verror: richer JavaScript errors

This module provides two classes:

• VError, for combining errors while preserving each one's error message, and
• WError, for wrapping errors.

Both support printf-style error messages using extsprintf.

printf-style Error constructor

At the most basic level, VError is just like JavaScript's Error class, but with printf-style arguments:

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var VError = require('verror');

var filename = '/etc/passwd';
var err = new VError('missing file: "%s"', filename);
console.log(err.message);

This prints:

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missing file: "/etc/passwd"

err.stack works the same as for built-in errors:

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console.log(err.stack);

This prints:

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missing file: "/etc/passwd"
    at Object.<anonymous> (/Users/dap/node-verror/examples/varargs.js:4:11)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.runMain (module.js:492:10)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)

Causes

You can also pass a cause argument, which is another Error. For example:

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var fs = require('fs');
var VError = require('verror');

var filename = '/nonexistent';
fs.stat(filename, function (err1) {
    var err2 = new VError(err1, 'stat "%s" failed', filename);
    console.error(err2.message);
});

This prints out:

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stat "/nonexistent" failed: ENOENT, stat '/nonexistent'

which resembles how Unix programs typically report errors:

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$ sort /nonexistent
sort: open failed: /nonexistent: No such file or directory

To match the Unixy feel, just prepend the program's name to the VError's message.

You can also get the next-level Error using err.cause():

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console.error(err2.cause().message);

prints:

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ENOENT, stat '/nonexistent'

Of course, you can nest these as many times as you want:

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var VError = require('verror');
var err1 = new Error('No such file or directory');
var err2 = new VError(err1, 'failed to stat "%s"', '/junk');
var err3 = new VError(err2, 'request failed');
console.error(err3.message);

This prints:

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request failed: failed to stat "/junk": No such file or directory

The idea is that each layer in the stack annotates the error with a description of what it was doing (with a printf-like format string) and the result is a message that explains what happened at every level.

WError: wrap layered errors

Sometimes you don't want an Error's "message" field to include the details of all of the low-level errors, but you still want to be able to get at them programmatically. For example, in an HTTP server, you probably don't want to spew all of the low-level errors back to the client, but you do want to include them in the audit log entry for the request. In that case, you can use a WError, which is created exactly like VError (and also supports both printf-style arguments and an optional cause), but the resulting "message" only contains the top-level error. It's also more verbose, including the class associated with each error in the cause chain. Using the same example above, but replacing err3's VError with WError, so that it looks like this:

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var verror = require('verror');
var err1 = new Error('No such file or directory');
var err2 = new verror.VError(err1, 'failed to stat "%s"', '/junk');
var err3 = new verror.WError(err2, 'request failed');
console.error(err3.message);

we get this output:

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request failed

That's what we wanted -- just a high-level summary for the client. But we can get the object's toString() for the full details:

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WError: request failed; caused by WError: failed to stat "/nonexistent";
caused by Error: No such file or directory

For a complete example, see examples/werror.js.

Contributing

Contributions welcome. Code should be "make check" clean. To run "make check", you'll need these tools:

• https://github.com/davepacheco/jsstyle
• https://github.com/davepacheco/javascriptlint

If you're changing something non-trivial or user-facing, you may want to submit an issue first.