JavaScript JSON.stringify无法正确处理原型吗？

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/

Answer 1

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).

Answer 2

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