CODEWITHSUNDEEP
javascriptjsonnode.jserror-handling
Jay
Jay

Reputation: 19857

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

Reproducing the problem

I'm running into an issue when trying to pass error messages around using web sockets. I can replicate the issue I am facing using JSON.stringify to cater to a wider audience:

// node v0.10.15
> var error = new Error('simple error message');
    undefined

> error
    [Error: simple error message]

> Object.getOwnPropertyNames(error);
    [ 'stack', 'arguments', 'type', 'message' ]

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

The problem is that I end up with an empty object.

What I've tried

Browsers

I first tried leaving node.js and running it in various browsers. Chrome version 28 gives me the same result, and interestingly enough, Firefox at least makes an attempt but left out the message:

>>> JSON.stringify(error); // Firebug, Firefox 23
{"fileName":"debug eval code","lineNumber":1,"stack":"@debug eval code:1\n"}

Replacer function

I then looked at the Error.prototype. It shows that the prototype contains methods such as toString and toSource. Knowing that functions can't be stringified, I included a replacer function when calling JSON.stringify to remove all functions, but then realized that it too had some weird behavior:

var error = new Error('simple error message');
JSON.stringify(error, function(key, value) {
    console.log(key === ''); // true (?)
    console.log(value === error); // true (?)
});

It doesn't seem to loop over the object as it normally would, and therefore I can't check if the key is a function and ignore it.

The Question

Is there any way to stringify native Error messages with JSON.stringify? If not, why does this behavior occur?

Methods of getting around this

Updates

@Ray Toal Suggested in a comment that I take a look at the property descriptors. It is clear now why it does not work:

var error = new Error('simple error message');
var propertyNames = Object.getOwnPropertyNames(error);
var descriptor;
for (var property, i = 0, len = propertyNames.length; i < len; ++i) {
    property = propertyNames[i];
    descriptor = Object.getOwnPropertyDescriptor(error, property);
    console.log(property, descriptor);
}

Output:

stack { get: [Function],
  set: [Function],
  enumerable: false,
  configurable: true }
arguments { value: undefined,
  writable: true,
  enumerable: false,
  configurable: true }
type { value: undefined,
  writable: true,
  enumerable: false,
  configurable: true }
message { value: 'simple error message',
  writable: true,
  enumerable: false,
  configurable: true }

Key: enumerable: false.

Accepted answer provides a workaround for this problem.

Upvotes: 626

Views: 242153

Answers (15)

Sanghyun Lee
Sanghyun Lee

Reputation: 22982

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 any enumerable properties, that's why it prints an empty object.

Background on enumerable properties

In Javascript, an object can have two types of properties:

  • enumerable properties
  • non-enumerable properties

The exact distinction is a bit tricky, but basically:

  • "normal" properties, such as the ones you create by assignment ( myobj= {}; myobj.prop1 = 4711;), are enumerable,
  • "internal" properties, such as the length property of an array, are non-enumerable

In particular, an Error has only non-enumerable properties.

For details, see for example Enumerability and ownership of properties on MDN.

Upvotes: 271

laggingreflex
laggingreflex

Reputation: 34627

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

seems to work

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

Also see the answer by "Sanghyun Lee" for an explanation why this is required.

Upvotes: 552

gvlax
gvlax

Reputation: 788

Usually I declare once:

  const cloneError = (err) => {
    return err ? { name: err.name, message: err.message, stack: err.stack, cause: err.cause } : {};
  };

Then everywhere I can use it, for instance:

...logger.log('An error occurred:', cloneError(err));

Upvotes: 2

Lukasz Czerwinski
Lukasz Czerwinski

Reputation: 15432

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';

const stringifiedError = serializeError(error);

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

Upvotes: 104

Elron
Elron

Reputation: 1455

I've extended this answer: Is it not possible to stringify an Error using JSON.stringify?

serializeError.ts

export function serializeError(err: unknown) {
    return JSON.parse(JSON.stringify(err, Object.getOwnPropertyNames(err)))
}

And I can use it like this:

import { serializeError } from '../helpers/serializeError'; // Change to your path

try {
    const res = await create(data);
    return { status: 201 };
} catch (err) {
    return { status: 400, error: serializeError(err) };
}

Upvotes: 3

Jonathan Lonowski
Jonathan Lonowski

Reputation: 123423

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));

Upvotes: 258

Deep Panchal
Deep Panchal

Reputation: 159

String constructor should be able to stringify error

try { 
  throw new Error("MY ERROR MSG")
} catch (e) {
  String(e) // returns 'Error: MY ERROR MSG'
}

Upvotes: 9

Pawel
Pawel

Reputation: 18212

Just convert to a regular object

// example error
let err = new Error('I errored')

// one liner converting Error into regular object that can be stringified
err = Object.getOwnPropertyNames(err).reduce((acc, key) => { acc[key] = err[key]; return acc; }, {})

If you want to send this object from child process, worker or though the network there's no need to stringify. It will be automatically stringified and parsed like any other normal object

Upvotes: 5

David Navrkal
David Navrkal

Reputation: 552

If using nodejs there is better reliable way by using native nodejs inspect. As well you can specify to print objects to unlimited depth.

Typescript example:

import { inspect }  from "util";

const myObject = new Error("This is error");
console.log(JSON.stringify(myObject)); // Will print {}
console.log(myObject); // Will print full error object
console.log(inspect(myObject, {depth: null})); // Same output as console.log plus it works as well for objects with many nested properties.

Link to documentation, link to example usage.

And as well discussed in the topic How can I get the full object in Node.js's console.log(), rather than '[Object]'? here in stack overflow.

Upvotes: 11

savram
savram

Reputation: 580

You can solve this with a one-liner( errStringified ) in plain javascript:

var error = new Error('simple error message');
var errStringified = (err => JSON.stringify(Object.getOwnPropertyNames(Object.getPrototypeOf(err)).reduce(function(accumulator, currentValue) { return accumulator[currentValue] = err[currentValue], accumulator}, {})))(error);
console.log(errStringified);

It works with DOMExceptions as well.

Upvotes: 0

Jason
Jason

Reputation: 3178

I was working on a JSON format for log appenders and ended up here trying to solve a similar problem. After a while, I realized I could just make Node do the work:

const util = require("util");
...
return JSON.stringify(obj, (name, value) => {
    if (value instanceof Error) {
        return util.format(value);
    } else {
        return value;
    }
}

Upvotes: 15

Joel Malone
Joel Malone

Reputation: 1314

We needed to serialise an arbitrary object hierarchy, where the root or any of the nested properties in the hierarchy could be instances of Error.

Our solution was to use the replacer param of JSON.stringify(), e.g.:

function jsonFriendlyErrorReplacer(key, value) {
  if (value instanceof Error) {
    return {
      // Pull all enumerable properties, supporting properties on custom Errors
      ...value,
      // Explicitly pull Error's non-enumerable properties
      name: value.name,
      message: value.message,
      stack: value.stack,
    }
  }

  return value
}

let obj = {
    error: new Error('nested error message')
}

console.log('Result WITHOUT custom replacer:', JSON.stringify(obj))
console.log('Result WITH custom replacer:', JSON.stringify(obj, jsonFriendlyErrorReplacer))

Upvotes: 20

Elliott Palermo
Elliott Palermo

Reputation: 61

None of the answers above seemed to properly serialize properties which are on the prototype of Error (because getOwnPropertyNames() does not include inherited properties). I was also not able to redefine the properties like one of the answers suggested.

This is the solution I came up with - it uses lodash but you could replace lodash with generic versions of those functions.

 function recursivePropertyFinder(obj){
    if( obj === Object.prototype){
        return {};
    }else{
        return _.reduce(Object.getOwnPropertyNames(obj), 
            function copy(result, value, key) {
                if( !_.isFunction(obj[value])){
                    if( _.isObject(obj[value])){
                        result[value] = recursivePropertyFinder(obj[value]);
                    }else{
                        result[value] = obj[value];
                    }
                }
                return result;
            }, recursivePropertyFinder(Object.getPrototypeOf(obj)));
    }
}


Error.prototype.toJSON = function(){
    return recursivePropertyFinder(this);
}

Here's the test I did in Chrome:

var myError = Error('hello');
myError.causedBy = Error('error2');
myError.causedBy.causedBy = Error('error3');
myError.causedBy.causedBy.displayed = true;
JSON.stringify(myError);

{"name":"Error","message":"hello","stack":"Error: hello\n    at <anonymous>:66:15","causedBy":{"name":"Error","message":"error2","stack":"Error: error2\n    at <anonymous>:67:20","causedBy":{"name":"Error","message":"error3","stack":"Error: error3\n    at <anonymous>:68:29","displayed":true}}}

Upvotes: 6

cheolgook
cheolgook

Reputation: 107

You can also just redefine those non-enumerable properties to be enumerable.

Object.defineProperty(Error.prototype, 'message', {
    configurable: true,
    enumerable: true
});

and maybe stack property too.

Upvotes: 9

Bryan Larsen
Bryan Larsen

Reputation: 10006

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'));

Upvotes: 69

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