JSON.parse and JSON.stringify are not idempotent and that is bad

Question

This question is multipart-

(1a) JSON is fundamental to JavaScript, so why is there no JSON type? A JSON type would be a string that is formatted as JSON. It would be marked as parsed/stringified until the data was altered. As soon as the data was altered it would not be marked as JSON and would need to be re-parsed/re-stringified.

(1b) In some software systems, isn't it possible to (accidentally) attempt to send a plain JS object over the network instead of a serialized JS object? Why not make an attempt to avoid that?

(1c) Why can't we call JSON.parse on a straight up JavaScript object without stringifying it first?

    var json = {   //JS object in properJSON format
        "baz":{
            "1":1,
            "2":true,
            "3":{}
        }
    };

    var json0 = JSON.parse(json); //will throw a parse error...bad...it should not throw an error if json var is actually proper JSON.

So we have no choice but to do this:

 var json0= JSON.parse(JSON.stringify(json));

However, there are some inconsistencies, for example:

JSON.parse(true); //works
JSON.parse(null); //works
JSON.parse({}); //throws error

(2) If we keep calling JSON.parse on the same object, eventually it will throw an error. For example:

var json = {   //same object as above
    "baz":{
        "1":1,
        "2":true,
        "3":{}
    }
};

var json1 = JSON.parse(JSON.stringify(json));
var json2 = JSON.parse(json1); //throws an error...why

(3) Why does JSON.stringify infinitely add more and more slashes to the input? It is not only hard to read the result for debugging, but it actually puts you in dangerous state because one JSON.parse call won't give you back a plain JS object, you have to call JSON.parse several times to get back the plain JS object. This is bad and means it is quite dangerous to call JSON.stringify more than once on a given JS object.

   var json = {
        "baz":{
            "1":1,
            "2":true,
            "3":{}
        }
    };

    var json2 = JSON.stringify(json);
    console.log(json2);

    var json3 = JSON.stringify(json2);
    console.log(json3);

    var json4 = JSON.stringify(json3);
    console.log(json4);

    var json5 = JSON.stringify(json4);
    console.log(json5); 

(4) What is the name for a function that we should be able to call over and over without changing the result (IMO how JSON.parse and JSON.stringify should behave)? The best term for this seems to be "idempotent" as you can see in the comments.

(5) Considering JSON is a serialization format that can be used for networked objects, it seems totally insane that you can't call JSON.parse or JSON.stringify twice or even once in some cases without incurring some problems. Why is this the case?

If you are someone who is inventing the next serialization format for Java, JavaScript or whatever language, please consider this problem.

IMO there should be two states for a given object. A serialized state and a deserialized state. In software languages with stronger type systems, this isn't usually a problem. But with JSON in JavaScript, if call JSON.parse twice on the same object, we run into fatal exceptions. Likewise, if we call JSON.stringify twice on the same object, we can get into an unrecoverable state. Like I said there should be two states and two states only, plain JS object and serialized JS object.

Answer 1

1) JSON.parse expects a string, you are feeding it a Javascript object.

2) Similar issue to the first one. You feed a string to a function that needs an object.

3) Stringfy actually expects a string, but you are feeding it a String object. Therefore, it applies the same measures to escape the quotes and slashes as it would for the first string. So that the language can understand the quotes, other special characters inside the string.

4) You can write your own function for this.

5) Because you are trying to do a conversion that is illegal. This is related to the first and second question. As long as the correct object types are fed, you can call it as many times as you want. The only problem is the extra slashes but it is in fact the standard.

Answer 2

We'll start with this nightmare of your creation: string input and integer output.
IJSON.parse(IJSON.stringify("5")); //=> 5

The built-in JSON functions would not fail us this way: string input and string output.
JSON.parse(JSON.stringify("5")); //=> "5"

JSON must preserve your original data types

Think of JSON.stringify as a function that wraps your data up in a box, and JSON.parse as the function that takes it out of a box.

Consider the following:

var a = JSON.stringify;
var b = JSON.parse;

var data = "whatever";

b(a(data)) === data; // true

b(b(a(a(data)))) === data; // true

b(b(b(a(a(a(data)))))) === data; // true

That is, if we put the data in 3 boxes, we have to take it out of 3 boxes. Right?

If I put my data in 2 boxes and take it out of 1, I'm not holding my data yet, I'm holding a box that contains my data. Right?

b(a(a(data))) === data; // false

Seems sane to me...

---

1. JSON.parse unboxes your data. If it is not boxed, it cannot unbox it. JSON.parse expects a string input and you're giving it a JavaScript object literal

2. The first valid call to JSON.parse would return an object. Calling JSON.parse again on this object output would result in the same failure as #1

3. repeated calls to JSON.stringify will "box" our data multiple times. So of course you have to use repeated calls to JSON.parse then to get your data out of each "box"

4. Idempotence

5. No, this is perfectly sane. You can't triple-stamp a double-stamp.

   You'd never make a mistake like this, would you?

   var json = IJSON.stringify("hi");
   IJSON.parse(json);
   //=> "hi"
   

   OK, that's idempotent, but what about

   var json = IJSON.stringify("5");
   IJSON.parse(json);
   //=> 5
   

   UH OH! We gave it a string each time, but the second example returns an integer. The input data type has been lost!

   Would the JSON functions have failed us here?

   var json = JSON.stringify("hi");
   JSON.parse(json);
   //=> "hi"
   

   All good. And what about the "5" ?

   var json = JSON.stringify("5");
   JSON.parse(json));
   //=> "5"
   

   Yay, the types have been preseved! JSON works, IJSON does not.

---

Maybe a more real-life example:

OK, so you have a busy app with a lot of developers working on it. It makes reckless assumptions about the types of your underlying data. Let's say it's a chat app that makes several transformations on messages as they move from point to point.

Along the way you'll have:

1. IJSON.stringify
2. data moves across a network
3. IJSON.parse
4. Another IJSON.parse because who cares? It's idempotent, right?
5. String.prototype.toUpperCase — because this is a formatting choice

Let's see the messages

bob: 'hi'
// 1) '"hi"', 2) <network>, 3) "hi", 4) "hi", 5) "HI"

Bob's message looks fine. Let's see Alice's.

alice: '5'
// 1) '5'
// 2) <network>
// 3) 5
// 4) 5
// 5) Uncaught TypeError: message.toUpperCase is not a function

Oh no! The server just crashed. You'll notice it's not even the repeated calling of IJSON.parse that failed here. It would've failed even if you called it once.

Seems like you were doomed from the start... Damned reckless devs and their careless data handling!

It would fail if Alice used any input that happened to also be valid JSON

alice: '{"lol":"pwnd"}'
// 1) '{"lol":"pwnd"}'
// 2) <network>
// 3) {lol:"pwnd"}
// 4) {lol:"pwnd"}
// 5) Uncaught TypeError: message.toUpperCase is not a function

---

OK, unfair example maybe, right? You're thinking, "I'm not that reckless, I wouldn't call IJSON.stringify or IJSON.parse on user input like that!" It doesn't matter. You've fundamentally broken JSON because the original types can no longer be extracted.

If I box up a string using IJSON, and then unbox it, who knows what I will get back? Certainly not you, and certainly not the developer using your reckless function.

• "Will I get a string type back?"
• "Will I get an integer?"
• "Maybe I'll get an object?"
• "Maybe I will get cake. I hope it's cake"

It's impossible to tell!

You're in a whole new world of pain because you've been careless with your data types from the start. Your types are important so start handling them with care.

JSON.stringify expects an object type and JSON.parse expects a string type.

Now do you see the light?

Answer 3

I'll try to give you one reason why JSON.parse cannot be called multiple time on the same data without us having a problem.

you might not know it but a JSON document does not have to be an object.

this is a valid JSON document:

"some text"

lets store the representation of this document inside a javascript variable:

var JSONDocumentAsString = '"some text"';

and work on it:

var JSONdocument = JSON.parse(JSONDocumentAsString);
JSONdocument === 'some text';

this will cause an error because this string is not the representation of a JSON document

JSON.parse(JSONdocument);
// SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

in this case how could have JSON.parse guessed that JSONdocument (being a string) was a JSON document and that it should have returned it untouched ?