Is this even legal JSON ? °

What you are showing is the JavaScript equivalent of a coderef in Perl, but IMHO JSON should be language agnostic.

If you really need to transport the JS code, maybe try serializing the function object on the JS side with .toString() or .toSource() to a string.

update

demo from my FF console:

>> var coderef = function() {return true;}
  undefined

>> typeof coderef
  "function"

>> coderef instanceof Function
  true

>> coderef.toString()
  "function() {return true;}"

>> coderef.toSource()
  "(function() {return true;})"

>> JSON= {a: coderef.toString() }
  {…}
  a: "function() {return true;}"
[download]

update

°) Nope!

From WP: JSON#Unsupported native data types

JavaScript syntax defines several native data types that are not included in the JSON standard: Map, Set, Date, Error, Regular Expression, Function, Promise, and undefined.

These JavaScript data types must be represented by some other data format, with the programs on both ends agreeing on how to convert between the types.

As of 2011, there are some de facto standards, e.g., converting from Date to String, but none universally recognized.

Other languages may have a different set of native types that must be serialized carefully to deal with this type of conversion.

(emphasize added)

PS: ok Wikipedia is probably not the most reliable source for citations but my resources for free consultation is limited. YMMV ;)