Merge an error with its cause.

This merges error.cause recursively with its parent error, including its message, stack, name and errors.

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Example

import mergeErrorCause from 'merge-error-cause'

const main = (userId) => {
  try {
    return createUser(userId)
  } catch (error) {
    throw mergeErrorCause(error)
    // Printed as:
    //   TypeError: Invalid user id: false
    //   Could not create user.
  }
}

const createUser = (userId) => {
  try {
    validateUserId(userId)
    return sendDatabaseRequest('create', userId)
  } catch (cause) {
    throw new Error('Could not create user.', { cause })
  }
}

const validateUserId = (userId) => {
  if (typeof userId !== 'string') {
    throw new TypeError(`Invalid user id: ${userId}.`)
  }
}

main(false)

Install

npm install merge-error-cause

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

mergeErrorCause(error)

error Error | any
Return value: Error

error is modified and returned.

If error's class is Error or if error.wrap is true, error.cause is modified and returned instead.

If error is not a valid Error, a new error is created and returned instead.

This never throws.

Background

error.cause is a recent JavaScript feature to wrap error messages and properties.

try {
  validateUserId(userId)
  sendDatabaseRequest('create', userId)
} catch (cause) {
  throw new Error('Could not create user.', { cause })
}

However, it comes with a few issues.

Traversing error.cause

Problem

Consumers need to traverse error.cause.

try {
  createUser(userId)
} catch (error) {
  if (error.code === 'E101' || (error.cause && error.cause.code === 'E101')) {
    // Checking for properties requires traversing `error.cause`
  }

  if (
    error.name === 'UserError' ||
    (error.cause && error.cause.name === 'UserError')
  ) {
    // So does checking for error class
  }
}

This is tricky to get right. For example:

• error.cause.cause might also exist (and so on)
• If error is not an Error instance, error.name might throw
• Recursing over error.cause might be an infinite cycle

Solution

This library merges error.cause recursively. It also ensures error is an Error instance. Consumers can then handle errors without checking its cause.

try {
  createUser(userId)
} catch (error) {
  if (error.code === 'E101') {
    /* ... */
  }

  if (error.name === 'UserError') {
    /* ... */
  }
}

Verbose stack trace

Problem

Stack traces with multiple error.cause can be quite verbose.

Error: Could not create user group.
    at createUserGroup (/home/user/app/user_group.js:19:9)
    at createGroups (/home/user/app/user_group.js:101:10)
    at startApp (/home/user/app/app.js:35:20)
    at main (/home/user/app/app.js:3:4) {
  [cause]: Error: Could not create user.
      at newUser (/home/user/app/user.js:52:7)
      at createUser (/home/user/app/user.js:43:5)
      at createUserGroup (/home/user/app/user_group.js:17:11)
      at createGroups (/home/user/app/user_group.js:101:10)
      at startApp (/home/user/app/app.js:35:20)
      at main (/home/user/app/app.js:3:4) {
    [cause]: Error: Invalid user.
        at validateUser (/home/user/app/user.js:159:8)
        at userInstance (/home/user/app/user.js:20:4)
        at newUser (/home/user/app/user.js:50:7)
        at createUser (/home/user/app/user.js:43:5)
        at createUserGroup (/home/user/app/user_group.js:17:11)
        at createGroups (/home/user/app/user_group.js:101:10)
        at startApp (/home/user/app/app.js:35:20)
        at main (/home/user/app/app.js:3:4) {
      [cause]: UserError: User "15" does not exist.
          at checkUserId (/home/user/app/user.js:195:3)
          at checkUserExist (/home/user/app/user.js:170:10)
          at validateUser (/home/user/app/user.js:157:23)
          at userInstance (/home/user/app/user.js:20:4)
          at newUser (/home/user/app/user.js:50:7)
          at createUser (/home/user/app/user.js:43:5)
          at createUserGroup (/home/user/app/user_group.js:17:11)
          at createGroups (/home/user/app/user_group.js:101:10)
          at startApp (/home/user/app/app.js:35:20)
          at main (/home/user/app/app.js:3:4)
    }
  }
}

Each error cause is indented and printed separately.

• The stack traces mostly repeat each other since the function calls are part of the same line execution
• The most relevant message (innermost) is harder to find since it is shown last

Solution

This library only keeps the innermost stack trace. Error messages are concatenated by default from innermost to outermost. This results in much simpler stack traces without losing any information.

TypeError: User "15" does not exist.
Invalid user.
Could not create user.
Could not create user group.
    at checkUserId (/home/user/app/user.js:195:3)
    at checkUserExist (/home/user/app/user.js:170:10)
    at validateUser (/home/user/app/user.js:157:23)
    at userInstance (/home/user/app/user.js:20:4)
    at newUser (/home/user/app/user.js:50:7)
    at createUser (/home/user/app/user.js:43:5)
    at createUserGroup (/home/user/app/user_group.js:17:11)
    at createGroups (/home/user/app/user_group.js:101:10)
    at startApp (/home/user/app/app.js:35:20)
    at main (/home/user/app/app.js:3:4)

Features

Stack traces

Only the innermost stack trace is kept.

Please make sure you use async/await instead of new Promise() or callbacks to prevent truncated stack traces.

Messages

Inner error messages are printed first.

try {
  throw new Error('Invalid user id.')
} catch (cause) {
  throw new Error('Could not create user.', { cause })
  // Printed as:
  //   Error: Invalid user id.
  //   Could not create user.
}

If the outer error message ends with :, it is prepended instead.

try {
  throw new Error('Invalid user id.')
} catch (cause) {
  throw new Error('Could not create user:', { cause })
  // Printed as:
  //   Error: Could not create user: Invalid user id.
}

: can optionally be followed by a newline.

try {
  throw new Error('Invalid user id.')
} catch (cause) {
  throw new Error('Could not create user:\n', { cause })
  // Printed as:
  //   Error: Could not create user:
  //   Invalid user id.
}

Error class

The outer error class is used.

try {
  throw new TypeError('User id is not a string.')
} catch (cause) {
  const error = new UserError('Could not create user.', { cause })
  const mergedError = mergeErrorCause(error)
  console.log(mergedError instanceof UserError) // true
  console.log(mergedError.name) // 'UserError'
}

If the parent error class is Error, the child class is used instead. This allows wrapping the error message or properties while keeping its class.

try {
  throw new TypeError('User id is not a string.')
} catch (cause) {
  const error = new Error('Could not create user.', { cause })
  console.log(mergeErrorCause(error) instanceof TypeError) // true
}

error.wrap: true has the same effect, but works with any parent error class.

try {
  throw new TypeError('User id is not a string.')
} catch (cause) {
  const error = new UserError('Could not create user.', { cause })
  error.wrap = true
  console.log(mergeErrorCause(error) instanceof TypeError) // true
}

Error properties

Error properties are shallowly merged.

// Both `userId` and `invalidUser` are kept
try {
  throw Object.assign(new Error('Invalid user id.'), { userId: '5' })
} catch (cause) {
  throw Object.assign(new Error('Could not create user.', { cause }), {
    invalidUser: true,
  })
}

Empty error messages are ignored. This is useful when wrapping error properties.

try {
  throw new Error('Invalid user id.')
} catch (cause) {
  throw Object.assign(new Error('', { cause }), { invalidUser: true })
}

Aggregate errors

Any aggregateError.errors[*].cause is processed recursively. However, aggregateError.errors are not merged with each other since those are different from each other.

If both error.errors and error.cause.errors exist, they are concatenated.

Normalization

Invalid errors are normalized to proper Error instances.

try {
  throw 'Invalid user id.'
} catch (error) {
  console.log(mergeErrorCause(error)) // Error: Invalid user id.
}

Related projects

• modern-errors: Handle errors in a simple, stable, consistent way
• error-custom-class: Create one error class
• error-class-utils: Utilities to properly create error classes
• error-serializer: Convert errors to/from plain objects
• normalize-exception: Normalize exceptions/errors
• is-error-instance: Check if a value is an Error instance
• set-error-class: Properly update an error's class
• set-error-message: Properly update an error's message
• wrap-error-message: Properly wrap an error's message
• set-error-props: Properly update an error's properties
• set-error-stack: Properly update an error's stack
• error-cause-polyfill: Polyfill error.cause
• handle-cli-error: 💣 Error handler for CLI applications 💥
• log-process-errors: Show some ❤ to Node.js process errors
• error-http-response: Create HTTP error responses
• winston-error-format: Log errors with Winston

Support

For any question, don't hesitate to submit an issue on GitHub.

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