Why does JSON.stringify returns "{}"?

Question

I have one prototype of this structure:

function MyObj() { }

MyObj.prototype = {
    prop1: {
        prop11: null,
        prop12: null,
        prop13: null,
    },
    prop2: {
        prop21: null,
        prop22: null,
        prop23: null,
    },
    prop3: {
        prop31: [],
        prop32: '',
        prop34: [],
    },
    prop4: {
        prop41: null,
    },
}

When I call JSON.stringify(myObjInstance), I get {}, why?

Similar to http://stackoverflow.com/questions/12369543/why-is-json-stringify-not-serializing-prototype-values. You can write your custom [`toJSON`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON_behavior) method to avoid that. — maxdec, Jan 14 '14 at 13:13

score 3 · Answer 1 · answered Jan 14 '14 at 13:12

This happens because prop1 through prop4 are properties of the prototype and not of the instantiated object.

You can compare it to something like:

for (var i in obj) {
    if (obj.hasOwnProperty(i)) {
        // add property to bag
    }
}

Only the properties of the object itself are used.

T.J. Crowder · Answer 2 · 2014-01-14T13:19:07.587

Because JSON.stringify only includes an object's own properties (specifically, own enumerable properties), not properties an object inherits from its prototypes or any of its own properties that are non-enumerable.

So for example: Live Copy | Live Source

function Foo() { }

Foo.prototype.inherited = true;

var f = new Foo();
Object.defineProperty(f, "ownNonEnumerable", {
    value: true
});

f.ownEnumerable = true;

console.log(f.inherited);        // true
console.log(f.ownNonEnumerable); // true
console.log(f.ownEnumerable);    // true
console.log(JSON.stringify(f));  // {"ownEnumerable": true}

score 1 · Answer 3 · answered Jan 14 '14 at 13:13

1

JSON.stringify will only include a property if it can be found with hasOwnProperty, and

new MyObj().hasOwnProperty("prop1") === false;

Try this instead:

JSON.stringify(MyObj.prototype);

answered Jan 14 '14 at 13:13

tckmn

57,719
27
114
156

score 1 · Answer 4 · answered Jan 14 '14 at 13:13

1

JSON.stringify(obj) will output the properties that the object itself has, not its prototype. Its prototype is a different object.

What you're looking for would be something like this

JSON.stringify(MyObj.prototype)

answered Jan 14 '14 at 13:13

scrblnrd3

7,228
9
33
64

score -1 · Answer 5 · answered Jan 14 '14 at 13:14

I had the same issue, when I start working with JSON.stringify. In order to create a string the object you create needs to of typeof == object. Otherwise it won't work.

Try to alert the typeOf of MyObj. If it not Object that won't work.

MyObj = {
prop1: {
    prop11: null,
    prop12: null,
    prop13: null,
},
prop2: {
    prop21: null,
    prop22: null,
    prop23: null,
},
prop3: {
    prop31: [],
    prop32: '',
    prop34: [],
},
prop4: {
    prop41: null,
},

}

Why does JSON.stringify returns "{}"?

5 Answers5