Best practices for logging error objects in JavaScript

As a beginner in web development, I've recently come across an interesting observation related to handling errors. When working with json.stringyfy(), I noticed that the message key is not displayed in statement 2. However, accessing error.message returns a value instead of undefined.

 try {
           //Some error occours
    } catch (error) {
        console.log(JSON.stringify(error)) //statement 1
        console.error(error) //statement 2
        console.log(error.message) //statement 3
        console.log(Object.keys(error)) //statement 4
    }

statement 1 logs

MongoServerError: E11000 duplicate key error collection: trendyApp.Markets index: name_1 dup key: { name: "murat market" }
    at D:\web projects\trendyApp\server\node_modules\mongodb\lib\operations\insert.js:51:33
    at D:\web projects\trendyApp\server\node_modules\mongodb\lib\cmap\connection_pool.js:273:25
    at handleOperationResult (D:\web projects\trendyApp\server\node_modules\mongodb\lib\sdam\server.js:363:9)
    at MessageStream.messageHandler (D:\web projects\trendyApp\server\node_modules\mongodb\lib\cmap\connection.js:474:9)
    at MessageStream.emit (events.js:375:28)
    at processIncomingData (D:\web projects\trendyApp\server\node_modules\mongodb\lib\cmap\message_stream.js:108:16)
    at MessageStream._write (D:\web projects\trendyApp\server\node_modules\mongodb\lib\cmap\message_stream.js:28:9)
    at writeOrBuffer (internal/streams/writable.js:358:12)
    at MessageStream.Writable.write (internal/streams/writable.js:303:10)
    at TLSSocket.ondata (internal/streams/readable.js:726:22) {
  index: 0,
  code: 11000,
  keyPattern: { name: 1 },
  keyValue: { name: 'murat market' }
}

statement 2 logs

{"index":0,"code":11000,"keyPattern":{"name":1},"keyValue":{"name":"murat market"}}

statement 3 logs

E11000 duplicate key error collection: trendyApp.Markets index: name_1 dup key: { name: "murat market" }

While developing an express application and encountering this error from Mongoose, I believe this behavior is prevalent across JavaScript.

statement 4 logs

[ 'index', 'code', 'keyPattern', 'keyValue' ]
javascriptexpressmongoose

Answer №1

"message" is not considered a key (refer to statement 4)

Although technically it is, Object.keys is specifically designed to display only enumerable properties, and message happens to be non-enumerable.

According to the ECMAScript specification, the Error constructor sets its message (and other properties) through the process described as:

Execute CreateNonEnumerableDataPropertyOrThrow(O, "message", msg).

Additional custom properties can be added to Error objects using JavaScript code, which clarifies why other properties do appear when using Object.keys().

Regarding the output of console.log: the way objects are displayed can vary significantly based on the console API implementation.

To display those non-enumerable properties as well, you can utilize:

console.log(Object.getOwnPropertyNames(error))

Answer №2

Like @trincot mentioned, the reason why message is not displayed is because it is a non-enumerable attribute for Error constructor. The MongoServerError overrides the MongoError, which in turn overrides the JS Error object. If you want to know the attributes of the error, you can refer to the provided links.

If you need to access all attributes, including the non-enumerable ones, you can use Object.getOwnProperties(error) to view them.

Alternatively, you can utilize Object.getOwnPropertyDescriptors(error) to understand the descriptor for each attribute defined in the object.

Answer №3

Unfortunately, the error stack and message fields cannot be enumerated. Here is a helpful function that can extract all properties from an error object, including any custom fields you may have added:

const err = new Error('An unexpected error occurred.');
err.details = { custom: { code: '123' } };

console.log(errorToPOJO(err)); // {"stack":"Error: An unexpected error occurred.\\n    at <anonymous>:1:13","message":"An unexpected error occurred.","details":{"custom":{"code":"123"}}

function errorToPOJO(error) {
  const ret = {};
  for (const propertyName of Object.getOwnPropertyNames(error)) {
    ret[propertyName] = error[propertyName];
  }
  return ret;
};

Answer №4

For those utilizing an API, give this a shot

console.log(error.message.toString())

Answer №5

Expanding on the previous answer, you can obtain the enumerable properties by running the following code:

console.log(JSON.stringify(error, Object.getOwnPropertyNames(error)))

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