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Easy error subclassing and stack customization

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Package description

What is error-ex?

The error-ex npm package is designed to create more customizable and informative error objects in JavaScript. It allows developers to extend the native Error type with additional properties and behaviors, making error handling and debugging more efficient.

What are error-ex's main functionalities?

Creating custom error types

This feature allows developers to create custom error types by extending the native Error object. The custom error can then be thrown with a specific message, making it easier to identify and handle specific kinds of errors in the code.

const ErrorEx = require('error-ex');
const MyError = ErrorEx('MyError');
throw new MyError('Something went wrong');

Adding properties to errors

This functionality enables the addition of custom properties to error objects. In this example, a 'code' property is added to a custom error type, which could be used to store HTTP status codes or other relevant error information.

const ErrorEx = require('error-ex');
const MyError = ErrorEx('MyError');
MyError.prototype.code = 404;
const errorInstance = new MyError('Resource not found');
console.log(errorInstance.code); // 404

Other packages similar to error-ex



node-error-ex Build Status Coverage Rating

Easily subclass and customize new Error types


To include in your project:

var errorEx = require('error-ex');

To create an error message type with a specific name (note, that will not reflect this):

var JSONError = errorEx('JSONError');

var err = new JSONError('error');; //-> JSONError
throw err; //-> JSONError: error

To add a stack line:

var JSONError = errorEx('JSONError', {fileName: errorEx.line('in %s')});

var err = new JSONError('error')
err.fileName = '/a/b/c/foo.json';
throw err; //-> (line 2)-> in /a/b/c/foo.json

To append to the error message:

var JSONError = errorEx('JSONError', {fileName: errorEx.append('in %s')});

var err = new JSONError('error');
err.fileName = '/a/b/c/foo.json';
throw err; //-> JSONError: error in /a/b/c/foo.json


errorEx([name], [properties])

Creates a new ErrorEx error type

  • name: the name of the new type (appears in the error message upon throw; defaults to
  • properties: if supplied, used as a key/value dictionary of properties to use when building up the stack message. Keys are property names that are looked up on the error message, and then passed to function values.
    • line: if specified and is a function, return value is added as a stack entry (error-ex will indent for you). Passed the property value given the key.
    • stack: if specified and is a function, passed the value of the property using the key, and the raw stack lines as a second argument. Takes no return value (but the stack can be modified directly).
    • message: if specified and is a function, return value is used as new .message value upon get. Passed the property value of the property named by key, and the existing message is passed as the second argument as an array of lines (suitable for multi-line messages).

Returns a constructor (Function) that can be used just like the regular Error constructor.

var errorEx = require('error-ex');

var BasicError = errorEx();

var NamedError = errorEx('NamedError');

// --

var AdvancedError = errorEx('AdvancedError', {
	foo: {
		line: function (value, stack) {
			if (value) {
				return 'bar ' + value;
			return null;

var err = new AdvancedError('hello, world'); = 'baz';
throw err;

	AdvancedError: hello, world
	    bar baz
	    at tryReadme() (readme.js:20:1)

Creates a stack line using a delimiter

This is a helper function. It is to be used in lieu of writing a value object for properties values.

  • str: The string to create
    • Use the delimiter %s to specify where in the string the value should go
var errorEx = require('error-ex');

var FileError = errorEx('FileError', {fileName: errorEx.line('in %s')});

var err = new FileError('problem reading file');
err.fileName = '/a/b/c/d/foo.js';
throw err;

	FileError: problem reading file
	    in /a/b/c/d/foo.js
	    at tryReadme() (readme.js:7:1)

Appends to the error.message string

This is a helper function. It is to be used in lieu of writing a value object for properties values.

  • str: The string to append
    • Use the delimiter %s to specify where in the string the value should go
var errorEx = require('error-ex');

var SyntaxError = errorEx('SyntaxError', {fileName: errorEx.append('in %s')});

var err = new SyntaxError('improper indentation');
err.fileName = '/a/b/c/d/foo.js';
throw err;

	SyntaxError: improper indentation in /a/b/c/d/foo.js
	    at tryReadme() (readme.js:7:1)


Licensed under the MIT License. You can find a copy of it in LICENSE.



Last updated on 19 Jun 2018

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