uncatch1.0.2 • Public • Published
Uncatch is a cooperative uncaught exception manager, which allows multiple modules to register listeners for uncaught exceptions, and all of them will be executed before the process finally exits. The module also handles emitting the error and stack trace to STDERR.
The reason for this module is that Node.js behaves differently when you register an uncaught exception handler. If any listeners are defined for this global event, then the stack trace is no longer emitted to the console, and the process does not exit. It is up to your listener code to handle both operations. This is bad because multiple modules may all want to execute their own emergency shutdown / cleanup code, especially in larger applications, but if any of them were to call
process.exit() (as they should) then not all the listeners get a chance to execute.
Uncatch solves this problem by offering its own global EventEmitter upon which multiple modules may all register listeners. Uncatch then registers a single global uncaught exception handler and fires its own
uncaughtException event, executing all of its listeners before finally emitting the error and stack trace to STDERR, and calling
All modules in your application have to agree to use Uncatch for this to really work. If any module in your stack registers a global uncaught exception handler and calls
process.exit() then all bets are off.
Imagine a modular Node.js daemon application that spawns and manages child processes. If the root daemon suffers an uncaught exception, it should kill off its children as it goes down. This is fine, but what happens when it adds a global uncaught exception handler? That handler needs to call
process.exit() as per best practices, but then no other modules can do anything. For example, the application may have another module that also needs to perform shutdown operations, like logging the exception to a crash log, etc.
If your application can manage this from a single global uncaught exception handler, then you probably don't need Uncatch. But if you have a modular application where multiple separate modules all need to cooperate to run multiple shutdown operations, with a single exit at the very end, then Uncatch may be for you.
What about the uncaught module? This module came close to what I needed, but it doesn't emit the error and stack trace to STDERR, and it doesn't call
process.exit() after all its listeners execute.
What about the uncaught-exception module? It has weird requirements, seems to be built around statsd, its usage is confusing, and the README says things like "v0.10 only". It rubbed me the wrong way.
What about the exception module? This module is rather nifty, but it doesn't solve the problem where multiple modules all need to run uncaught exception code. Also, it has native module requirements (like heapdump).
Download via npm:
npm install uncatch --save
Use it in your modules like this:
var Uncatch = ;Uncatch;//// ...in some other module...//var Uncatch = ;Uncatch;
You do not have to call
process.exit() as Uncatch will do it for you, once all the event listeners are executed. If you need to log the crash, use synchronous filesystem operations like fs.appendFileSync(). Nothing async will execute. It's sync or die during a crash.
Custom Exit Code
By default Uncatch calls
process.exit(1) which signals an unclean exit to whatever calling process or shell spawned your Node.js app. You can change this exit code by setting
Uncatch.exitCode. You can even do this from inside an uncaught exception listener. Example:
Please note that the latter prevails here. If multiple modules all fight to customize the
exitCode then whichever one did it last is the winner.
Uncatch recognizes the following environment variables if set to any true value:
||If set, do not register an uncaught exception handler.|
||If set, do not emit the error and stack trace to STDERR.|
||If set, do not call
For example, to temporarily disable Uncatch, run your app thusly:
NO_UNCATCH=1 node your-app.js
The MIT License (MIT)
Copyright (c) 2017 Joseph Huckaby
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