Inherit from Error object

[Branko Vukelić]

Is it possible to make a custom error object that inherits from the Error
object? I would like to associate a few extra pieces of data and still
keep the ability to have stack traces and such if the custom object is
thrown.

I've tried this method:

function MyError(msg, someData) {
this.msg = msg;
this.data = someData;
}
MyError.prototype.toString = function() {
return 'ERR: ' + this.msg + ' ' + this.data.toString();
};
MyError.prototype = Error;

Of course, the above doesn't really work. So how do I do this properly?
Or do I avoid this pattern?

--
Branko Vukelic
bra...@herdhound.com
bg.b...@gmail.com

Lead Developer
Herd Hound (tm) - Travel that doesn't bite
www.herdhound.com

Love coffee? You might love Loveffee, too.
loveffee.appspot.com

[Bradley Meck]

best way:

use require('util').inherits(myError, Error);

manual way:

do not set properties on the prototype before you set the prototype to a new object (MyError.prototype = ... should occur before you add any properties to the prototype)

set the prototype to new Error not Error

set the prototype.constructor to MyError

call Error.call(this) in the constructor

[Branko Vukelić]

Thanks for the tips!

[Marcel Laverdet]

If you are interested in stack traces, be sure to do this:

Error.captureStackTrace(this, MyError);

Even calling the Error ctor manually won't get you a stack trace, and error.stack will be empty.

2011/8/11 Branko Vukelić <bg.b...@gmail.com>

--

Job Board: http://jobs.nodejs.org/
Posting guidelines: https://github.com/joyent/node/wiki/Mailing-List-Posting-Guidelines
You received this message because you are subscribed to the Google
Groups "nodejs" group.
To post to this group, send email to nod...@googlegroups.com
To unsubscribe from this group, send email to
nodejs+un...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/nodejs?hl=en?hl=en

[Isaac Schlueter]

Inheriting from the Error class is a bit unusual. To do it properly,
you have to do some extra steps to get the stack trace properly. The
good news is that the API gives you quite a bit of control over this.

Check out node's lib/assert.js where it defines the AssertionError class.

[Branko Vukelić]

On 2011-08-11 08:56 -0700, Isaac Schlueter wrote:
> Check out node's lib/assert.js where it defines the AssertionError class.

Thanks.

[Branko Vukelić]

On 2011-08-11 10:43 -0500, Marcel Laverdet wrote:
> Error.captureStackTrace(this, MyError);

Is this done in the constructor or somewhere else?

[Aseem Kishore]

Yep, constructor.

2011/8/11 Branko Vukelić <bg.b...@gmail.com>

[Branko Vukelić]

On 2011-08-11 10:14 -0700, Aseem Kishore wrote:
> Yep, constructor.

Thanks everyone, it's working beatifully now. If anyone stumbles upon
this convo later looking for the same things, here's the V8
documentation on stack traces that might be useful:

http://code.google.com/p/v8/wiki/JavaScriptStackTraceApi

[Michael Jackson]

Check out the custom Error subclass that Link uses:

https://github.com/mjijackson/link/blob/master/lib/link.js#L153

It should be exactly what you need, plus it allows itself to be
subclassed and contain nested errors, while preserving the full stack
trace.

Michael

2011/8/11 Branko Vukelić <bg.b...@gmail.com>:

[Branko Vukelić]

On 2011-08-11 15:46 -0700, Michael Jackson wrote:
> Check out the custom Error subclass that Link uses:
>
> https://github.com/mjijackson/link/blob/master/lib/link.js#L153
>
> It should be exactly what you need, plus it allows itself to be
> subclassed and contain nested errors, while preserving the full stack
> trace.

Awesome! :)

[Tim Smart]

Some more examples: https://github.com/Tim-Smart/express-aid/blob/master/index.js#L66-113

Tim.

2011/8/12 Branko Vukelić <bg.b...@gmail.com>

[Michael Jackson]

Tim,

It's slightly better to use

Error.captureStackTrace(this, this.constructor)

instead of

Error.captureStackTrace(this, arguments.callee)

if you want to omit the call to the constructor from the stack trace.
"this.constructor" will always be a reference to the error's
constructor function, but "arguments.callee" will always be a
reference to the same parent constructor, even when calling it from
inside a "subclass" constructor.

Michael

[Nuno Job]

This is what I did in the driver I'm writing:

https://github.com/dscape/nuvem/blob/master/lib/nuvem/error.js