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1# verror: rich JavaScript errors
2
3This module provides several classes in support of Joyent's [Best Practices for
4Error Handling in Node.js](http://www.joyent.com/developers/node/design/errors).
5If you find any of the behavior here confusing or surprising, check out that
6document first.
7
8The error classes here support:
9
10* printf-style arguments for the message
11* chains of causes
12* properties to provide extra information about the error
13* creating your own subclasses that support all of these
14
15The classes here are:
16
17* **VError**, for chaining errors while preserving each one's error message.
18 This is useful in servers and command-line utilities when you want to
19 propagate an error up a call stack, but allow various levels to add their own
20 context. See examples below.
21* **WError**, for wrapping errors while hiding the lower-level messages from the
22 top-level error. This is useful for API endpoints where you don't want to
23 expose internal error messages, but you still want to preserve the error chain
24 for logging and debugging.
25* **SError**, which is just like VError but interprets printf-style arguments
26 more strictly.
27* **MultiError**, which is just an Error that encapsulates one or more other
28 errors. (This is used for parallel operations that return several errors.)
29
30
31# Quick start
32
33First, install the package:
34
35 npm install verror
36
37If nothing else, you can use VError as a drop-in replacement for the built-in
38JavaScript Error class, with the addition of printf-style messages:
39
40```javascript
41var err = new VError('missing file: "%s"', '/etc/passwd');
42console.log(err.message);
43```
44
45This prints:
46
47 missing file: "/etc/passwd"
48
49You can also pass a `cause` argument, which is any other Error object:
50
51```javascript
52var fs = require('fs');
53var filename = '/nonexistent';
54fs.stat(filename, function (err1) {
55 var err2 = new VError(err1, 'stat "%s"', filename);
56 console.error(err2.message);
57});
58```
59
60This prints out:
61
62 stat "/nonexistent": ENOENT, stat '/nonexistent'
63
64which resembles how Unix programs typically report errors:
65
66 $ sort /nonexistent
67 sort: open failed: /nonexistent: No such file or directory
68
69To match the Unixy feel, when you print out the error, just prepend the
70program's name to the VError's `message`. Or just call
71[node-cmdutil.fail(your_verror)](https://github.com/joyent/node-cmdutil), which
72does this for you.
73
74You can get the next-level Error using `err.cause()`:
75
76```javascript
77console.error(err2.cause().message);
78```
79
80prints:
81
82 ENOENT, stat '/nonexistent'
83
84Of course, you can chain these as many times as you want, and it works with any
85kind of Error:
86
87```javascript
88var err1 = new Error('No such file or directory');
89var err2 = new VError(err1, 'failed to stat "%s"', '/junk');
90var err3 = new VError(err2, 'request failed');
91console.error(err3.message);
92```
93
94This prints:
95
96 request failed: failed to stat "/junk": No such file or directory
97
98The idea is that each layer in the stack annotates the error with a description
99of what it was doing. The end result is a message that explains what happened
100at each level.
101
102You can also decorate Error objects with additional information so that callers
103can not only handle each kind of error differently, but also construct their own
104error messages (e.g., to localize them, format them, group them by type, and so
105on). See the example below.
106
107
108# Deeper dive
109
110The two main goals for VError are:
111
112* **Make it easy to construct clear, complete error messages intended for
113 people.** Clear error messages greatly improve both user experience and
114 debuggability, so we wanted to make it easy to build them. That's why the
115 constructor takes printf-style arguments.
116* **Make it easy to construct objects with programmatically-accessible
117 metadata** (which we call _informational properties_). Instead of just saying
118 "connection refused while connecting to 192.168.1.2:80", you can add
119 properties like `"ip": "192.168.1.2"` and `"tcpPort": 80`. This can be used
120 for feeding into monitoring systems, analyzing large numbers of Errors (as
121 from a log file), or localizing error messages.
122
123To really make this useful, it also needs to be easy to compose Errors:
124higher-level code should be able to augment the Errors reported by lower-level
125code to provide a more complete description of what happened. Instead of saying
126"connection refused", you can say "operation X failed: connection refused".
127That's why VError supports `causes`.
128
129In order for all this to work, programmers need to know that it's generally safe
130to wrap lower-level Errors with higher-level ones. If you have existing code
131that handles Errors produced by a library, you should be able to wrap those
132Errors with a VError to add information without breaking the error handling
133code. There are two obvious ways that this could break such consumers:
134
135* The error's name might change. People typically use `name` to determine what
136 kind of Error they've got. To ensure compatibility, you can create VErrors
137 with custom names, but this approach isn't great because it prevents you from
138 representing complex failures. For this reason, VError provides
139 `findCauseByName`, which essentially asks: does this Error _or any of its
140 causes_ have this specific type? If error handling code uses
141 `findCauseByName`, then subsystems can construct very specific causal chains
142 for debuggability and still let people handle simple cases easily. There's an
143 example below.
144* The error's properties might change. People often hang additional properties
145 off of Error objects. If we wrap an existing Error in a new Error, those
146 properties would be lost unless we copied them. But there are a variety of
147 both standard and non-standard Error properties that should _not_ be copied in
148 this way: most obviously `name`, `message`, and `stack`, but also `fileName`,
149 `lineNumber`, and a few others. Plus, it's useful for some Error subclasses
150 to have their own private properties -- and there'd be no way to know whether
151 these should be copied. For these reasons, VError first-classes these
152 information properties. You have to provide them in the constructor, you can
153 only fetch them with the `info()` function, and VError takes care of making
154 sure properties from causes wind up in the `info()` output.
155
156Let's put this all together with an example from the node-fast RPC library.
157node-fast implements a simple RPC protocol for Node programs. There's a server
158and client interface, and clients make RPC requests to servers. Let's say the
159server fails with an UnauthorizedError with message "user 'bob' is not
160authorized". The client wraps all server errors with a FastServerError. The
161client also wraps all request errors with a FastRequestError that includes the
162name of the RPC call being made. The result of this failed RPC might look like
163this:
164
165 name: FastRequestError
166 message: "request failed: server error: user 'bob' is not authorized"
167 rpcMsgid: <unique identifier for this request>
168 rpcMethod: GetObject
169 cause:
170 name: FastServerError
171 message: "server error: user 'bob' is not authorized"
172 cause:
173 name: UnauthorizedError
174 message: "user 'bob' is not authorized"
175 rpcUser: "bob"
176
177When the caller uses `VError.info()`, the information properties are collapsed
178so that it looks like this:
179
180 message: "request failed: server error: user 'bob' is not authorized"
181 rpcMsgid: <unique identifier for this request>
182 rpcMethod: GetObject
183 rpcUser: "bob"
184
185Taking this apart:
186
187* The error's message is a complete description of the problem. The caller can
188 report this directly to its caller, which can potentially make its way back to
189 an end user (if appropriate). It can also be logged.
190* The caller can tell that the request failed on the server, rather than as a
191 result of a client problem (e.g., failure to serialize the request), a
192 transport problem (e.g., failure to connect to the server), or something else
193 (e.g., a timeout). They do this using `findCauseByName('FastServerError')`
194 rather than checking the `name` field directly.
195* If the caller logs this error, the logs can be analyzed to aggregate
196 errors by cause, by RPC method name, by user, or whatever. Or the
197 error can be correlated with other events for the same rpcMsgid.
198* It wasn't very hard for any part of the code to contribute to this Error.
199 Each part of the stack has just a few lines to provide exactly what it knows,
200 with very little boilerplate.
201
202It's not expected that you'd use these complex forms all the time. Despite
203supporting the complex case above, you can still just do:
204
205 new VError("my service isn't working");
206
207for the simple cases.
208
209
210# Reference: VError, WError, SError
211
212VError, WError, and SError are convenient drop-in replacements for `Error` that
213support printf-style arguments, first-class causes, informational properties,
214and other useful features.
215
216
217## Constructors
218
219The VError constructor has several forms:
220
221```javascript
222/*
223 * This is the most general form. You can specify any supported options
224 * (including "cause" and "info") this way.
225 */
226new VError(options, sprintf_args...)
227
228/*
229 * This is a useful shorthand when the only option you need is "cause".
230 */
231new VError(cause, sprintf_args...)
232
233/*
234 * This is a useful shorthand when you don't need any options at all.
235 */
236new VError(sprintf_args...)
237```
238
239All of these forms construct a new VError that behaves just like the built-in
240JavaScript `Error` class, with some additional methods described below.
241
242In the first form, `options` is a plain object with any of the following
243optional properties:
244
245Option name | Type | Meaning
246---------------- | ---------------- | -------
247`name` | string | Describes what kind of error this is. This is intended for programmatic use to distinguish between different kinds of errors. Note that in modern versions of Node.js, this name is ignored in the `stack` property value, but callers can still use the `name` property to get at it.
248`cause` | any Error object | Indicates that the new error was caused by `cause`. See `cause()` below. If unspecified, the cause will be `null`.
249`strict` | boolean | If true, then `null` and `undefined` values in `sprintf_args` are passed through to `sprintf()`. Otherwise, these are replaced with the strings `'null'`, and '`undefined`', respectively.
250`constructorOpt` | function | If specified, then the stack trace for this error ends at function `constructorOpt`. Functions called by `constructorOpt` will not show up in the stack. This is useful when this class is subclassed.
251`info` | object | Specifies arbitrary informational properties that are available through the `VError.info(err)` static class method. See that method for details.
252
253The second form is equivalent to using the first form with the specified `cause`
254as the error's cause. This form is distinguished from the first form because
255the first argument is an Error.
256
257The third form is equivalent to using the first form with all default option
258values. This form is distinguished from the other forms because the first
259argument is not an object or an Error.
260
261The `WError` constructor is used exactly the same way as the `VError`
262constructor. The `SError` constructor is also used the same way as the
263`VError` constructor except that in all cases, the `strict` property is
264overriden to `true.
265
266
267## Public properties
268
269`VError`, `WError`, and `SError` all provide the same public properties as
270JavaScript's built-in Error objects.
271
272Property name | Type | Meaning
273------------- | ------ | -------
274`name` | string | Programmatically-usable name of the error.
275`message` | string | Human-readable summary of the failure. Programmatically-accessible details are provided through `VError.info(err)` class method.
276`stack` | string | Human-readable stack trace where the Error was constructed.
277
278For all of these classes, the printf-style arguments passed to the constructor
279are processed with `sprintf()` to form a message. For `WError`, this becomes
280the complete `message` property. For `SError` and `VError`, this message is
281prepended to the message of the cause, if any (with a suitable separator), and
282the result becomes the `message` property.
283
284The `stack` property is managed entirely by the underlying JavaScript
285implementation. It's generally implemented using a getter function because
286constructing the human-readable stack trace is somewhat expensive.
287
288## Class methods
289
290The following methods are defined on the `VError` class and as exported
291functions on the `verror` module. They're defined this way rather than using
292methods on VError instances so that they can be used on Errors not created with
293`VError`.
294
295### `VError.cause(err)`
296
297The `cause()` function returns the next Error in the cause chain for `err`, or
298`null` if there is no next error. See the `cause` argument to the constructor.
299Errors can have arbitrarily long cause chains. You can walk the `cause` chain
300by invoking `VError.cause(err)` on each subsequent return value. If `err` is
301not a `VError`, the cause is `null`.
302
303### `VError.info(err)`
304
305Returns an object with all of the extra error information that's been associated
306with this Error and all of its causes. These are the properties passed in using
307the `info` option to the constructor. Properties not specified in the
308constructor for this Error are implicitly inherited from this error's cause.
309
310These properties are intended to provide programmatically-accessible metadata
311about the error. For an error that indicates a failure to resolve a DNS name,
312informational properties might include the DNS name to be resolved, or even the
313list of resolvers used to resolve it. The values of these properties should
314generally be plain objects (i.e., consisting only of null, undefined, numbers,
315booleans, strings, and objects and arrays containing only other plain objects).
316
317### `VError.fullStack(err)`
318
319Returns a string containing the full stack trace, with all nested errors recursively
320reported as `'caused by:' + err.stack`.
321
322### `VError.findCauseByName(err, name)`
323
324The `findCauseByName()` function traverses the cause chain for `err`, looking
325for an error whose `name` property matches the passed in `name` value. If no
326match is found, `null` is returned.
327
328If all you want is to know _whether_ there's a cause (and you don't care what it
329is), you can use `VError.hasCauseWithName(err, name)`.
330
331If a vanilla error or a non-VError error is passed in, then there is no cause
332chain to traverse. In this scenario, the function will check the `name`
333property of only `err`.
334
335### `VError.hasCauseWithName(err, name)`
336
337Returns true if and only if `VError.findCauseByName(err, name)` would return
338a non-null value. This essentially determines whether `err` has any cause in
339its cause chain that has name `name`.
340
341### `VError.errorFromList(errors)`
342
343Given an array of Error objects (possibly empty), return a single error
344representing the whole collection of errors. If the list has:
345
346* 0 elements, returns `null`
347* 1 element, returns the sole error
348* more than 1 element, returns a MultiError referencing the whole list
349
350This is useful for cases where an operation may produce any number of errors,
351and you ultimately want to implement the usual `callback(err)` pattern. You can
352accumulate the errors in an array and then invoke
353`callback(VError.errorFromList(errors))` when the operation is complete.
354
355
356### `VError.errorForEach(err, func)`
357
358Convenience function for iterating an error that may itself be a MultiError.
359
360In all cases, `err` must be an Error. If `err` is a MultiError, then `func` is
361invoked as `func(errorN)` for each of the underlying errors of the MultiError.
362If `err` is any other kind of error, `func` is invoked once as `func(err)`. In
363all cases, `func` is invoked synchronously.
364
365This is useful for cases where an operation may produce any number of warnings
366that may be encapsulated with a MultiError -- but may not be.
367
368This function does not iterate an error's cause chain.
369
370
371## Examples
372
373The "Demo" section above covers several basic cases. Here's a more advanced
374case:
375
376```javascript
377var err1 = new VError('something bad happened');
378/* ... */
379var err2 = new VError({
380 'name': 'ConnectionError',
381 'cause': err1,
382 'info': {
383 'errno': 'ECONNREFUSED',
384 'remote_ip': '127.0.0.1',
385 'port': 215
386 }
387}, 'failed to connect to "%s:%d"', '127.0.0.1', 215);
388
389console.log(err2.message);
390console.log(err2.name);
391console.log(VError.info(err2));
392console.log(err2.stack);
393```
394
395This outputs:
396
397 failed to connect to "127.0.0.1:215": something bad happened
398 ConnectionError
399 { errno: 'ECONNREFUSED', remote_ip: '127.0.0.1', port: 215 }
400 ConnectionError: failed to connect to "127.0.0.1:215": something bad happened
401 at Object.<anonymous> (/home/dap/node-verror/examples/info.js:5:12)
402 at Module._compile (module.js:456:26)
403 at Object.Module._extensions..js (module.js:474:10)
404 at Module.load (module.js:356:32)
405 at Function.Module._load (module.js:312:12)
406 at Function.Module.runMain (module.js:497:10)
407 at startup (node.js:119:16)
408 at node.js:935:3
409
410Information properties are inherited up the cause chain, with values at the top
411of the chain overriding same-named values lower in the chain. To continue that
412example:
413
414```javascript
415var err3 = new VError({
416 'name': 'RequestError',
417 'cause': err2,
418 'info': {
419 'errno': 'EBADREQUEST'
420 }
421}, 'request failed');
422
423console.log(err3.message);
424console.log(err3.name);
425console.log(VError.info(err3));
426console.log(err3.stack);
427```
428
429This outputs:
430
431 request failed: failed to connect to "127.0.0.1:215": something bad happened
432 RequestError
433 { errno: 'EBADREQUEST', remote_ip: '127.0.0.1', port: 215 }
434 RequestError: request failed: failed to connect to "127.0.0.1:215": something bad happened
435 at Object.<anonymous> (/home/dap/node-verror/examples/info.js:20:12)
436 at Module._compile (module.js:456:26)
437 at Object.Module._extensions..js (module.js:474:10)
438 at Module.load (module.js:356:32)
439 at Function.Module._load (module.js:312:12)
440 at Function.Module.runMain (module.js:497:10)
441 at startup (node.js:119:16)
442 at node.js:935:3
443
444You can also print the complete stack trace of combined `Error`s by using
445`VError.fullStack(err).`
446
447```javascript
448var err1 = new VError('something bad happened');
449/* ... */
450var err2 = new VError(err1, 'something really bad happened here');
451
452console.log(VError.fullStack(err2));
453```
454
455This outputs:
456
457 VError: something really bad happened here: something bad happened
458 at Object.<anonymous> (/home/dap/node-verror/examples/fullStack.js:5:12)
459 at Module._compile (module.js:409:26)
460 at Object.Module._extensions..js (module.js:416:10)
461 at Module.load (module.js:343:32)
462 at Function.Module._load (module.js:300:12)
463 at Function.Module.runMain (module.js:441:10)
464 at startup (node.js:139:18)
465 at node.js:968:3
466 caused by: VError: something bad happened
467 at Object.<anonymous> (/home/dap/node-verror/examples/fullStack.js:3:12)
468 at Module._compile (module.js:409:26)
469 at Object.Module._extensions..js (module.js:416:10)
470 at Module.load (module.js:343:32)
471 at Function.Module._load (module.js:300:12)
472 at Function.Module.runMain (module.js:441:10)
473 at startup (node.js:139:18)
474 at node.js:968:3
475
476`VError.fullStack` is also safe to use on regular `Error`s, so feel free to use
477it whenever you need to extract the stack trace from an `Error`, regardless if
478it's a `VError` or not.
479
480# Reference: MultiError
481
482MultiError is an Error class that represents a group of Errors. This is used
483when you logically need to provide a single Error, but you want to preserve
484information about multiple underying Errors. A common case is when you execute
485several operations in parallel and some of them fail.
486
487MultiErrors are constructed as:
488
489```javascript
490new MultiError(error_list)
491```
492
493`error_list` is an array of at least one `Error` object.
494
495The cause of the MultiError is the first error provided. None of the other
496`VError` options are supported. The `message` for a MultiError consists the
497`message` from the first error, prepended with a message indicating that there
498were other errors.
499
500For example:
501
502```javascript
503err = new MultiError([
504 new Error('failed to resolve DNS name "abc.example.com"'),
505 new Error('failed to resolve DNS name "def.example.com"'),
506]);
507
508console.error(err.message);
509```
510
511outputs:
512
513 first of 2 errors: failed to resolve DNS name "abc.example.com"
514
515See the convenience function `VError.errorFromList`, which is sometimes simpler
516to use than this constructor.
517
518## Public methods
519
520
521### `errors()`
522
523Returns an array of the errors used to construct this MultiError.
524
525
526# Contributing
527
528See separate [contribution guidelines](CONTRIBUTING.md).
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