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  3. JavaScript — How to JSON Stringify Errors

JavaScript — How to JSON Stringify Errors

You can create a JSON string from anything in JavaScript using the JSON.stringify(anything) method. However, the resulting JSON string may not contain the expected values. For example, if you’re JSON serializing a JavaScript error, you’ll notice that its JSON string doesn’t contain any properties.

JavaScript doesn’t include internal, non-enumerable properties when using JSON.stringify. And all properties of an error are non-enumerable. This tutorial shows you how to JSON-serialize errors in JavaScript.

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Implement a toJSON Method on Error Classes

When creating and using your own error classes, you can implement a toJSON() method. This toJSON() method returns an object or JSON string that exposes the selected properties.

The default JSON serialization won’t include non-enumerable class properties. It only includes properties that you’re assigning within the class itself, for example within the constructor.

Here’s an example of an HttpError class that doesn’t implement a toJSON method. You’ll only see the assigned statusCode variable in the JSON string:

class HttpError extends Error {  
  constructor(message, statusCode) {
    super(message)
    this.statusCode = statusCode
  }
}

const error = new HttpError('Failed to find user', 404)  
console.log(JSON.stringify(error))  
// {"statusCode":404}

The default JSON-serialization of a JavaScript error doesn’t contain all properties. If possible, you should create your own error class that extends the built-in Error. Your new error class then exposes a toJSON method that returns the error’s properties:

class HttpError extends Error {  
  constructor(message, statusCode, data) {
    super(message)
    this.data = data
    this.statusCode = statusCode
  }

  toJSON() {
    return {
      ...this.data
      message: this.message,
      status: this.statusCode,
    }
  }
}

const error = new HttpError('Failed to find user', 404)  
console.log(JSON.stringify(error))  
// {"message":"Failed to find user","status":404}

const errorWithData = new HttpError('Failed to find user', 404, {  
  userId: 123,
})
console.log(JSON.stringify(errorWithData))  
// {"userId":123,"message":"Failed to find user","status":404}

Notice: we use a data property to inject an object of data into the error itself. This can be useful when you want to provide more context than a string, like validation errors with the failed field and their list of errors.

JSON Stringify Errors You’re Not in Control Of

You can’t extend error classes used in third-party packages. Well, you could do it with a global patch on the Error class. But please don’t do this! Globally patching JavaScript objects has unexpected side-effects on code.

Yet, you can still serialize generic JavaScript errors to JSON. What you need is an error handler that catches all errors within your application and then does the handling. For example, your error handler may check if an error implements a toJSON method, and then call that method. If not, you could use the serialize-error package from Sindre Sorhus.

npm install serialize-error

The serialize-error package exposes a serializeError() method. Use this method to create a JSON string from a given error:

import { serializeError } from 'serialize-error';

const error = new Error('Failed to find user');  
const serialized = serializeError(error);

console.log(JSON.stringify(serialized));  
// {"name":"Error","message":"Failed to find user","stack":"Error: Failed to find user\n    at Object.<anonymous> …"}

The error’s JSON string contains all non-enumerable properties. All enumerable error fields are included.

Notice: serialize-error will also include the error.stack property in the resulting JSON. Please don’t expose the stack to the outside in production environments. The stack can be helpful during development and internal pre-production environments. In production, you shouldn’t leak server paths or details.

That’s it!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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