Mastering Node.js（Second Edition）
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Errors and exceptions

Generally in programming, the terms error and exception are often used interchangeably. Within the Node environment, these two concepts are not identical. Errors and exceptions are different. Additionally, the definition of error and exception within Node does not necessarily align with similar definitions in other languages and development environments.

Conventionally, an error condition in a Node program is a non-fatal condition that should be caught and handled, seen most explicitly in the Error as first argument convention displayed by the typical Node callback pattern. An exception is a serious error (a system error) that a sane environment should not ignore or try to handle.

One comes across four common error contexts in Node, and should respond predictably:

  • A synchronous context: This will normally happen in the context of a function, where a bad call signature or another non-fatal error is detected. The function should simply return an error object; new Error(…), or some other consistent indicator that the function call has failed.
  • An asynchronous context: When expected to respond by firing a callback function, the execution context should pass an Error object, with appropriate message, as the first argument to that callback.
  • An event context: Quoting the Node documentation: "When an EventEmitter instance experiences an error, the typical action is to emit an error event. Error events are treated as a special case in node. If there is no listener for it, then the default action is to print a stack trace and exit the program." Use events where events are expected.
  • A Promise context: A Promise throws or is otherwise rejected, and this error is caught within a .catch block. Important note: you should always reject Promises with true Error objects. Petka Antonov, author of the popular Bluebird Promises implementation, discusses why: https://github.com/petkaantonov/bluebird/blob/master/docs/docs/warning-explanations.md

Clearly, these are situations where an error is caught in a controlled manner, prior to it destabilizing the entire application. Without falling too far into defensive coding, an effort should be made to check inputs and other sources for errors, and properly dismiss them.

An additional benefit of always returning a proper Error object is access to the stack property of that object. The error stack shows the provenance of an error, each link in the chain of function, and calls the function that led to the error. A typical Error.stack trace would look like this:

> console.log(new Error("My Error Message").stack);
 Error: My Error Message
     at Object.<anonymous> (/js/errorstack.js:1:75)
     at Module._compile (module.js:449:26)
     at Object.Module._extensions..js (module.js:467:10)
     ...

Similarly, the stack is always available via the console.trace method:

> console.trace("The Stack Head")
 Trace: The Stack Head
     at Object.<anonymous> (/js/stackhead.js:1:71)
     at Module._compile (module.js:449:26)
     at Object.Module._extensions..js (module.js:467:10)
     ...

It should be clear how this information aids in debugging, helping to ensure that the logical flow of our application is sound.

A normal stack trace truncates after a dozen or so levels. If longer stack traces are useful to you, try Matt Insler's longjohn: https://github.com/mattinsler/longjohn

As well, run and examine the js/stacktrace.js file in your bundle for some ideas on how stack information might be used when reporting errors, or even test results.

Exception handling is different. Exceptions are unexpected or fatal errors that have destabilized the application. These should be handled with care; a system in an exception state is unstable, with indeterminate future states, and should be gracefully shut down and restarted. This is the smart thing to do.

Typically, exceptions are caught in try/catch blocks:

try {
  something.that = wontWork;
} catch (thrownError) {
  // do something with the exception we just caught
}

Peppering a codebase with try/catch blocks and trying to anticipate all errors can become unmanageable and unwieldy. Additionally, what if an exception you didn't anticipate, an uncaught exception, occurs? How do you pick up from where you left off?

Node does not have a standard built-in way to handle uncaught critical exceptions. This is a weakness of the platform. An exception that is uncaught will continue to bubble up through the execution stack until it hits the event loop where, like a wrench in the gears of a machine, it will take down the entire process. The best we have is to attach an uncaughtException handler to the process itself:

process.on('uncaughtException', (err) => {
  console.log('Caught exception: ' + err);
 });
 
setTimeout(() => {
  console.log("The exception was caught and this can run.");
}, 1000);
 
throwAnUncaughtException();
 
// > Caught exception: ReferenceError: throwAnUncaughtException is not defined
// > The exception was caught and this can run.

While nothing that follows our exception code will execute, the timeout will still fire, as the process managed to catch the exception, saving itself. However, this is a very clumsy way of handling exceptions. The domain module aimed to fix this hole in Node's design, but it has since been deprecated. Properly handling and reporting errors remains a real weakness in the Node platform. Work continues by the core team to address this problem: https://nodejs.org/en/docs/guides/domain-postmortem/

Recently, a similar mechanism was introduced to catch runaway Promises, which occur when you do not attach a catch handler to your Promise chain:

process.on('unhandledRejection', (reason, Prom) => {
  console.log(`Unhandled Rejection: ${p} reason: ${reason}`);
});

The unhandledRejection handler is fired whenever a Promise is rejected and no error handler is attached to the Promise within one turn of the event loop.

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