JSON: How to create an undefined property ?

Or is result of 1/0 ‘Infinity’ as this science called mathematics tells us? In any case and at this moment in time, and looking into this JSON conundrum, I started to think I had no idea.

[first published 2013JAN01]
Consider for example this little “number” :


Property “b” is simply and quietly not created, or so it seems? Perhaps it is me who is wrong or I need to go back to ES5 specification?

JSON NOT to rescue

Problem is not in some esoteric ES5 “feature” (aka omission) but it is an “issue” with JSON. JSON was created to ignore undefined (and functions, NaN’s and Infinity-es). And yes I knew this. I started this post in this way, because I also wanted to show how this kind of “feature” can catch anybody off-guard. And repeatedly at that.

In the snippet above there is yet another fundamental rule of JavaScript/DOM programming broken:

Never use JSON.stringify() for debugging

Object literal above is not a mistake. It really produces object with properties “a” and “b”, where “b” is indeed undefined. And crucially key “b” does exist, and its value is undefined. So, back to senses, hit F12 in your venerable Chrome or IE, and try this:

And voila, output is the actual object created, with two properties present:

Not out of this wood, yet

This might be one happy ending to one blog post, but alas it is not. Far from that.

The big worm in that can is this: How is this reflected on AJAX ? How are XMLHTTPREQUEST implementations out there handling this JSON “feature”? Namely: how is one passing a JSON objects where some keys might have values as e.g. undefined ? JSON encoded object like this one:

is of course NOT a good JSON and resident JSON.parse() method on the receiving side will throw an syntax error:

I am not even sure I can hope that sending side will allow me to come to the above. That is I am not sure how to pack the offending object into the valid JSON string.
Valid requirement. Although 1/0 is 'Infinity'.
1/0 is ‘Infinity’.

So, what gives? I see no other way but to perhaps use ‘null’ instead of undefined’? But then I have no simple/native/natural way of JSON trustingly encoding the object which (for example) represents the calculation of the table from the image in this post ).

if x is zero then 1/x is not ‘null’. It is mathematically represented as ‘infinity’, not as ‘null’. And not as “undefined” either, as table on the image is wrong. Strictly 1/0 is Infinity, but then JSON encoding rules do not allow that too: Infinity is not legal value in JSON.

We are definitely not in the “good parts” territory here.

In pure JavaScript, I can compute NaN or Infinity values or hold undefined ones, but how do I express in JSON that something is undefined, Infinity or NaN ? Beside putting it in a string: “undefined”, “NaN”, “Infinity”.

If I have to “decode” these values on the receiving side, this introduces yet another level of encoding of “special” values and that is not a good thing.

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