Is it not possible to stringify an Error using JSON stringify

Question

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  gt  var error   new Error  simple error message        undefined   gt  error      Error  simple error message    gt  Object getOwnPropertyNames error          stack    arguments    type    message      gt  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    gt  gt  gt  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   Stick with simple string-based error messages  or create personal error objects and don t rely on the native Error object  Pull properties  JSON stringify   message  error message  stack  error stack      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  lt  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

User · Answer

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.

User · Answer

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     x000D   x000D  function jsonFriendlyErrorReplacer key  value    x000D    if  value instanceof Error    x000D      return   x000D           Pull all enumerable properties  supporting properties on custom Errors x000D           value  x000D           Explicitly pull Error s non-enumerable properties x000D        name  value name  x000D        message  value message  x000D        stack  value stack  x000D        x000D      x000D   x000D    return value x000D    x000D   x000D  let obj     x000D      error  new Error  nested error message   x000D    x000D   x000D  console log  Result WITHOUT custom replacer    JSON stringify obj   x000D  console log  Result WITH custom replacer    JSON stringify obj  jsonFriendlyErrorReplacer   x000D   x000D   x000D

User · Answer

As no one is talking about the why part  I m gonna answer it   Why this JSON stringify returns an empty object    gt  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

User · Answer

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

User · Answer

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  key                error key    value key                        return error             return value     var error   new Error  testing    error detail    foo bar    console log JSON stringify error  replaceErrors

User · Answer

JSON stringify err  Object getOwnPropertyNames err     seems to work   from a comment by  u ub3rgeek on  r javascript  and  felixfbecker s comment below

User · Answer

You can solve this with a one-liner  errStringified   in plain javascript  var error   new Error  simple error message    var errStringified    err   gt  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

User · Answer

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    gt        if  value instanceof Error            return util format value         else           return value

User · Answer

Make it serializable    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    gt    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

User · Answer

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

User · Answer

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  lt anonymous gt  66 15   causedBy    name   Error   message   error2   stack   Error  error2 n    at  lt anonymous gt  67 20   causedBy    name   Error   message   error3   stack   Error  error3 n    at  lt anonymous gt  68 29   displayed  true

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

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