Why does JSON.stringify not serialize non-enumerable properties?

Question

I\'m serializing objects to JSON strings with JavaScript,

I noticed only enumerable object properties get serialized:

var a = Object.create(null,{
          


        

Answer 1

It's specified in the ES5 spec.

If Type(value) is Object, and IsCallable(value) is false

If the [[Class]] internal property of value is "Array" then

    Return the result of calling the abstract operation JA with argument value.

Else, return the result of calling the abstract operation JO with argument value.

So, let's look at JO. Here's the relevant section:

Let K be an internal List of Strings consisting of the names of all the own properties of value whose [[Enumerable]] attribute is true. The ordering of the Strings should be the same as that used by the Object.keys standard built-in function.

Answer 2

As @ThiefMaster answered above, it's specified in the spec

however, if you know the names of the non-enumerable properties you like to be serialized ahead of time, you can achieve it by passing a replacer function as the second param to JSON.stringify() (documentation on MDN), like so

var o = {
  prop: 'propval',
}

Object.defineProperty(o, 'propHidden', {
  value: 'propHiddenVal',
  enumerable: false,
  writable: true,
  configurable: true
});

var s = JSON.stringify(o, (key, val) => {
  if (!key) {
    // Initially, the replacer function is called with an empty string as key representing the object being stringified. It is then called for each property on the object or array being stringified.
    if (typeof val === 'object' && val.hasOwnProperty('propHidden')) {
      Object.defineProperty(val, 'propHidden', {
        value: val.propHidden,
        enumerable: true,
        writable: true,
        configurable: true
      });

    }
  }
  return val;
});

console.log(s);