(example);

I was able to fix it by replacing all undefineds with empty strings. Are undefineds not part of JSON?

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4 Answers 4

up vote 4 down vote accepted

If you can wrap your head around this, the token undefined is actually undefined.

Allow me to elaborate: even though JavaScript has a special primitive value called undefined, undefined is not a JavaScript keyword nor does it have any special meaning. You can break code which tests for the existance of an object by comparing to undefined by defining it.

var obj = { BKEY: 'I exist!' };
if (obj.AKEY == undefined) console.log ('no AKEY');
if (obj.BKEY == undefined) console.log ('should not happen');

undefined='uh oh';

if (obj.AKEY == undefined) console.log ('oops!'); // Logically, we want this to execute, but it will not!
if (obj.BKEY == undefined) console.log ('should not happen');

The only console output will be 'no AKEY'. After we've assigned to the global variable undefined, obj.AKEY == undefined becomes false because undefined != 'uh oh'. obj.BKEY == undefined still returns false, but only because we're lucky. If I had set obj.BKEY='uh oh', then obj.BKEY == undefined would be true, even though it actually exists!

You probably want to explicity set AKEY to null. (By the way, null is a keyword; null='uh oh' throws an exception).

You could also simply omit AKEY from your JSON, in which case you would find:

typeof(example.AKEY) == 'undefined'

(If you set AKEY to null, then typeof(example.AKEY) == 'object'.)

The only real difference between setting to null and omitting is whether you want the key to appear in a foreach loop.

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up vote 4 down vote

No, but null is. RFC 4627 §2.1:

A JSON value MUST be an object, array, number, or string, or one of the following three literal names:

  false null true

var example = '{ "AKEY": null }';
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up vote 1 down vote

Correct. Undefined and functions are not represented in JSON. http://www.json.org/js.html

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up vote 1 down vote

They're not permitted in JSON...look at the alternative and it's clear as to why:

var example = '{}';
var obj = jQuery.parseJSON(example);
obj.AKEY //undefined

If it's undefined, when you go to access it, it's the same as the key not even being present. Since JSON's primary purpose is for transmitting data (otherwise the broader object literal syntax is fine)...it's better to leave it out altogether.

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There is a difference between {} and {AKEY: undefined}. With the latter, AKEY will show up in for each enumeration. –  Matthew Flaschen Nov 19 '10 at 14:39
    
@Matthew - When you're accessing it though, there's no difference...can you think of a valid use case where you'd want to see it enumerated and care, given it's not defined? –  Nick Craver♦ Nov 19 '10 at 14:40
    
No. And it definitely simplifies things for JSON just to have null. I just wanted to note that there is a difference. –  Matthew Flaschen Nov 19 '10 at 14:43

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