errs

errs

Simple error creation and passing utilities focused on:

• Creating Errors
• Reusing Error Types
• Merging with Existing Errors
• Optional Callback Invocation
• Piping Error Events

## Creating Errors

You should know by now that a String is not an Error. Unfortunately the Error constructor in Javascript isn't all that convenient either. How often do you find yourself in this situation?

  var err = new Error('This is an error. There are many like it.');
  err.someProperty = 'more syntax';
  err.someOtherProperty = 'it wont stop.';
  err.notEven = 'for the mayor';
  
  throw err;

Rest your fingers, errs is here to help. The following is equivalent to the above:

  var errs = require('errs');
  
  throw errs.create({
    message: 'This is an error. There are many like it.',
    someProperty: 'more syntax',
    someOtherProperty: 'it wont stop.',
    notEven: 'for the mayor'
  });

## Reusing Custom Error Types

errs also exposes an inversion of control interface for easily reusing custom error types across your application. Custom Error Types registered with errs will transparently invoke Error constructor and Error.captureStackTrace to attach transparent stack traces:

  /*
   * file-a.js: Create and register your error type.
   *
   */
   
  var util = require('util'),
      errs = require('errs');

  function MyError() {
    this.message = 'This is my error; I made it myself. It has a transparent stack trace.';
  }

  //
  // Alternatively `MyError.prototype.__proto__ = Error;`
  //
  util.inherits(MyError, Error);

  //
  // Register the error type
  //
  errs.register('myerror', MyError);



  /*
   * file-b.js: Use your error type.
   *
   */
   
  var errs = require('errs');
  
  console.log(
    errs.create('myerror')
      .stack
      .split('\n')
  );

The output from the two files above is shown below. Notice how it contains no references to errs.js:

[ 'MyError: This is my error; I made it myself. It has a transparent stack trace.',
  '    at Object.<anonymous> (/file-b.js:19:8)',
  '    at Module._compile (module.js:441:26)',
  '    at Object..js (module.js:459:10)',
  '    at Module.load (module.js:348:31)',
  '    at Function._load (module.js:308:12)',
  '    at Array.0 (module.js:479:10)',
  '    at EventEmitter._tickCallback (node.js:192:40)' ]

## Merging with Existing Errors

When working with errors you catch or are returned in a callback you can extend those errors with properties by using the errs.merge method. This will also create a human readable error message and stack-trace:

process.on('uncaughtException', function(err) {
  console.log(errs.merge(err, {namespace: 'uncaughtException'}));
});

var file = fs.createReadStream('FileDoesNotExist.here');

{ [Error: Unspecified error]
  name: 'Error',
  namespace: 'uncaughtException',
  errno: 34,
  code: 'ENOENT',
  path: 'FileDoesNotExist.here',
  description: 'ENOENT, no such file or directory \'FileDoesNotExist.here\'',
  stacktrace: [ 'Error: ENOENT, no such file or directory \'FileDoesNotExist.here\'' ] }

## Optional Callback Invocation

Node.js handles asynchronous IO through the elegant EventEmitter API. In many scenarios the callback may be optional because you are returning an EventEmitter for piping or other event multiplexing. This complicates code with a lot of boilerplate:

  function importantFeature(callback) {
    return someAsyncFn(function (err) {
      if (err) {
        if (callback) {
          return callback(err);
        }
        
        throw err;
      }
    });
  }

errs it presents a common API for both emitting error events and invoking continuations (i.e. callbacks) with errors. If a callback is supplied to errs.handle() it will be invoked with the error. It no callback is provided then an EventEmitter is returned which emits an error event on the next tick:

  function importantFeature(callback) {
    return someAsyncFn(function (err) {
      if (err) {
        return errs.handle(err, callback);
      }
    });
  }

## Piping Errors

Often when working with streams (especially when buffering for whatever reason), you may have already returned an EventEmitter or Stream instance by the time an error is handled.

  function pipeSomething(callback) {
    //
    // You have a stream (e.g. http.ResponseStream) and you 
    // have an optional `callback`.
    //
    var stream = new require('stream').Stream;

    //
    // You need to do something async which may respond with an
    // error
    //
    getAnotherStream(function (err, source) {
      if (err) {
        if (callback)
          callback(err);
        }
        
        stream.emit('error', err);
        return;
      }
      
      source.pipe(stream);
    })

    return stream;
  }

You may pass either a function or EventEmitter instance to errs.handle.

  function pipeSomething(callback) {
    //
    // You have a stream (e.g. http.ResponseStream) and you 
    // have an optional `callback`.
    //
    var stream = new require('stream').Stream;

    //
    // You need to do something async which may respond with an
    // error
    //
    getAnotherStream(function (err, source) {
      if (err) {
        //
        // Invoke the callback if it exists otherwise the stream.
        //
        return errs.handle(err, callback || stream);
      }
      
      source.pipe(stream);
    })

    return stream;
  }

If you wish to invoke both a callback function and an error event simply pass both:

  errs.handle(err, callback, stream);

Methods

The errs modules exposes some simple utility methods:

• .create(type, opts): Creates a new error instance for with the specified type and opts. If the type is not registered then a new Error instance will be created.
• .register(type, proto): Registers the specified proto to type for future calls to errors.create(type, opts).
• .unregister(type): Unregisters the specified type for future calls to errors.create(type, opts).
• .handle(err, callback): Attempts to instantiate the given error. If the error is already a properly formed error object (with a stack property) it will not be modified.
• .merge(err, type, opts): Merges an existing error with a new error instance for with the specified type and opts.

Installation

Installing npm (node package manager)

  $ curl http://npmjs.org/install.sh | sh

Installing errs

  $ [sudo] npm install errs

Tests

All tests are written with vows and should be run with npm:

  $ npm test

Author: Nodejitsu Inc.

Contributors: Charlie Robbins, Nuno Job

License: MIT