new-error

new-error

A production-grade error creation library designed for Typescript. Useful for direct printing of errors to a client or for internal development / logs.

• All created errors extend Error with additional methods added on.
• Show errors that are safe for client / user-consumption vs internal only.
• Create your own custom error types with custom messaging, status codes and metadata.
• Attach an error to your error object to get the full error chain.
• Selectively expose error metadata based on internal or external use.
• Built-in auto-completion for Typescript when searching for registered error types.
• 100% test coverage

Table of Contents

• Installation
• Example Usage
  • With the error registry
  • Class-based with low level errors without a registry
  • Bare-bones class-based error
• Error Registry
  • Creating errors
    • Create a well-defined error
    • Create an error without a low-level error
  • instanceOf / comparisons
    • Comparing a custom error
    • Native instanceof
• Error API
  • Getters
  • Attaching errors
  • Format messages
  • Adding metadata
    • Safe metadata
    • Internal metadata
  • Serializing errors
    • Safe serialization
    • Internal serialization
• Example Express Error Handling

Installation

$ npm i new-error --save

Example Usage

With the error registry

• Define a set of high level errors
  • Common high level error types could be 4xx/5xx HTTP codes
• Define a set of low level errors
  • Think of low level errors as a fine-grained sub-code/category to a high level error
• Initialize the error registry with the errors

// This is a working example
import { ErrorRegistry } from 'new-error'

// Define high level errors
// Do *not* assign a Typescript type to the object
// or IDE autocompletion will not work!
const errors = {
  INTERNAL_SERVER_ERROR: {
   /**
    * The class name of the generated error
    */
    className: 'InternalServerError',
    /**
     * A user-friendly code to show to a client.
     */
    code: 'ERR_INT_500',
   /**
    * (optional) Protocol-specific status code, such as an HTTP status code. Used as the
    * default if a Low Level Error status code is not specified or defined.
    */
    statusCode: 500
  }
}

// Define low-level errors
// Do *not* assign a Typescript type to the object
// or IDE autocompletion will not work!
const errorCodes = {
  // 'type' of error
  DATABASE_FAILURE: {
    /**
     * Full description of the error. sprintf() flags can be applied
     * to customize it.
     * @see https://www.npmjs.com/package/sprintf-js
     */
    message: 'There was a database failure, SQL err code %s',
    /**
     * (optional) A user-friendly code to show to a client.
     */
    subCode: 'DB_0001',
    /**
     * (optional) Protocol-specific status code, such as an HTTP status code.
     */
    statusCode: 500
  }
}

// Create the error registry by registering your errors and codes
// you will want to memoize this as you will be using the
// reference throughout your application
const errRegistry = new ErrorRegistry(errors, errorCodes)

// Create an instance of InternalServerError
// Typescript autocomplete should show the available definitions as you type the error names
// and type check will ensure that the values are valid
const err = errRegistry.newError('INTERNAL_SERVER_ERROR', 'DATABASE_FAILURE').formatMessage('SQL_1234')
console.log(err.toJSON())

Produces:

(You can omit fields you do not need - see usage section below.)

{
  name: 'InternalServerError',
  code: 'ERR_INT_500',
  message: 'There was a database failure, SQL err code SQL_1234',
  type: 'DATABASE_FAILURE',
  subCode: 'DB_0001',
  statusCode: 500,
  meta: {},
  stack: 'InternalServerError: There was a database failure, SQL err code %s\n' +
    '    at ErrorRegistry.newError (new-error/src/ErrorRegistry.ts:128:12)\n' +
    '    at Object.<anonymous> (new-error/src/test.ts:55:25)\n' +
    '    at Module._compile (internal/modules/cjs/loader.js:1158:30)\n' +
    '    at Module._compile (new-error/node_modules/source-map-support/source-map-support.js:541:25)\n' +
    '    at Module.m._compile (/private/var/folders/mx/b54hc2lj3fbfsndkv4xmz8d80000gn/T/ts-node-dev-hook-20649714243977457.js:57:25)\n' +
    '    at Module._extensions..js (internal/modules/cjs/loader.js:1178:10)\n' +
    '    at require.extensions.<computed> (/private/var/folders/mx/b54hc2lj3fbfsndkv4xmz8d80000gn/T/ts-node-dev-hook-20649714243977457.js:59:14)\n' +
    '    at Object.nodeDevHook [as .ts] (new-error/node_modules/ts-node-dev/lib/hook.js:61:7)\n' +
    '    at Module.load (internal/modules/cjs/loader.js:1002:32)\n' +
    '    at Function.Module._load (internal/modules/cjs/loader.js:901:14)'
}

Class-based with low level errors without a registry

You can create concrete error classes by extending the BaseRegistryError class, which extends the BaseError class.

The registry example can be also written as:

import { BaseRegistryError, LowLevelError } from 'new-error'

class InternalServerError extends BaseRegistryError {
  constructor (errDef: LowLevelError) {
    super({
      code: 'ERR_INT_500',
      statusCode: 500
    }, errDef)
  }
}

const err = new InternalServerError({
  type: 'DATABASE_FAILURE',
  message: 'There was a database failure, SQL err code %s',
  subCode: 'DB_0001',
  statusCode: 500
})

console.log(err.formatMessage('SQL_1234').toJSON())

Bare-bones class-based error

If you want a native-style Error, you can use BaseError.

The registry example can be written as:

import { BaseError } from 'new-error'

class InternalServerError extends BaseError {}

const err = new InternalServerError('There was a database failure, SQL err code %s')
  .withErrorType('DATABASE_FAILURE')
  .withErrorCode('ERR_INT_500')
  .withErrorSubCode('DB_0001')
  .withStatusCode(500)

console.log(err.formatMessage('SQL_1234').toJSON())

Error Registry

The ErrorRegistry is responsible for the registration and creation of errors.

Creating errors

Errors generated by the registry extends BaseError.

Create a well-defined error

Method: ErrorRegistry#newError(highLevelErrorName, LowLevelErrorName)

This is the method you should generally use as you are forced to use your well-defined high and low level error definitions. This allows for consistency in how errors are defined and thrown.

// Creates an InternalServerError error with a DATABASE_FAILURE code and corresponding
// message and status code
const err = errRegistry.newError('INTERNAL_SERVER_ERROR', 'DATABASE_FAILURE')

Create an error without a low-level error

Method: ErrorRegistry#newBareError(highLevelErrorName, message)

This method does not include a low level error code, and allows direct specification of an error message.

// Creates an InternalServerError error with a custom message
const err = errRegistry.newBareError('INTERNAL_SERVER_ERROR', 'An internal server error has occured.')

instanceOf / comparisons

Comparing a custom error

Method: ErrorRegistry#instanceOf(classInstance, highLevelErrorName)

Performs an instanceof operation against a custom error.

// creates an InternalServerError error instance
const err = errRegistry.newError('INTERNAL_SERVER_ERROR', 'DATABASE_FAILURE')

if (errRegistry.instanceOf(err, 'INTERNAL_SERVER_ERROR')) {
  // resolves to true since err is an InternalServerError instance
}

Native instanceof

You can also check if the error is custom-built using this check:

import { BaseError } from 'new-error'

function handleError(err) {
  if (err instanceof BaseError) {
    // err is a custom error
  }
}

Error API

Except for the serialization methods, all methods are chainable.

Generated errors extend the BaseError class, which supplies the manipulation methods.

Getters

• BaseError#getCode()
• BaseError#getSubCode()
• BaseError#getStatusCode()
• BaseError#getCausedBy()
• BaseError#getMetadata()
• BaseError#getSafeMetadata()

Attaching errors

Method: BaseError#causedBy(err)

You can attach another error to the error.

const externalError = new Error('Some thrown error')
err.causedBy(externalError)

Format messages

Method: BaseError#formatMessage(...formatParams)

See the sprintf-js package for usage.

// specify the database specific error code
// Transforms the message to:
// 'There was a database failure, SQL err code %s' ->
// 'There was a database failure, SQL err code SQL_ERR_1234',
err.formatMessage('SQL_ERR_1234')

Adding metadata

Safe metadata

Method: BaseError#withSafeMetadata(data = {})

Safe metadata would be any kind of data that you would be ok with exposing to a client, like an HTTP response.

err.withSafeMetadata({
  errorId: 'err-12345',
  moreData: 1234
})
// can be chained to append more data
.withSafeMetadata({
  requestId: 'req-12345'
})

This can also be written as:

err.withSafeMetadata({
  errorId: 'err-12345',
  moreData: 1234
})

// This will append requestId to the metadata
err.withSafeMetadata({
  requestId: 'req-12345'
})

Internal metadata

Method: BaseError#withMetadata(data = {})

Internal metadata would be any kind of data that you would not be ok with exposing to a client, but would be useful for internal development / logging purposes.

err.withMetadata({
  email: '[e-mail]'
})
// can be chained to append more data
.withMetadata({
  userId: 'user-abcd'
})

Serializing errors

Safe serialization

Method: BaseError#toJSONSafe(fieldsToOmit = [])

Generates output that would be safe for client consumption.

• Omits name
• Omits message
• Omits causedBy
• Omits type
• Omits the stack trace
• Omits any data defined via BaseError#withMetadata()

err.withSafeMetadata({
  errorId: 'err-12345',
  requestId: 'req-12345'
})
// you can remove additional fields by specifying property names in an array
//.toJSONSafe(['code']) removes the code field from output
.toJSONSafe()

Produces:

{
  code: 'ERR_INT_500',
  subCode: 'DB_0001',
  statusCode: 500,
  meta: { errorId: 'err-12345', requestId: 'req-12345' }
}

Internal serialization

Method: BaseError#toJSON(fieldsToOmit = [])

Generates output that would be suitable for internal use.

• Includes name
• Includes type
• Includes message
• Includes causedBy
• Includes the stack trace
• All data from BaseError#withMetadata() and BaseError#withJSONMetadata() is included

err.withSafeMetadata({
  errorId: 'err-12345',
}).withMetadata({
  email: '[e-mail]'
})
// you can remove additional fields by specifying property names in an array
//.toJSON(['code', 'statusCode']) removes the code and statusCode field from output
.toJSON()

Produces:

{
  name: 'InternalServerError',
  code: 'ERR_INT_500',
  message: 'There was a database failure, SQL err code %s',
  type: 'DATABASE_FAILURE',
  subCode: 'DB_0001',
  statusCode: 500,
  meta: { errorId: 'err-12345', requestId: 'req-12345' },
  stack: 'InternalServerError: There was a database failure, SQL err code %s\n' +
    '    at ErrorRegistry.newError (new-error/src/ErrorRegistry.ts:128:12)\n' +
    '    at Object.<anonymous> (new-error/src/test.ts:55:25)\n' +
    '    at Module._compile (internal/modules/cjs/loader.js:1158:30)\n' +
    '    at Module._compile (new-error/node_modules/source-map-support/source-map-support.js:541:25)\n' +
    '    at Module.m._compile (/private/var/folders/mx/b54hc2lj3fbfsndkv4xmz8d80000gn/T/ts-node-dev-hook-17091160954051898.js:57:25)\n' +
    '    at Module._extensions..js (internal/modules/cjs/loader.js:1178:10)\n' +
    '    at require.extensions.<computed> (/private/var/folders/mx/b54hc2lj3fbfsndkv4xmz8d80000gn/T/ts-node-dev-hook-17091160954051898.js:59:14)\n' +
    '    at Object.nodeDevHook [as .ts] (new-error/node_modules/ts-node-dev/lib/hook.js:61:7)\n' +
    '    at Module.load (internal/modules/cjs/loader.js:1002:32)\n' +
    '    at Function.Module._load (internal/modules/cjs/loader.js:901:14)'
}

Example Express Error Handling

import express from 'express'
import { ErrorRegistry, BaseError } from 'new-error'
const app = express()
const port = 3000

const errors = {
  INTERNAL_SERVER_ERROR: {
    className: 'InternalServerError',
    code: 'ERR_INT_500',
    statusCode: 500
  }
}

const errorCodes = {
  DATABASE_FAILURE: {
    message: 'There was a database failure.',
    subCode: 'DB_0001',
    statusCode: 500
  }
}

const errRegistry = new ErrorRegistry(errors, errorCodes)

// middleware definition
app.get('/', async (req, res, next) => {
  try {
    // simulate a failure
    throw new Error('SQL issue')
  } catch (e) {
    const err = errRegistry.newError('INTERNAL_SERVER_ERROR', 'DATABASE_FAILURE')
    err.causedBy(err)
    // errors must be passed to next()
    // to be caught when using an async middleware
    return next(err)
  }
})

// catch errors
app.use((err, req, res, next) => {
  // error was sent from middleware
  if (err) {
    // check if the error is a generated one
    if (err instanceof BaseError) {
      // get the status code, if the status code is not defined, default to 500
      res.status(err.getStatusCode() ?? 500)
      // spit out the error to the client
      return res.json({
        err: err.toJSONSafe()
      })
    }
  }

  // no error, proceed
  next()
})

app.listen(port, () => console.log(`Example app listening at http://localhost:${port}`))

If you visit http://localhost:3000, you'll get a 500 status code, and the following response:

{"err": {"code":"ERR_INT_500","subCode":"DB_0001","statusCode":500,"meta":{}}}

Changelog

1.0.6 - Fri May 15 2020 23:34:12

Contributor: Theo Gravity

• [minor] Concrete class-based support (#1)

• Added examples on how to work with the library without the registry

• Updated and exported some interfaces to assist with class-based creation