Per previous threads, I'm implementing a transform stream around a particular
command line program for use in a web service. How should the stream report
errors?
In the case of the _transform and _flush functions that I'm supposed to
implement, they're given a callback parameter, so I can "callback(new
Error(…))". That's easy, but what about in other situations, like the
constructor?
In the constructor of my custom transform stream, I spawn a process:
var process = this._process = spawn(options.command, options.args,
options.env);
If the process closes with a non-zero exit code or crashes, I want to be able
to report that, not just to the server's console, but also to the web browser
of the user who made the request that started the process:
process.on('close', function(code, signal) {
if (signal !== null) {
…
} else if (code !== 0) {
…
}
self.emit('close');
});
If a problem occurred, should I "throw new Error(…)" or "self.emit('error', new
Error(…)"?
I was emitting the error, but am having trouble making that work in code
running directly in the constructor. Let's say I want to allow only a few
different commands ("foo" and "bar") to be run:
if (!~['foo', 'bar'].indexOf(options.command)) {
this.emit('error', new Error('Invalid command: ' + options.command));
}
If I emit an error like that, then the node program exits saying there was no
listener attached to the stream listening for the error event. But I cannot
attach a listener to the stream until after the constructor has finished and
has returned the stream object to me.
Should I instead throw the error? If so, do I just use a try{}catch{} block
whenever I create such a stream?
Passing Error objects to a callback is straightforward, but other methods of
error handling in node are still very confusing to me, especially try/catch.
Guidance would be appreciated.
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