121 lines
4.4 KiB
Markdown
121 lines
4.4 KiB
Markdown
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# verror: richer JavaScript errors
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This module provides two classes: VError, for accretive errors, and WError, for
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wrapping errors. Both support printf-style error messages using extsprintf.
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## Printf-style errors
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At the most basic level, VError is just like JavaScript's Error class, but with
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printf-style arguments:
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var verror = require('verror');
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var opname = 'read';
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var err = new verror.VError('"%s" operation failed', opname);
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console.log(err.message);
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console.log(err.stack);
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This prints:
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"read" operation failed
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"read" operation failed
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at Object.<anonymous> (/Users/dap/node-verror/examples/varargs.js:4:11)
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at Module._compile (module.js:449:26)
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at Object.Module._extensions..js (module.js:467:10)
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at Module.load (module.js:356:32)
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at Function.Module._load (module.js:312:12)
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at Module.runMain (module.js:492:10)
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at process.startup.processNextTick.process._tickCallback (node.js:244:9)
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## VError for accretive error messages
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More interestingly, you can use VError to build up an error describing what
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happened at various levels in the stack. For example, suppose you have a
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request handler that stats a file and fails if it doesn't exist:
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var fs = require('fs');
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var verror = require('verror');
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function checkFile(filename, callback) {
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fs.stat(filename, function (err) {
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if (err)
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/* Annotate the "stat" error with what we were doing. */
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return (callback(new verror.VError(err,
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'failed to check "%s"', filename)));
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/* ... */
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});
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}
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function handleRequest(filename, callback) {
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checkFile('/nonexistent', function (err) {
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if (err) {
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/* Annotate the "checkFile" error with what we were doing. */
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return (callback(new verror.VError(err, 'request failed')));
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}
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/* ... */
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});
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}
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handleRequest('/nonexistent', function (err) {
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if (err)
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console.log(err.message);
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/* ... */
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});
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Since the file "/nonexistent" doesn't exist, this prints out:
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request failed: failed to check "/nonexistent": ENOENT, stat '/nonexistent'
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The idea here is that the lowest level (Node's "fs.stat" function) generates an
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arbitrary error, and each higher level (request handler and stat callback)
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creates a new VError that annotates the previous error with what it was doing,
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so that the result is a clear message explaining what failed at each level.
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This plays nicely with extsprintf's "%r" specifier, which prints out a
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Java-style stacktrace with the whole chain of exceptions:
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EXCEPTION: VError: request failed: failed to check "/nonexistent": ENOENT, stat '/nonexistent'
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at /Users/dap/work/node-verror/examples/levels.js:21:21
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at /Users/dap/work/node-verror/examples/levels.js:9:12
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at Object.oncomplete (fs.js:297:15)
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Caused by: EXCEPTION: VError: failed to check "/nonexistent": ENOENT, stat '/nonexistent'
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at /Users/dap/work/node-verror/examples/levels.js:9:21
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at Object.oncomplete (fs.js:297:15)
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Caused by: EXCEPTION: Error: Error: ENOENT, stat '/nonexistent'
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## WError for wrapped errors
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Sometimes you don't want an Error's "message" field to include the details of
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all of the low-level errors, but you still want to be able to get at them
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programmatically. For example, in an HTTP server, you probably don't want to
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spew all of the low-level errors back to the client, but you do want to include
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them in the audit log entry for the request. In that case, you can use a
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WError, which is created exactly like VError (and also supports both
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printf-style arguments and an optional cause), but the resulting "message" only
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contains the top-level error. It's also more verbose, including the class
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associated with each error in the cause chain. Using the same example above,
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but replacing the VError in handleRequest with WError, we get this output:
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request failed
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That's what we wanted -- just a high-level summary for the client. But we can
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get the object's toString() for the full details:
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WError: request failed; caused by WError: failed to check "/nonexistent";
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caused by Error: ENOENT, stat '/nonexistent'
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# Contributing
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Contributions welcome. Code should be "make check" clean. To run "make check",
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you'll need these tools:
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* https://github.com/davepacheco/jsstyle
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* https://github.com/davepacheco/javascriptlint
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If you're changing something non-trivial or user-facing, you may want to submit
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an issue first.
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