JSON: How to create an undefined or NaN value?

Developers Kindergarten

Or is the 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 way back to ES5 specification?

JSON NOT to rescue

The 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 an 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, the 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 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 the resident JSON.parse() method on the receiving side will throw a syntax error:

SyntaxError: Unexpected token 

If anybody cares: (2014 SEP 11 Update ) In IE11 Console for some reason this is "Invalid character", Error?  In case you are laughing here is the Opera response as of today (2021 MAR)

VM5325:1 Uncaught SyntaxError:
Unexpected token u in JSON at position 17
at JSON.parse (<anonymous>)

at <anonymous>:1:6

Many years have passed. It seems we have no solution yet.

I am not even sure I can hope that the sending side (in the wild there are very strange senders) will allow me to come to the above point where I can use JavaScript code. That is:

I am not sure how to pack the offending object into the valid JSON string, that can be parsed safely.

Valid requirement. Although 1/0 is 'Infinity'.

1/0 is ‘Infinity’. It is not ‘undefined’. This image is wrong.

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 a table on the image is wrong. Strictly 1/0 is Infinity, but then JSON encoding rules do not allow that too: Infinity is not a legal value in JSON.

Undefined “is” null but its identity is not null … We are definitely not in the “good parts”  of JavaScript, a territory in 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”. NaN, undefined and null is three distinctive values. And just like any other values in JavaScript

Solution? Direction? Perhaps, but solution no.

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

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