Javascript JSON.stringify doesn't handle prototype correctly?

Question

I've been initializing my reusable classes like this (constructor is usually a copy-constructor):

function Foo() {}
Foo.prototype.a = "1";
Foo.prototype.b = "2";
Foo.prototype.c = [];
var obj = new Foo();
obj.c.push("3");

but the JSON.stringify does not produce the expected result:

JSON.stringify(obj);

{}

The variables work as expected for everything else.
If toJSON is overridden, it works fine:

Foo.prototype.toJSON = function () {
    return {
        a: this.a,
        b: this.b,
        c: this.c
    };
};
JSON.stringify(obj);

{"a":"1","b":"2","c":["3"]}

It also works fine if the variables are defined inside the constructor:

function Alt() {
    this.a = 1;
    this.b = "2";
    this.c = [];
}
JSON.stringify(obj);

{"a":1,"b":"2","c":["3"]}

What's going on?

Example here: http://jsfiddle.net/FdzB6/

The JSON serializer only pays attention to properties directly on the objects being serialized ("own" properties). — Pointy
– Pointy, Commented Feb 12, 2014 at 15:34
@MikeEmery for "Foo", the function?!? No, the function "Foo" doesn't have those properties, and neither will an object created by calling new Foo - they'll be readable from the prototype via an instance, of course, but a hasOwnProperty("a") test will return false. — Pointy
– Pointy, Commented Feb 12, 2014 at 15:49
@RocketHazmat yes; there's really no difference between a simple string or number-valued prototype property and a function property. The lookup process for a property reference is the same. I should add an answer I guess :) — Pointy
– Pointy, Commented Feb 12, 2014 at 15:50
For reference, here's how is implemented Crockford's stringify : github.com/douglascrockford/JSON-js/blob/master/json2.js#L326 — Denys Séguret
– Denys Séguret, Commented Feb 12, 2014 at 15:51
@MikeEmery well, it depends on what you're trying to do. If the ability to serialize objects with JSON is really important, then yes I'd say it's a bad idea :) — Pointy
– Pointy, Commented Feb 12, 2014 at 16:10

Pointy · Accepted Answer · 2014-02-12 15:53:46Z

3

Properties on an object's prototype (that is, the prototype of its constructor) are readable via a reference to an the object:

function Constructor() { }
Constructor.prototype.a = "hello world";

var x = new Constructor();
alert(x.a); // "hello world"

However, those properties really are "stuck" on the prototype object:

alert(x.hasOwnProperty("a")); // false

The JSON serializer only pays attention to properties that directly appear on objects being processed. That's kind-of painful, but it makes a little sense if you think about the reverse process: you certainly don't want JSON.parse() to put properties back onto a prototype (which would be pretty tricky anyway).

answered Feb 12, 2014 at 15:53

Pointy

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1 Comment

Mike Emery Over a year ago

Thanks for this explanation - the bit about using JSON.parse later is especially helpful.

Community · Accepted Answer · 2017-05-23 12:05:31Z

1

your answer lies Why is JSON.stringify not serializing prototype values?

JSON.stringify only does "own" properties.

In your first example: prototype members are being set, and then calling Stringify on the object itself, which does not have its own properties.

In your second: this.a will climb the chain until it finds the property

In the third: You are setting properties directly on the object and not its prototype

edited May 23, 2017 at 12:05

CommunityBot

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answered Feb 12, 2014 at 15:57

michael truong

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