What is the criteria for representing null values ​​in JSON?

I can comment you with an example;

    var json1 = '{}';
    var json2 = '{"myCount": null}';
    var json4 = '{"myString": ""}';
    var json5 = '{"myString": "null"}';
    
    console.log(JSON.parse(json1)); // {}
    console.log(JSON.parse(json2)); // {myCount: null}
    console.log(JSON.parse(json4)); // {myString: ""}
    console.log(JSON.parse(json5)); // {myString: "null"}

JSON1 {} : Returns an empty object.

JSON2 {"myCount": null}: This is the definitive way to represent nulls according to the official documentation http://www.json.org/

JSON4 : Returns an empty string.

JSON5 {"myString": "null"}: With this you will have a literal string that contains NULL as a string.

I got this info some time ago from this post in English: https://stackoverflow.com/questions/21120999/representing-null-in-json

In the second edition of the final draft for the JSON syntax standard (dated December 2017), in section 5 of Values ​​in JSON (page 10 in the linked PDF) it is stated that (my translation):

A value in JSON can be an object, an array, a number, a string, true, false, or null.

Therefore null(lowercase) would be allowed as a value and would be most appropriate for defining null values ​​in JSON.

In the case exposed in the question, the values ​​will not be interpreted as nullbut as the text string "NULL" which could cause problems because being a non-empty string it will be evaluated as truein a conditional while the value nullevaluates as false, being able to generate unexpected behaviors. For example:

var miJSON = {
  nulo: null,
  textoNULL: "NULL"
}

if (miJSON.nulo) {
  console.log("nulo");
}

if (miJSON.textoNULL) {
  console.log("textoNULL");
}

More than worrying about how to represent the values ​​in the .json, in this case it would be how to represent them in the application and that it works correctly when containing this type of value.

You can add the values ​​however you want to represent them in your .json, for example:

{"myValue": null}
{"myValue": "null"}
{"myValue": ""}

or simply not add it, since your parser is the one in charge of representing those values ​​in the objects that your application will use, obviously you have to validate when these values ​​do not exist in the .json.

From experience the best approach to handle this type of values ​​is not to add value or not to add the property when it has a null value, one of the reasons is:

• Reduce the size of the file to download, bandwidth, etc.

by not reading these properties you won't have to add them to the object:

• These properties of your objects will therefore have a null value, which would be a benefit if your number of objects to build is considerable, so your application will not consume unnecessary memory.