Creating custom Error classes in Node.js

custom-error.js

'use strict';

module.exports = function CustomError(message, extra) {
  Error.captureStackTrace(this, this.constructor);
  this.name = this.constructor.name;
  this.message = message;
  this.extra = extra;
};

require('util').inherits(module.exports, Error);

error.md

It's nice to be able to distinguish error types by classes. But it's a bit tricky to correctly create a custom error class in Node.js, so here is an example.

The example also shows how to add an extra parameter called extra that will be stored as a property on the error.

Usage

var CustomError = require('./errors/custom-error');

function doSomethingBad() {
  throw new CustomError('It went bad!', 42);
}

Features

• Name appears once - less editing if you have to create lots of custom error classes
• Easy to subclass - just change the last line to inherit from another custom error class you created
• Correct stack trace - no extra stack frames, no double capturing of the stack trace

Anti-patterns

These are some things that I've seen in other proposed solutions that you should avoid.

• Error.call(this) - creates another error object (wasting a bunch of time) and doesn't touch this at all
• Error.captureStackTrace(this, arguments.callee); - works, but arguments.callee is deprecated, so don't use it
• this.stack = (new Error).stack - this... I don't even...

test-error.js

// Mini test suite for our custom error

var assert = require('assert');
var CustomError = require('./errors/custom-error');

function doSomethingBad() {
  throw new CustomError('It went bad!', 42);
}

try {
  doSomethingBad();
} catch (err) {
  // The name property should be set to the error's name
  assert(err.name = 'CustomError');

  // The error should be an instance of its class
  assert(err instanceof CustomError);

  // The error should be an instance of builtin Error
  assert(err instanceof Error);

  // The error should be recognized by Node.js' util#isError
  assert(require('util').isError(err));

  // The error should have recorded a stack
  assert(err.stack);

  // toString should return the default error message formatting
  assert.strictEqual(err.toString(),
                     'CustomError: It went bad!');

  // The stack should start with the default error message formatting
  assert.strictEqual(err.stack.split('\n')[0],
                     'CustomError: It went bad!');

  // The first stack frame should be the function where the error was thrown.
  assert.strictEqual(err.stack.split('\n')[1].indexOf('doSomethingBad'), 7);

  // The extra property should have been set
  assert.strictEqual(err.extra, 42);
}

// Spoiler: It passes!

In the test suite, there is a small typo: assert(err.name = 'CustomError'); should be assert(err.name === 'CustomError'); or assert.strictEqual(err.name, 'CustomError');

Error.captureStackTrace is V8 exclusive, if you want to be extra careful call it this way:

Error.captureStackTrace && Error.captureStackTrace(this, this.constructor);

(great gist by the way!)

👍

This is excellent, thank you.

I'd previously been following suggestions from MDN, but was not really satisfied.

How do you match the string representation of CustomError to that of Error object?

Object.prototype.toString.call(new Error); // '[object Error]'
Object.prototype.toString.call(new CustomError); //  '[object Object]'

@hassansin in ES5, you cannot change the toString representation of an Object.

I'm using Visual Studio Code and am getting a Warning when creating the Error with new:

Only a void function can be called with the 'new' keyword.

Any idea whether there's an easy way to fix that?

@netmikey are you returning something from the constructor? If you are you shouldn't have to and it sounds like that's what the warning means.

By default an object called with the new keyword returns 'this'

👍

And here is a working snippet for use with ES2015 classes:

class CustomError extends Error {
  constructor ( message, extra ) {
    super()
    Error.captureStackTrace( this, this.constructor )
    this.name = 'CustomError'
    this.message = message
    if ( extra ) this.extra = extra
  }
}

FYI - be careful using this.constructor.name - if you minify/obfuscate your code for any reason, the name gets mangled.

Thanks for the code snippet.
I have updated my project errorable with this method.

https://github.com/calidion/errorable

Errorable is a simple tool to define errors and can be easily adopted for APIs.
Hope it helps.

I'm using TypeScript and came up with the following solution:

class MyError extends Error {

  static name: string;

  constructor(public message?: string, public extra?: number) {
    super(message);
    Error.captureStackTrace(this, MyError);
    this.name = (this as any).constructor.name; // OR this.name = (<any>this).constructor.name;
  }

};

Note that you must have the node typings installed in order to use Error.captureStackTrace.

Thank you for this great Gist!

I've made a custom version of it to be used in ES6/ES2015:
https://gist.github.com/slavafomin/b164e3e710a6fc9352c934b9073e7216

Would love to hear your opinion on it.

In Node 7.1 and TypeScript, this works:

export class ErrorBase extends Error {
  public name: string;

  constructor(message: string) {
    super(message);
    this.name = (this.constructor as any).name;
    this.message = message;
    Error.captureStackTrace(this, this.constructor); // after initialize properties
  }
}

Updated:

this features was added in typescript 2.1, so this implementation is better (see also below comment):

export interface ErrorBase extends Error {
  readonly name: string;
  readonly message: string;
  readonly stack: string;
};
export interface ErrorBaseConstructor {
  new (message: string): ErrorBase;
  readonly prototype: ErrorBase;
}

export const ErrorBase: ErrorBaseConstructor = <any>class ErrorBase {
  public constructor(message: string) {
    Object.defineProperty(this, 'name', {
      get: () => (this.constructor as any).name,
    });
    Object.defineProperty(this, 'message', {
      get: () => message,
    });
    Error.captureStackTrace(this, this.constructor);
  }
};
(ErrorBase as any).prototype = Object.create(Error.prototype);
ErrorBase.prototype.constructor = ErrorBase;

In Node (7.2) console.log will print custom errors a bit differently unless all properties are defined as non-enumerable:

class CustomError extends Error {
  constructor(...args) {    
    super(...args);    
    Object.defineProperty(this, "name", {           
      value: this.constructor.name
    });        
    Error.captureStackTrace(this, this.constructor);
  }
}

@urugator: do you have a link about this? Thank you!

I avoid classes (classical-inheritance is flawed) and types (google: instanceof lies).
The best solution is just to duck-type. See http://gunargessner.com/subclassing-exception/.

It's more better move Error.captureStackTrace(this, this.constructor); after this.message = message;, then the e.stack will output like this:
CustomError: custom ...
Otherwise it will output:
Error ...
that output of stack will miss error type and message

So tricky

node --version  
v7.10.0

module.exports = function CustomError(message, extra) {
  this.name = this.constructor.name
  this.message = message
  Error.captureStackTrace(this, this.constructor)
  this.extra = extra
}

module.exports.prototype.inspect = function () {
  return this.stack
}

Error.test.js

class MyCustomError extends require('./Errors') {}

try {
  throw new MyCustomError ('message')
} catch (err) {
  console.log(err);
  console.log(err.constructor.name);
  console.log(err instanceof MyCustomError)
  console.log(err.stack);
}

node Error.test.js

MyCustomError: message
    at Object.<anonymous> (Error.test.js:4:9)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.runMain (module.js:605:10)
    at run (bootstrap_node.js:427:7)
    at startup (bootstrap_node.js:151:9)
    at bootstrap_node.js:542:3

https://gist.github.com/mrenrich84/f499c4697a5f3e650f37def3ec8acc22

Why Not

     function MyCustomError(message, extra) {
          const error  = new Error(message);
          error.name = 'MyCustomError';
          if (extra) {
                error.extra = extra;
           }

          return error;
     }


throw MyCustomError('Something Went Wrong');

Because

var error = new MyCustomError('Something Went Wrong');
error instanceof MyCustomError // false

Which is not expected. You can't check the type of error for example in this case

try { }
catch(err){
  switch(true){
    case err instanceof MyCustomError: break;
    case err instanceof Error: break;
  }
}

https://github.com/joyent/node-verror

These were all showing up in the console as an object, not as the usual Error:

{ CustomError: test
    at ... 
        name: 'CustomError' }

Instead if I use the name getter, I get the usual error-like appearance:

class CustomError { 
    get name () { return this.constructor.name } 
}

CustomError: test
    at ...

How do you match the string representation of CustomError to that of Error object?

Object.prototype.toString.call(new Error); // '[object Error]'
Object.prototype.toString.call(new CustomError); //  '[object Object]'

You need to define the toStringTag for the CustomError:

Object.defineProperty(CustomError.prototype, Symbol.toStringTag, {
	value: 'Error',
	writable: false, configurable: false, enumerable: false
});

This consequently help with libraries like lodash and it's _.isError().

Thanks for the gist, and thanks to all the commenters for the suggestions.