@namecheap/error-extender

Simplifies creation of custom Error classes for Node.js and Browser!

...which then produces stack with appended stacks of supplied cause (very much like in Java)!

const extendError = require('@namecheap/error-extender');

const CustomError = extendError('CustomError');

const rootCause = new Error('the root cause');

console.log(new CustomError({ message: 'An error has occurred.', cause: rootCause }));

Shall output:

CustomError: An error has occurred.
    at Object.<anonymous> (/opt/app/index.js:7:13)
    at Module._compile (internal/modules/cjs/loader.js:702:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
    at Module.load (internal/modules/cjs/loader.js:612:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
    at Function.Module._load (internal/modules/cjs/loader.js:543:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:744:10)
    at startup (internal/bootstrap/node.js:240:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:564:3)
Caused by: Error: the root cause
    at Object.<anonymous> (/opt/app/index.js:5:19)
    at Module._compile (internal/modules/cjs/loader.js:702:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:713:10)
    at Module.load (internal/modules/cjs/loader.js:612:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:551:12)
    at Function.Module._load (internal/modules/cjs/loader.js:543:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:744:10)
    at startup (internal/bootstrap/node.js:240:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:564:3)

100% Code Coverage

Oh, by the way, 100% test coverage. See for yourself (via npm test)!

Features

"Extending" Errors

It's quite simple! See below:

const extendError = require('@namecheap/error-extender');

const AppError = extendError('AppError'); // extends `Error` (default)

Or... A bit more complex using the second argument (options):

const extendError = require('@namecheap/error-extender');

const AppError = extendError('AppError', {
  defaultMessage: 'An unhandled error has occurred.',
  defaultData: { status: 503, message: 'An unhandled error has occurred.' }
});

const ServiceError = extendError('ServiceError', {
  parent: AppError, // extends `AppError`
  defaultMessage: 'A service error has occurred.',
  defaultData: { status: 500, message: 'A service error has occurred.' }
});

const DatabaseError = extendError('DatabaseError', {
  parent: ServiceError, // extends `ServiceError`
  defaultMessage: 'A service database error has occurred.',
  defaultData: { message: 'A service database error has occurred.' }
});

require('assert').deepStrictEqual(
  DatabaseError.defaultData, {
    status: 500,
    message: 'A service database error has occurred.'
  });
// no error

Yes, defaultData merges!

error-extender Arguments

error-extender accepts a single object literal as second argument.

The options (object literal keys) are as follows:

key	expected type
parent	Error.prototype or one that extends it
defaultMessage	string
defaultData	any

"Extended Errors"

1. Creates prototype-based Error classes (child/subclass) : "Extended Errors".
2. Those "Extended Errors", accepts cause (Error); very much like how it is with Java Exception.
3. Appends stack of cause to the bottom (or top) of instantiated "Extended Errors" stack.
4. "Extended Errors" constructor & argument (w/ optional new):
   1. new ExtendedError(options)
   2. ExtendedError(options)

Yes, much like JavaScript's native Error, "Extended Errors" can be written/used "factory-like" (without the new keyword).

"Extended Errors" Arguments (constructor)

"Extended Errors" accepts a single object literal as argument:

const extendError = require('@namecheap/error-extender');
const ServiceError = extendError('ServiceError');
try {
  // ... something throws something
} catch (error) {
  throw new ServiceError({
    message: 'An error has occurred',
    data: { ref: '7e9f876ca116' },
    cause: error
  });
}

The options (object literal keys) are as follows:

key	alias	expected type
message	m	string
data	d	any
cause	c	instancedof Error

Given the alias, you may construct extended errors by:

const extendError = require('@namecheap/error-extender');
const ServiceError = extendError('ServiceError');
try {
  // ... something throws something
} catch (error) {
  throw new ServiceError({
    m: 'An error has occurred',
    d: { ref: '7e9f876ca116' },
    c: error
  });
}

Note: Aliases are evaluated first; hence if you have both m and message, if m's value is truthy, then m's value will be used.

Instance Properties

As with Error, "Extended Errors" would have the following properties:

• name
• message
• stack

... "Extended Errors" shall have the following additiona properties:

• data - (as set in constructor args)
• cause - (as set in constructor args)

data merging w/ defaultData

Yes, you heard right, instance data merges with defaultData!!!

See example below:

const extendError = require('@namecheap/error-extender');

const AppError = extendError('ServiceError', {
  defaultData: { status: 503, message: 'An unhandled error has occurred.' }
});

const appError = new AppError({ d: { status: 401 } });

require('assert').deepStrictEqual(
  appError.data, {
    status: 401,
    message: 'An unhandled error has occurred.'
  });
// no error

The inspiration (thanks bluebird!):

const Promise = require('bluebird');
// ...
const extendError = require('@namecheap/error-extender');
// ...
const ServiceError = extendError('ServiceError');
const ServiceStateError = extendError(
  'ServiceStateError',
  { parent: ServiceError });
// ...
function aServiceFunction() {
  return new Promise(
    function (resolve, reject) {
      // ... multiple things that may throw your
      //     custom "expected" errors
    })
    .catch(ServiceStateError, function (error) {
      // ... your "common way" of handling
      //     ServiceStateError
      // ... then propagate
    })
    .catch(ServiceError, function (error) {
      // ... your "common generc way" of handling
      //     ServiceError
      // ... then propagate
    })
    .catch(function (error) {
      // ... the "catch all"
      // ... then propagate
    });
}

With JavaScript, I felt quite stifled when I was limited to:

1. Do selective/custom handling based on matching messages from throw new Error('..').
2. Return/propagate JSend-like responses to function "callers"/"users".
3. ... or whatever error possible passing/handling could be done, throughout functions and callers/users.

With error-extender with help from syntactic-sugar from bluebird, you can improve (or even standardize) your way of propagating/handling errors throughout your application. callers.