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Errati is a flexible errors index for NodeJS that helps you to use the Node callback pattern to its fullest. It helps the developer to use errors as active messangers that are consistent through the application.

At its simplest use, it helps you use a standardized set of errors and ease development, debugging and feedback. In more complex use, it enables custom values to be passed in an expected manner, to process errors and take action through the backtracking of callbacks and automatically take appropriate actions when errors are instantiated.

All of this is done "define once, use everywhere" with straight forward Javascript syntax.

Errati is style agnostic and aims to be adapted to the developers needs instead of the other way round. There are no dependencies and no learning curve to speak of. Register your errors from a dictionary (or use the provided HTTP Standard Codes) and you are ready to go.

The design goals are:

  1. Efficiency
  2. Flexibility
  3. Ease of use

Use case example:
A module instantiates an error due to a failed user request. The error is populated with default and custom values. It is propagated back through the callback chain where actions may be triggered and additional data is added to the error instace. Finally one set of error data is returned to the user, while another set of data is logged. For severe actions, instantiation is extended with a function to notify somebody responsible to take immediate action.

Released under BSD licence.
Mikael Konttinen / Somewhat Original.

Table of contents


Start with registering errors from a dictionary, either the provided http codes or a custom dictionary. Errati uses a singletonish pattern to have the indexed errors be consistently provided wherever required. Errors can be used interchangably as reference and/or instances.

The dictionary can be a Javascript Object or Array, depending on your preferences and needs. The dictionary specifies error codes, names, custom values and functions. All definitions are made by the order of generic to specific, meaning you can define values or functions for the entire dictionary and progressively override them for the class and the error instance.

Errati adds a couple of helpers for common needs, all of which can be overridden. The helpers are toString(), toObject() and toJson(). You can easily add custom helpers in your desired scope. Errati also comes with internal parsers for instantiation arguments and stack traces, and a post-instantiation hook. All of these can also be overridden with custom functions.

Errati comes with two dictionaries of HTTP Standard Codes. One "short" dictionary of most used HTTP Standard Errors, and one "full" dictionary of HTTP Standard Codes. Using one of these is optional, as is extending the index with custom dictionaries. Multiple dictionaries can be registered and separated in the index by prefixes.

Errati aims to be as straightforward as possible. All registered Errors are Javascript functions that inherit the prototype from the Javascript Error.

The code is well documented in case you would need additional functionality that is not provided out of the box.

Tests are included to verify the functionality of Errati.


    npm install errati
        var options = { /* ... */ };
        var Errati = require('errati').setup( options );
        var Errati = require('errati');

...and you are ready to go. Note that you must call the .setup() function to initialize Errati. If no options are provided, the included "http-short" dictionary of the most common HTTP Standard Errors is loaded.

To load the "http-full" dictionary of HTTP Standard Codes, use the following options.

    var options = { dictionary:'http-full' };
    var Errati = require('errati').setup( options );
    console.log( Errati );
    // List of registered errors 

Note that the HTTP dictionaries are not more than collections of standardized codes and names. You can use them for any desired application.

And to load your custom list of errors:

    var myErrors = { '101':'First error', '102':'Second error' };
    var options = { dictionary: myErrors };
    var Errati = require('errati').setup( options );
    console.log( Errati )
    // { FirstError: { [Function] _name: 'First error', code: '101' }, 
    //   SecondError: { [Function] _name: 'Second error', code: '102' } } 
    console.log( Errati.FirstError );
    // { _name:'First error', code:'101' } 
    console.log( new Errati.FirstError( 'This is a message') );
    // { name:'First error', code:'101', message:'This is a message' } 

Note that 'name' is a reserved field for javascript functions, so error classes use the underscore prefixed field '_name' instead.


Configuration of Errati is done by passing a configuration object (from here; 'options') to the Errati.setup() function. The options specify how the jointly passed dictionary is parsed.

The options are divided in the following main parts:

    var options = {
      /* Defines the errors that are to be parsed and indexed */
      dictionary: {},
      /* Defines how the errors are parsed and indexed */
      fields: {
        map: {}
      reflow: '',
      /* Defines functionality for indexed errors */
      extend: {},
      override: {},
      /* Defines the functionality of Errati itself */
      errati: {}

Options passed to Errati.setup() are a configuration for that particular dictionary. No changes are made to errors already indexed by Errati.

The only exception is the field options.errati that adds functionality to Errati itself.


A dictionary of Errors.

dictionary: {}|[]|'http-short'|'http-full'|false
  • 'http-short': Most common HTTP Errors (default)
  • 'http-full': Full set of HTTP Status Codes

See: Dictionaries.

.fields and

Configuration of mapping and indexing of dictionary data.

fields: {
    // Mapping 
    map: {
        _index: 'field1', // Dictionary indexes to field1 
        _value: 'field2', // Dictionary string values to field2 
    // Indexing and prefixing 
    'field1': true, // Index field1 
    'field2': 'prefix1', // Index field2 with 'prefix1' 
    'field3': { // Index field3 with 'prefix2' 
        prefix: 'prefix2'
    //'field4': false // Superfluous 
} determines how the index and string values (not objects) from the dictionary are to be interpreted. The mapping uses two special fields, _index and _value, prefixed with an underscore to signify that they are not referring to the fields 'index' and 'value', but references to the dictionary.

dictionary[ _index ] = _value

By default, _index is set to 'code' and _value is set to 'name'. Hence with the default settings:

    var options = {
        dictionary: { '100': 'My Error' }
    Errati.setup( options );
    console.log( Errati );
    // { MyError: { _name: 'My Error', code: '100' } } 
    console.log( Errati.MyError );
    // { _name: 'My Error', code: '100' } 

Say we'd like to preserve the default mapping of dictionary index to the field 'code' but remap dictionary string values to 'title'. Also, we'd like to index the field 'code' and prefix it with 'code_'.

    var options = {
        dictionary: { '100': 'My Error' },
        fields: {
            map: {
                _value: 'title'
            'code': 'code_'
    Errati.setup( options );
    console.log( Errati );
    // { code_100: { title: 'My Error', code: '100' } } 
    console.log( Errati.code_100 );
    // { title: 'My Error', code: '100' } 

We can also map the dictionary index and values to something totally different:

    var myErrors = { 'emergency': 'Tommy' };
    var options = {
        dictionary: myErrors,
        fields: {
            map: {
                _index: 'state',
                _value: 'blame'
            'state': 'stateOf'
    Errati.setup( options );
    console.log( Errati );
    // { stateOfEmergency: { blame: 'Tommy', state: 'emergency' } } 
    console.log( Errati.stateOfEmergency );
    // { blame: 'Tommy', state: 'emergency' } 


Reflowing of error fields to be indexed to code friendly indexes.

reflow: 'CamelCase'|'underscore'|'nospace'|function|false
Reflow 'error name '
CamelCase 'ErrorName'
Underscore 'error_name'
Nospace 'errorname'

Example of a custom function:

    reflow: function( string ){
        return string;


Extending all errors in dictionary with fields and functions.

extend: {
    // Extend all classes and instances with field 
    myField: 'value',
    // Extend all instances with function 
    myHelper: function(){
        /* this... */
    // Custom function called after error instantiation 
    init: function(){
        /* this... */

As all fields are populated in the order from generic to specific (Dictionary > Class > Instance), you can also add default values that are later specified per class or instance.

For example:

    var myErrors = {
        '101': { name: 'Error One'  },
        '102': { name: 'Error Two', message: 'Class message' }
    var options = {
        dictionary: myErrors,
        extend: {
            message: 'Dictionary message'
    Errati.setup( options );
    console.log( Errati.ErrorOne.message );
    // Dictionary message 
    console.log( Errati.ErrorTwo.message );
    // Class message 
    console.log( new Errati.ErrorTwo('Instance message').message );
    // Instance message 


There is also the "hook" options.extends.init (actually just a field) for a function to be run post instantiation of an error.

The init function takes no arguments, but is scoped to the instance, meaning you have the full instance with the keyword "this". Init follows the same chain of population, so you can define generic and custom init functions. Note that init is called at instantiation only.

For example:

    var myErrors = {
        '101': { name: 'Error One'  },
        '102': { 
            name: 'Error Two',
            init: function(){
                console.log( 'I am totally second:', this.toString() );
    var options = {
        dictionary: myErrors,
        extend: {
            init: function(){
                console.log( 'I am error:', this.toString() );
    Errati.setup( options );
    new Errati.ErrorOne();
    // I am error: Error One 
    new Errati.ErrorTwo();
    // I am totally second: Error Two 


Overriding default functions for error instances.

    toString:function(){ /* this... */ return string; },
    toObject:function(){ /* this... */ return object; },
    toJson:function(){ /* this... */ return json; },
    parseArgs:function(args){ /* this... */ },
    parseStack:function(stacktrace){ /* this... */ this.stack = stacktrace }

The sections extend and override function in a near identical fashion, meaning options.override also populates from generic to specific, with the exception that functions in the options.override are already populated by default functions. The separation of these in to two fields is somewhat by principle but mostly for legibility.

These helper functions are also available for class use in the class prototype (Errati.MyError.prototype).


This section is for configuring Errati itself. Currently the only field is options.errati.extend that extends Errati with custom functions.

For example:

    extend: {
            var names = [];
            for( var key in this ){
                if( this.hasOwnProperty( key ) )
                    names.push( key );
            return names;


The dictionary can be either an Object or an Array, depending on needs and preferences.

    var myErrors = [ 'First error', 'Second error', 'Third error' ];
    var myErrors = {
        // Plain 
        '101':'Error one',
        // Extends 
            name: 'Error two',
            some: 'value',
            my: function(){ /* code */ }
        // Overrides  
            name: 'Error four',
            code: '104',
            toString: function(){ /* code */ }

Errati.setup() can be called multiple times for multiple dictionaries, sets with different values/functions and/or for some other structural divisions.

Be sure to have unique indexes for all errors. Make use of prefixes (see Fields) for different sets with identical error codes or names. Errati throws an error if you attempt to register duplicates with identical index.

Instance parsers

Arguments parser

The instatiation arguments parser aims to provide a flexible error constructor with the assumption that errors are commonly instantiated with a message.

It works by the following principles, in order:

  1. If the first argument is a String, it is the message.
  2. If the first argument is an Object with the field 'message', all its fields are merged to the instance.
  3. Additional arguments are passed to an array in the field 'args'.
  4. If the first argument is not a String and has no field 'message', all arguments are passed to the field 'args'.


    new Errati.MyError( 'My message' );
    -> { name:'My Error', message:'My message', ... }
    new Errati.MyError( 'My message', { some:'value' } );
    -> { name:'My Error', message:'My message', args:[{ some:'value' }], ...  }
    new Errati.MyError( { message: 'My message', some:'value' } );
    -> { name:'My Error', message:'My message', some:'value', ... }
    new Errati.MyError( { some:'value' }, { another:'value' } );
    -> { name:'My Error', args:[{some:'value'}, {another:'value'}], ... }

Note that the arguments parser sidesteps the regular Error instantiation behaviour.

If you wish to retain this, simply override the arguments parser with:

    override: {
            Error.apply( this, arguments );

Stack parser

The single purpose of the instantiation stack parser is to provide a stack trace that tops on where the error was instantiated. It simply removes the lines that refer to Errati from the stack trace.

It can be overridden as:

    override: {
        parseStack:function( stacktrace ){
            /* do something */
            this.stack = stacktrace;

Instance Helpers

Instanced errors have by default the following helper functions with the following behaviour. All of these can be overridden with custom functions.


Returns a string representation that is limited to the error name and a colon separated message, if a such exists.


    console.log( new Errati.MyError('I am message').toString() );
    // My error: I am string. 
    console.log( new Errati.MyError() );
    // My error 



filters	{ in:[] || ex:[] }  (optional)

Returns a cloned object without the error prototype. Note that Errati returns a shallow (- only relevant if you want to work with the error instance or nested the values in intact form later).

This helper also has a simple filtering function for first level fields. The filtering is done either by including or excluding field names, but not both. Filtering is set by passing an Object with the fields 'in' or 'ex', containing an Array of field names to include or exclude.


    console.log( myError.toObject( { in:['name'] } ) );
    // { name:'My error' } 
    console.log( myError.toObject( { ex:['name'] } ) );
    // { code:'100', some:'value' } 



filters	{ in:[] | ex:[] }   (optional)

This is a wrapper for JSON.stringify that also applies the filtering function for toObject.


    console.log( myError.toJson( { in:['name'] } ) );
    // { "name":"My error" } 
    console.log( myError.toJson( { ex:['name'] } ) );
    // { "code":"100", "some":"value" } 

Errati helpers



tabulator	string  (optional)

This helper returns a string representation of the Errati errorsIndex. You can pass a custom string for the tabulator. By default the tabulator is two spaces.


    console.log( Errati.toString() );
    // ErrorOne: { 
    //   _name: "Error One", 
    //   code: "101" 
    // }, 
    // ErrorTwo: { 
    //   _name: "Error Two", 
    //   code: "102" 
    // } 



callback	function
arguments	(optional)

Iterate through all registered errors. The callback should be formulated as:

function( error, params ){}

And 'params' contain the following fields:

  • index: The errors indexing value
  • errati: The errati instance
  • args: An array of addtional arguments


    Errati.forEach( function( err, params){
        console.log( params.args, params.index, err._name );
    }, 'Hello' );
    // [ 'Hello' ] 'ErrorOne' 'Error One' 
    // [ 'Hello' ] 'ErrorTwo' 'Error Two' 



prefix	string  (optional)

Counts the number of registered errors. If a prefix string is provided, it returns the number of errors that starts with the provided string.


    // 15 
    // 4 

Final notes

Take a look at the examples directory and the code for in depth understanding of how Errati works. Some of you may find individual prototypes for errors classes an anti-pattern. I partly agree with this, but it was a conscious decision due to the following factors:

  1. Efficiency. Less lookup, less prototype chains to traverse.
  2. Legibility of code is largely improved by the current solution.
  3. The overhead is neglible.

I know there be Javascript-fu to name dynamic functions (, but them being eval'y hacks, I decided to stick with the _name field in error classes. Please let me know if you know of a better way to do this.

I'm considering a plugin system to make Errati easier to extend with third party modules. Not sure if anyone would use it. Let me know if this would be a big improvement.

Change log

Changes to the
Bumped version to adhere to NPM standards.

0.9.0 Going public! Release candidate for version 1.0.0