Literally Can't

A wise man once said:

Error messages should be of the form "Can't x because of y."

This package provides a clean API for creating errors with this format. These custom errors can also be associated with a particular stream to alter logging behavior based on severity.

$ npm install cant

Like. Really. Literally Can't

Cant is designed for you to create a whole bunch of custom errors, all in one place, which you then use throughout your application.

// errors.js

var Cant = require('cant');

var errors = Cant.errors({

  SheTotally: Cant('say %s to her').because('she\'s so %s')
    .streams(process.stderr),

  HeDidWhat: Cant('even believe %s').because('he is such a %s')

});

module.exports = errors;

// dosomethingcool.js

var errors = require('./errors');
var ughWhat = new errors.SheTotally('anything', 'stupid');

// now you can
ughWhat.log();

// or
throw ughWhat;

Error Levels and Streams

Before creating your errors with Cant.errors, you can map severity levels to output streams using Cant.streams

Cant.streams({
  'info': process.stdout,
  'warn': [process.stdout, '/var/log/warn.log'],
  'error': '/var/log/error.log',
  'totes_ugh': process.stderr
});

Severity levels are arbitrary (but "info", "warn" and "error" are pretty standard). Each level maps to a stream (or a file path which is turned into a stream), or an array of streams.

Then, when creating errors with Cant.errors, each custom error will be assigned output streams based on their level.

var errors = Cant.errors({

  AccessDenied: Cant('access the %s').because('your %s was declined')
    .level('warn')

});

var ohNo = new errors.AccessDenied('mainframe', 'username');
ohNo.log();

// or, to print a stack trace:
ohNo.log(true);

This would print an error message to stdout and /var/log/warn.log.

Currently any tty.WriteStream, stream.Writable or net.Socket will be supported as an output stream.

Combining Errors

When writing large, asynchronous applications in Node there's are many times when you don't have all the information to construct an informative error message at the same time.

Instead, you may know details about the desired action in one place, and details about the cause of error in another.

Well, Cant can help!

Let's take an example from a typical Express/Mongoose app:

var Cant = require('cant');

var RegisterError = Cant('register user %s')
  .because(Error)
  .name('RegisterError')
  .level('error')
  .http(500)
  .create();

var NotUniqueError = Cant('save Model')
  .because('%s is not unique')
  .name('NotUniqueError')
  .create();


route.get(function(req, res, next) {
  var username = req.body.username;
  var password = req.body.password;

  User.register(username, password, function(err) {
    if (err) return next(new RegisterError(username, err));
    
    res.status(201).send('registered!');
  });

});

User.statics.register = function register(username, password, callback) {

  User.findOne({username: username}, function(err, user) {
    if (user) return callback(new NotUniqueError('username'));
  });
};

If a user gives a non-unique username "bob", the resulting error message will be a combination of the two errors: "Can't register user bob because username is not unique."

When combining errors like this, the deeper "source" error does not have to be a custom Cant error. So you could do something like this:

var DBSaveError = Cant('save instance of model %s')
  .because(Error)
  .name('DBSaveError')
  .level('error')
  .http(500)
  .create();

User.save(function(err) {
  if (err) throw new DBSaveError('User', err);
});

Another module (like Mongoose) generates err, whose message is then used as the "because y" clause of the custom DBSaveError.

Cant ErrorFactory API

Here's all the options you have when defining your custom Cant errors.

ErrorFactory#name

Set the name of the custom error.

var ErrorFactory = require('cant').ErrorFactory;
var factory = new ErrorFactory();

var CantError = factory.name('CantError').create();

ErrorFactory#cant

Provide a format string for the "Cant x" clause.

This string will be used by Node's util.format to construct the final "Cant" clause of the error.

var CantError = factory.cant('do action %s').create();

throw new CantError('find');

// "Can't do action find"

ErrorFactory#because

Provide a format string (as in ErrorFactory#cant), OR the Error constructor for the "because y" clause.

Passing Error allows for combining error messages.

var SpecificActionError = factory.cant('do a specific action')
  .because(Error)
  .create();

var SpecificReasonError = factory.cant('do a generic action')
  .because('a specific thing went wrong')
  .create();

throw new SpecificActionError(new SpecificReasonError());

// "Can't do a specific action because a specific thing went wrong."

Here you can see how this enables a combination of two kinds of errors - those that are specific about the desired action that cannot be performed, and those that are specific about the reason for the error.

This can be helpful in a typical application, where information about the desired action and the root cause of an error may not be available in the same location.

ErrorFactory#http

Set an HTTP status code for the error.

var ServerError = factory.http(500).create();

ErrorFactory#level

Set a short severity level string.

var WarnError = factory.level('warn').create();

ErrorFactory#streams

Add a writable stream (or an array of such), allowing the generated error to log to a custom location.

Any tty.WriteStream, stream.Writable or net.Socket will be supported as an output stream.

var StreamError = factory.streams(process.stderr).create();

ErrorFactory#create

Creates a new custom CantError. Call this method last, after defining your custom error using the previous methods.

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© 2014 Weston Fribley

This software is MIT licensed - please see LICENSE for details.