XError

A utility class extending error that provides other useful features for standardized
error codes and handling. It inherits from Error and is compatible with it.

Error Codes

XError objects contain a machine-readable error code as well as a human-readable message. By convention,
error codes are lowercase, underscore_separated strings, such as internal_error. Registered error
codes are available as constants on the XError object, with the names uppercased. The common error
codes are listed here
and you can register your own (see below).

Data

XError objects contain both a data field and a privateData field. These are intended to be set to
objects that contain extra data about the error. privateData is used for information that should
not be displayed to a user.

Cause

XError objects can contain a "cause" error, similar to exception chains in languages such as Java. Causes
can be either an XError or an Error.

Fields

* xerror.code - The string error code, like internal_error
* xerror.message - Human-readable error message
* xerror.data - Object containing extra data about the error
* xerror.privateData - Object containing sensitive data about the error
* xerror.cause - XError or Error instance that triggered this error
* xerror.stack - Stack trace of where this error is constructed

Constructing

```javascript
var XError = require('xerror');

// Constructs an XError object with the NOT_FOUND error code and
// default message for that error code.
new XError(XError.NOT_FOUND);

// Constructs an XError object with just a message and the default
// code of 'internal_error'. This is distinguished from a code
// because it contains spaces.
new XError('Something bad happened');

// Constructs an XError object with both a code and a message
// The message need not contain spaces if a code is supplied
new XError(XError.BAD_REQUEST, 'You did something wrong');

// Constructs an XError object with a cause
new XError(XError.BAD_REQUEST, 'Something was wrong with your call', causeError);

// Constructs an XError object with data
new XError(XError.INVALID_ARGUMENT, { name: 'id', value: 'somethinginvalid' });

// Constructs an XError object with everything
new XError(
XError.ACCESS_DENIED,
'Error validating account credentials',
{ username: 'crispy1989' },
{ password: 'password123' },
new Error('validation error')
);

// Wrap an existing XError or Error
// This replicates all of the fields, but creates a new stack trace and sets
// the original error as the cause.
// This is useful when tracing errors through callback chains
XError.wrap(otherXError);
// Use it like this:
doSomething(function(error) {
if(error) return cb(XError.wrap(error));
// ...
});
``

Registering custom error codes

Error codes should be registered before being used. You can also register additional properties with the error code.

``javascript
// Register a new custom error code
XError.registerErrorCode('my_custom_code', {
// default error message to use with this error code
message: 'Custom Default Error Message',
// the http response code to map from this error code
http: 499
// you can add other custom fields as well
});
var error = new XError(XError.MY_CUSTOM_CODE, ...);

// Register an alias of another error code
XError.registerErrorCode('item_missing', { aliasOf: XError.NOT_FOUND });

// Register multiple at once
XError.registerErrorCodes({
custom_code_1: { message: 'Message for Custom Code 1' },
custom_code_2: { message: 'Message for Custom Code 2' }
});
``

You can also fetch these informational object about error codes:

``javascript
var info = XError.getErrorCode(XError.NOT_FOUND);

var listOfCodes = XError.listErrorCodes();
```