Is it not possible to stringify an Error using JSON.stringify?

JSON.stringify(err, Object.getOwnPropertyNames(err))

seems to work

[from a comment by /u/ub3rgeek on /r/javascript] and felixfbecker's comment below

You can define a Error.prototype.toJSON to retrieve a plain Object representing the Error:

if (!('toJSON' in Error.prototype))
Object.defineProperty(Error.prototype, 'toJSON', {
    value: function () {
        var alt = {};

        Object.getOwnPropertyNames(this).forEach(function (key) {
            alt[key] = this[key];
        }, this);

        return alt;
    },
    configurable: true,
    writable: true
});

var error = new Error('testing');
error.detail = 'foo bar';

console.log(JSON.stringify(error));
// {"message":"testing","detail":"foo bar"}

Using Object.defineProperty() adds toJSON without it being an enumerable property itself.

Regarding modifying Error.prototype, while toJSON() may not be defined for Errors specifically, the method is still standardized for objects in general (ref: step 3). So, the risk of collisions or conflicts is minimal.

Though, to still avoid it completely, JSON.stringify()'s replacer parameter can be used instead:

function replaceErrors(key, value) {
    if (value instanceof Error) {
        var error = {};

        Object.getOwnPropertyNames(value).forEach(function (propName) {
            error[propName] = value[propName];
        });

        return error;
    }

    return value;
}

var error = new Error('testing');
error.detail = 'foo bar';

console.log(JSON.stringify(error, replaceErrors));

As no one is talking about the why part, I'm gonna answer it.

Why this JSON.stringify returns an empty object?

> JSON.stringify(error);
'{}'

Answer

From the document of JSON.stringify(),

For all the other Object instances (including Map, Set, WeakMap and WeakSet), only their enumerable properties will be serialized.

and Error object doesn't have its enumerable properties, that's why it prints an empty object.

Modifying Jonathan's great answer to avoid monkey patching:

var stringifyError = function(err, filter, space) {
  var plainObject = {};
  Object.getOwnPropertyNames(err).forEach(function(key) {
    plainObject[key] = err[key];
  });
  return JSON.stringify(plainObject, filter, space);
};

var error = new Error('testing');
error.detail = 'foo bar';

console.log(stringifyError(error, null, '\t'));

There is a great Node.js package for that: serialize-error.

npm install serialize-error

It handles well even nested Error objects.

import {serializeError} from 'serialize-error';

JSON.stringify(serializeError(error));

Docs: https://www.npmjs.com/package/serialize-error