Error-serializer NPM

Convert errors to/from plain objects.

Hire me

Please reach out if you're looking for a Node.js API or CLI engineer (11 years of experience). Most recently I have been Netlify Build's and Netlify Plugins' technical lead for 2.5 years. I am available for full-time remote positions.

Features

Ensures errors are safe to serialize with JSON
Can be used as error.toJSON()
Deep serialization/parsing, including transforming
Custom serialization/parsing (e.g. YAML or process.send())
Keeps both native (TypeError, etc.) and custom error classes
Preserves errors' additional properties
Can keep constructor's arguments
Works recursively with error.cause and AggregateError
Normalizes invalid errors
Safe: this never throws

Example

import { serialize, parse } from 'error-serializer'

const error = new TypeError('example')
const errorObject = serialize(error)
// Plain object: { name: 'TypeError', message: 'example', stack: '...' }

const errorString = JSON.stringify(errorObject)
const newErrorObject = JSON.parse(errorString)

const newError = parse(newErrorObject)
// Error instance: 'TypeError: example ...'

Install

npm install error-serializer

This package works in both Node.js >=18.18.0 and browsers.

This is an ES module. It must be loaded using an import or import() statement, not require(). If TypeScript is used, it must be configured to output ES modules, not CommonJS.

API

serialize(errorInstance, options?)

errorInstance any\ options Options?\ Return value: ErrorObject

Convert an Error instance into a plain object.

Options

Object with the following optional properties.

shallow

Type: boolean\ Default: false

Unless this option is true, nested errors are also serialized. They can be inside other errors, plain objects or arrays.

const error = new Error('example')
error.inner = new Error('inner')
serialize(error).inner // { name: 'Error', message: 'inner', ... }
serialize(error, { shallow: true }).inner // Error: inner ...

loose

Type: boolean\ Default: false

By default, when the argument is not an Error instance, it is converted to one. If this option is true, it is kept as is instead.

serialize('example') // { name: 'Error', message: 'example', ... }
serialize('example', { loose: true }) // 'example'

include

Type: string[]

Only pick specific properties.

serialize(error, { include: ['message'] }) // { message: 'example' }

exclude

Type: string[]

Omit specific properties.

serialize(error, { exclude: ['stack'] }) // { name: 'Error', message: 'example' }

transformObject(errorObject, errorInstance)

Type: (errorObject, errorInstance) => void

Transform each error plain object.

errorObject is the error after serialization. It must be directly mutated.

errorInstance is the error before serialization.

parse(errorObject, options?)

errorObject any\ options Options?\ Return value: Error

Convert an error plain object into an Error instance.

Options

Object with the following optional properties.

classes

Type: object

Custom error classes to keep when parsing.

Each key is an errorObject.name
Each value is the error class to use

const errorObject = serialize(new CustomError('example'))
// `CustomError` class is kept
const error = parse(errorObject, { classes: { CustomError } })
// Map `CustomError` to another class
const otherError = parse(errorObject, { classes: { CustomError: TypeError } })

shallow

Type: boolean\ Default: false

Unless this option is true, nested error plain objects are also parsed. They can be inside other errors, plain objects or arrays.

const error = new Error('example')
error.inner = new Error('inner')
const errorObject = serialize(error)

parse(errorObject).inner // Error: inner ...
parse(errorObject, { shallow: true }).inner // { name: 'Error', message: ... }

loose

Type: boolean\ Default: false

By default, when the argument is not an error plain object, it is converted to one. If this option is true, it is kept as is instead.

parse('example') // Error: example
parse('example', { loose: true }) // 'example'

transformArgs(constructorArgs, errorObject, ErrorClass)

Type: (constructorArgs, errorObject, ErrorClass) => void

Transform the arguments passed to each new Error().

constructorArgs is the array of arguments. Usually, constructorArgs[0] is the error message and constructorArgs[1] is the constructor options object. constructorArgs must be directly mutated.

errorObject is the error before parsing. ErrorClass is its class.

transformInstance(errorInstance, errorObject)

Type: (errorInstance, errorObject) => void

Transform each Error instance.

errorInstance is the error after parsing. It must be directly mutated.

errorObject is the error before parsing.

Usage

JSON safety

Error plain objects are always safe to serialize with JSON.

const error = new Error('example')
error.cycle = error

// Cycles make `JSON.stringify()` throw, so they are removed
serialize(error).cycle // undefined

`error.toJSON()`

serialize() can be used as error.toJSON().

class CustomError extends Error {
  /* constructor(...) { ... } */

  toJSON() {
    return serialize(this)
  }
}
const error = new CustomError('example')

error.toJSON()
// { name: 'CustomError', message: 'example', stack: '...' }
JSON.stringify(error)
// '{"name":"CustomError","message":"example","stack":"..."}'

Custom serialization/parsing

Errors are converted to/from plain objects, not strings. This allows any serialization/parsing logic to be performed.

import { dump, load } from 'js-yaml'

const error = new Error('example')
const errorObject = serialize(error)
const errorYamlString = dump(errorObject)
// name: Error
// message: example
// stack: Error: example ...
const newErrorObject = load(errorYamlString)
const newError = parse(newErrorObject) // Error: example

Additional error properties

const error = new TypeError('example')
error.prop = true

const errorObject = serialize(error)
console.log(errorObject.prop) // true
const newError = parse(errorObject)
console.log(newError.prop) // true

Omit additional error properties

const error = new Error('example')
error.prop = true

const errorObject = serialize(error, { include: ['name', 'message', 'stack'] })
console.log(errorObject.prop) // undefined
console.log(errorObject) // { name: 'Error', message: 'example', stack: '...' }

Omit stack traces

const error = new Error('example')

const errorObject = serialize(error, { exclude: ['stack'] })
console.log(errorObject.stack) // undefined
console.log(errorObject) // { name: 'Error', message: 'example' }

Deep serialization/parsing

The loose option can be used to deeply serialize/parse objects and arrays.

const error = new Error('example')
const deepArray = serialize([{}, { error }], { loose: true })

const jsonString = JSON.stringify(deepArray)
const newDeepArray = JSON.parse(jsonString)

const newError = parse(newDeepArray, { loose: true })[1].error // Error: example

Transforming

const errors = [new Error('test secret')]
errors[0].date = new Date()

const errorObjects = serialize(errors, {
  loose: true,
  // Serialize `Date` instances as strings
  transformObject: (errorObject) => {
    errorObject.date = errorObject.date.toString()
  },
})
console.log(errorObjects[0].date) // Date string

const newErrors = parse(errorObjects, {
  loose: true,
  // Transform error message
  transformArgs: (constructorArgs) => {
    constructorArgs[0] = constructorArgs[0].replace('secret', '***')
  },
  // Parse date strings as `Date` instances
  transformInstance: (error) => {
    error.date = new Date(error.date)
  },
})
console.log(newErrors[0].message) // 'test ***'
console.log(newErrors[0].date) // `Date` instance

`error.cause` and `AggregateError`

const innerErrors = [new Error('one'), new Error('two')]
const cause = new Error('three')
const error = new AggregateError(innerErrors, 'four', { cause })

const errorObject = serialize(error)
// {
//   name: 'AggregateError',
//   message: 'four',
//   stack: '...',
//   cause: { name: 'Error', message: 'three', stack: '...' },
//   errors: [{ name: 'Error', message: 'one', stack: '...' }, ...],
// }
const newError = parse(errorObject)
// AggregateError: four
//   [cause]: Error: three
//   [errors]: [Error: one, Error: two]

Constructors

By default, when an error with custom classes is parsed, its constructor is not called. In most cases, this is not a problem since any property previously set by that constructor is still preserved, providing it is serializable and enumerable.

However, the error.constructorArgs property can be set to call the constructor with those arguments. It it throws, Error will be used as a fallback error class.

class CustomError extends Error {
  constructor(prefix, message) {
    super(`${prefix} - ${message}`)
    this.constructorArgs = [prefix, message]
  }
}
CustomError.prototype.name = 'CustomError'

const error = new CustomError('Prefix', 'example')

const errorObject = serialize(error)
// This calls `new CustomError('Prefix', 'example')`
const newError = parse(errorObject, { classes: { CustomError } })