By momo


2011-09-05 16:15:20 8 Comments

I just started trying out node.js a few days ago. I've realized that the Node is terminated whenever I have an unhandled exception in my program. This is different than the normal server container that I have been exposed to where only the Worker Thread dies when unhandled exceptions occur and the container would still be able to receive the request. This raises a few questions:

  • Is process.on('uncaughtException') the only effective way to guard against it?
  • Will process.on('uncaughtException') catch the unhandled exception during execution of asynchronous processes as well?
  • Is there a module that is already built (such as sending email or writing to a file) that I could leverage in the case of uncaught exceptions?

I would appreciate any pointer/article that would show me the common best practices for handling uncaught exceptions in node.js

10 comments

@balupton 2011-09-05 21:45:12

Update: Joyent now has their own guide. The following information is more of a summary:

Safely "throwing" errors

Ideally we'd like to avoid uncaught errors as much as possible, as such, instead of literally throwing the error, we can instead safely "throw" the error using one of the following methods depending on our code architecture:

  • For synchronous code, if an error happens, return the error:

    // Define divider as a syncrhonous function
    var divideSync = function(x,y) {
        // if error condition?
        if ( y === 0 ) {
            // "throw" the error safely by returning it
            return new Error("Can't divide by zero")
        }
        else {
            // no error occured, continue on
            return x/y
        }
    }
    
    // Divide 4/2
    var result = divideSync(4,2)
    // did an error occur?
    if ( result instanceof Error ) {
        // handle the error safely
        console.log('4/2=err', result)
    }
    else {
        // no error occured, continue on
        console.log('4/2='+result)
    }
    
    // Divide 4/0
    result = divideSync(4,0)
    // did an error occur?
    if ( result instanceof Error ) {
        // handle the error safely
        console.log('4/0=err', result)
    }
    else {
        // no error occured, continue on
        console.log('4/0='+result)
    }
    
  • For callback-based (ie. asynchronous) code, the first argument of the callback is err, if an error happens err is the error, if an error doesn't happen then err is null. Any other arguments follow the err argument:

    var divide = function(x,y,next) {
        // if error condition?
        if ( y === 0 ) {
            // "throw" the error safely by calling the completion callback
            // with the first argument being the error
            next(new Error("Can't divide by zero"))
        }
        else {
            // no error occured, continue on
            next(null, x/y)
        }
    }
    
    divide(4,2,function(err,result){
        // did an error occur?
        if ( err ) {
            // handle the error safely
            console.log('4/2=err', err)
        }
        else {
            // no error occured, continue on
            console.log('4/2='+result)
        }
    })
    
    divide(4,0,function(err,result){
        // did an error occur?
        if ( err ) {
            // handle the error safely
            console.log('4/0=err', err)
        }
        else {
            // no error occured, continue on
            console.log('4/0='+result)
        }
    })
    
  • For eventful code, where the error may happen anywhere, instead of throwing the error, fire the error event instead:

    // Definite our Divider Event Emitter
    var events = require('events')
    var Divider = function(){
        events.EventEmitter.call(this)
    }
    require('util').inherits(Divider, events.EventEmitter)
    
    // Add the divide function
    Divider.prototype.divide = function(x,y){
        // if error condition?
        if ( y === 0 ) {
            // "throw" the error safely by emitting it
            var err = new Error("Can't divide by zero")
            this.emit('error', err)
        }
        else {
            // no error occured, continue on
            this.emit('divided', x, y, x/y)
        }
    
        // Chain
        return this;
    }
    
    // Create our divider and listen for errors
    var divider = new Divider()
    divider.on('error', function(err){
        // handle the error safely
        console.log(err)
    })
    divider.on('divided', function(x,y,result){
        console.log(x+'/'+y+'='+result)
    })
    
    // Divide
    divider.divide(4,2).divide(4,0)
    

Safely "catching" errors

Sometimes though, there may still be code that throws an error somewhere which can lead to an uncaught exception and a potential crash of our application if we don't catch it safely. Depending on our code architecture we can use one of the following methods to catch it:

  • When we know where the error is occurring, we can wrap that section in a node.js domain

    var d = require('domain').create()
    d.on('error', function(err){
        // handle the error safely
        console.log(err)
    })
    
    // catch the uncaught errors in this asynchronous or synchronous code block
    d.run(function(){
        // the asynchronous or synchronous code that we want to catch thrown errors on
        var err = new Error('example')
        throw err
    })
    
  • If we know where the error is occurring is synchronous code, and for whatever reason can't use domains (perhaps old version of node), we can use the try catch statement:

    // catch the uncaught errors in this synchronous code block
    // try catch statements only work on synchronous code
    try {
        // the synchronous code that we want to catch thrown errors on
        var err = new Error('example')
        throw err
    } catch (err) {
        // handle the error safely
        console.log(err)
    }
    

    However, be careful not to use try...catch in asynchronous code, as an asynchronously thrown error will not be caught:

    try {
        setTimeout(function(){
            var err = new Error('example')
            throw err
        }, 1000)
    }
    catch (err) {
        // Example error won't be caught here... crashing our app
        // hence the need for domains
    }
    

    If you do want to work with try..catch in conjunction with asynchronous code, when running Node 7.4 or higher you can use async/await natively to write your asynchronous functions.

    Another thing to be careful about with try...catch is the risk of wrapping your completion callback inside the try statement like so:

    var divide = function(x,y,next) {
        // if error condition?
        if ( y === 0 ) {
            // "throw" the error safely by calling the completion callback
            // with the first argument being the error
            next(new Error("Can't divide by zero"))
        }
        else {
            // no error occured, continue on
            next(null, x/y)
        }
    }
    
    var continueElsewhere = function(err, result){
            throw new Error('elsewhere has failed')
    }
    
    try {
            divide(4, 2, continueElsewhere)
            // ^ the execution of divide, and the execution of 
            //   continueElsewhere will be inside the try statement
    }
    catch (err) {
            console.log(err.stack)
            // ^ will output the "unexpected" result of: elsewhere has failed
    }
    

    This gotcha is very easy to do as your code becomes more complex. As such, it is best to either use domains or to return errors to avoid (1) uncaught exceptions in asynchronous code (2) the try catch catching execution that you don't want it to. In languages that allow for proper threading instead of JavaScript's asynchronous event-machine style, this is less of an issue.

  • Finally, in the case where an uncaught error happens in a place that wasn't wrapped in a domain or a try catch statement, we can make our application not crash by using the uncaughtException listener (however doing so can put the application in an unknown state):

    // catch the uncaught errors that weren't wrapped in a domain or try catch statement
    // do not use this in modules, but only in applications, as otherwise we could have multiple of these bound
    process.on('uncaughtException', function(err) {
        // handle the error safely
        console.log(err)
    })
    
    // the asynchronous or synchronous code that emits the otherwise uncaught error
    var err = new Error('example')
    throw err
    

@momo 2011-09-05 22:21:25

So as a general pattern in node.js, you are saying that I will get err for every object that we call asynchronously (and error is normally returned for synchronous call), so we will always have to handle it there itself and there is no such thing as equivalent of RuntimeException in Java where the error just automatically bubble up to the top? In a sense the exception is always transparent for us to handle so I don't really need to worry about uncaughtException?

@balupton 2011-09-05 23:40:15

@momo I've updated the question to be more explanatory :-) Let me know if you're still not sure about anything :)

@momo 2011-09-05 23:48:01

Thanks for taking the time to answer in details! If I could assume that will be a general pattern that I will see from the modules in node.js, then there should be few uncaught exception that I will have to deal. Thanks again

@Raynos 2011-09-05 23:48:21

I recommend defaulting the error value to null rather then false since a lot of applications use null. Also try catch is evil. It should be avoided.

@balupton 2011-09-05 23:50:41

Thanks Raynos, updated. Do you have a source which explains the evils of try catch? As I'd love to back that up with evidence. Also fixed the sync example.

@David Erwin 2012-08-20 17:40:27

@balupton try catch in itself isn't evil, but improper use of it in a large app/team environment can make debugging a nightmarish experience. The catch block must always make clear exactly what is happening and where.

@Gabriel Llamas 2013-02-04 13:54:28

This answer is no longer valid. Domains solves this problem (recommended by node.js)

@balupton 2013-02-05 11:07:07

@GabrielLlamas uhhh.... domains help with catching otherwise uncaught exceptions, but they don't help with handling them correctly in the first place by making them the first argument of the completion callback. Please explain further if you disagree.

@Gabriel Llamas 2013-02-05 12:14:25

You can attach an error event listener to the domain so if your app fails to catch an error with try-catch or with regular callbacks the error listener is executed and the app doesn't crash. Domains doesn't catch errors produced at compile time, e.g., cannot find a module. Concluding, if a request throws an error and is not catched the server doesn't crash. nodejs.org/api/process.html#process_event_uncaughtexception "Don't use it, use domains isntead"

@B T 2013-04-10 19:30:08

This was out of date a decade before it was written. Return values are not appropriate for errors. This answer tells you nothing about how to properly isolate code when an error happens asynchronously. Also, this advocates the terrible use of an uncaughtException handler that simply lets your program continue in a bad state. This answer should have negative points.

@balupton 2013-04-11 02:37:27

Updated it for domains and events. Feedback welcome.

@kamituel 2013-08-17 14:17:36

@balupton - I've made small improvement to avoid any confusion - second example given is not asynchronous code per se (although it uses callbacks).

@Dave Kerr 2013-12-28 20:20:29

Superb answer, thanks very much indeed for putting the time into this.

@enl8enmentnow 2014-02-08 13:25:36

Hi, I prefer to separate my error handling code from my regular use case code. So in your first example, rather than returning an Error object, I would always return either a valid value or throw an exception. If it was important to the caller that they know whether it is going to throw in advance (in essense have your if/else statement) then they would check the arguments before passing them in. This is standard programming practice since exception handling had good support in most languages.

@enl8enmentnow 2014-02-08 13:25:57

The only reason that "err" is passed into the callback in Node.js library as the first parameter is because it's an asynch call. What you are proposing is throwback to how the programming community handled errors circa 1993: error return codes.

@Timo Huovinen 2014-06-27 18:05:35

makes a lot more sense with domains

@jfriend00 2014-09-05 23:50:10

Great answer, but would be nice to see it udpated with info about async error handling when using promises (my specific interest is node.js async file I/O).

@balupton 2015-02-01 23:12:09

@jfriend00 promises would be awesome, but there are many variations of how to do it due to the variations in spec and libraries, so best bet would be referring to your promise library's docs on error handling.

@balupton 2015-02-01 23:14:25

@enl8enmentnow throwing errors should be avoided at all costs, they break execution of the app, and could be caught in random places - potentially putting your app in an unknown state, which is entirely the thing we want to avoid at all costs. For instance see the new try catch warning example.

@enl8enmentnow 2015-02-03 00:48:44

@balupton Errors should be thrown for error handling. They should definitely NOT be avoided. There is nothing about them that breaks the execution of the app or anything else. Java and most other modern languages have excellent support for exceptions. My only conclusion after reading some of the misonformed posts here is that people don't understand them very well And so are afraid of them. Fear Uncertain Doubt. This debate was decided conclusively in favor of exceptions at least 20 years ago.

@balupton 2015-03-04 05:38:46

@enl8enmentnow updated the try catch examples once more to illustrate how they are dangerous in asynchronous javascript code

@Timothy Gu 2015-04-07 23:44:53

Now domains are deprecated by io.js: "This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated… Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future."

@balupton 2015-04-09 06:05:11

Thanks Timothy, Perhaps worth mentioning io.js definition of depreaction as well "Stability: 0 - Deprecated. This feature is known to be problematic, and changes are planned. Do not rely on it. Use of the feature may cause warnings. Backwards compatibility should not be expected." - it's strange they have deprecated it with nothing to replace it. It's probably to continue the ideal that you shouldn't need domains, but if you do need them, they exist, even if they are problematic.

@Olga 2015-07-28 16:25:42

Great answer, but I was wondering what if you want to catch errors in order to log them in desired format but then still exit. Is there any straightforward beginner way to do it using domains? I probably lack research. went researching further...

@Olga 2015-08-05 16:44:57

I actually found what needs to be done, after catching your error in domain and logging it nicely you can then process.exit(1)

@UpTheCreek 2015-10-08 13:20:37

The domain api is deprecated now? They mention a replacement API - anyone know when this will come out, and what it will look like?

@balupton 2015-10-09 14:31:08

@UpTheCreek that's been there for over a year now, no replacement API has been setup, it's safe to say there is no need to worry about it at the current time. Will update the answer when there actually is a replacement API. See previous comment for more information: stackoverflow.com/questions/7310521/…

@Akansh Gulati 2016-11-01 10:22:00

I would like to add that for callback async functions, handling error just by logging it is neither a write way nor we can handle those errors in parent functions. The best practise in that case could be to make it a promise and return the error for handling.

@NessDan 2018-09-27 02:55:31

@user905864 I think you're wrong, no where on that page does it mention that uncaughtException was deprecated, only instructions on how to use it properly. nodejs.org/api/process.html#process_event_uncaughtexception

@Yonatan 2016-04-18 20:10:13

Following is a summarization and curation from many different sources on this topic including code example and quotes from selected blog posts. The complete list of best practices can be found here


Best practices of Node.JS error handling


Number1: Use promises for async error handling

TL;DR: Handling async errors in callback style is probably the fastest way to hell (a.k.a the pyramid of doom). The best gift you can give to your code is using instead a reputable promise library which provides much compact and familiar code syntax like try-catch

Otherwise: Node.JS callback style, function(err, response), is a promising way to un-maintainable code due to the mix of error handling with casual code, excessive nesting and awkward coding patterns

Code example - good

doWork()
.then(doWork)
.then(doError)
.then(doWork)
.catch(errorHandler)
.then(verify);

code example anti pattern – callback style error handling

getData(someParameter, function(err, result){
    if(err != null)
      //do something like calling the given callback function and pass the error
    getMoreData(a, function(err, result){
          if(err != null)
            //do something like calling the given callback function and pass the error
        getMoreData(b, function(c){ 
                getMoreData(d, function(e){ 
                    ...
                });
            });
        });
    });
});

Blog quote: "We have a problem with promises" (From the blog pouchdb, ranked 11 for the keywords "Node Promises")

"…And in fact, callbacks do something even more sinister: they deprive us of the stack, which is something we usually take for granted in programming languages. Writing code without a stack is a lot like driving a car without a brake pedal: you don’t realize how badly you need it, until you reach for it and it’s not there. The whole point of promises is to give us back the language fundamentals we lost when we went async: return, throw, and the stack. But you have to know how to use promises correctly in order to take advantage of them."


Number2: Use only the built-in Error object

TL;DR: It pretty common to see code that throws errors as string or as a custom type – this complicates the error handling logic and the interoperability between modules. Whether you reject a promise, throw exception or emit error – using Node.JS built-in Error object increases uniformity and prevents loss of error information

Otherwise: When executing some module, being uncertain which type of errors come in return – makes it much harder to reason about the coming exception and handle it. Even worth, using custom types to describe errors might lead to loss of critical error information like the stack trace!

Code example - doing it right

    //throwing an Error from typical function, whether sync or async
 if(!productToAdd)
 throw new Error("How can I add new product when no value provided?");

//'throwing' an Error from EventEmitter
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));

//'throwing' an Error from a Promise
 return new promise(function (resolve, reject) {
 DAL.getProduct(productToAdd.id).then((existingProduct) =>{
 if(existingProduct != null)
 return reject(new Error("Why fooling us and trying to add an existing product?"));

code example anti pattern

//throwing a String lacks any stack trace information and other important properties
if(!productToAdd)
    throw ("How can I add new product when no value provided?");

Blog quote: "A string is not an error" (From the blog devthought, ranked 6 for the keywords “Node.JS error object”)

"…passing a string instead of an error results in reduced interoperability between modules. It breaks contracts with APIs that might be performing instanceof Error checks, or that want to know more about the error. Error objects, as we’ll see, have very interesting properties in modern JavaScript engines besides holding the message passed to the constructor.."


Number3: Distinguish operational vs programmer errors

TL;DR: Operations errors (e.g. API received an invalid input) refer to known cases where the error impact is fully understood and can be handled thoughtfully. On the other hand, programmer error (e.g. trying to read undefined variable) refers to unknown code failures that dictate to gracefully restart the application

Otherwise: You may always restart the application when an error appear, but why letting ~5000 online users down because of a minor and predicted error (operational error)? the opposite is also not ideal – keeping the application up when unknown issue (programmer error) occurred might lead unpredicted behavior. Differentiating the two allows acting tactfully and applying a balanced approach based on the given context

Code example - doing it right

    //throwing an Error from typical function, whether sync or async
 if(!productToAdd)
 throw new Error("How can I add new product when no value provided?");

//'throwing' an Error from EventEmitter
const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));

//'throwing' an Error from a Promise
 return new promise(function (resolve, reject) {
 DAL.getProduct(productToAdd.id).then((existingProduct) =>{
 if(existingProduct != null)
 return reject(new Error("Why fooling us and trying to add an existing product?"));

code example - marking an error as operational (trusted)

//marking an error object as operational 
var myError = new Error("How can I add new product when no value provided?");
myError.isOperational = true;

//or if you're using some centralized error factory (see other examples at the bullet "Use only the built-in Error object")
function appError(commonType, description, isOperational) {
    Error.call(this);
    Error.captureStackTrace(this);
    this.commonType = commonType;
    this.description = description;
    this.isOperational = isOperational;
};

throw new appError(errorManagement.commonErrors.InvalidInput, "Describe here what happened", true);

//error handling code within middleware
process.on('uncaughtException', function(error) {
    if(!error.isOperational)
        process.exit(1);
});

Blog Quote: "Otherwise you risk the state" (From the blog debugable, ranked 3 for the keywords "Node.JS uncaught exception")

"…By the very nature of how throw works in JavaScript, there is almost never any way to safely “pick up where you left off”, without leaking references, or creating some other sort of undefined brittle state. The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, you might have many connections open, and it is not reasonable to abruptly shut those down because an error was triggered by someone else. The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker"


Number4: Handle errors centrally, through but not within middleware

TL;DR: Error handling logic such as mail to admin and logging should be encapsulated in a dedicated and centralized object that all end-points (e.g. Express middleware, cron jobs, unit-testing) call when an error comes in.

Otherwise: Not handling errors within a single place will lead to code duplication and probably to errors that are handled improperly

Code example - a typical error flow

//DAL layer, we don't handle errors here
DB.addDocument(newCustomer, (error, result) => {
    if (error)
        throw new Error("Great error explanation comes here", other useful parameters)
});

//API route code, we catch both sync and async errors and forward to the middleware
try {
    customerService.addNew(req.body).then(function (result) {
        res.status(200).json(result);
    }).catch((error) => {
        next(error)
    });
}
catch (error) {
    next(error);
}

//Error handling middleware, we delegate the handling to the centrzlied error handler
app.use(function (err, req, res, next) {
    errorHandler.handleError(err).then((isOperationalError) => {
        if (!isOperationalError)
            next(err);
    });
});

Blog quote: "Sometimes lower levels can’t do anything useful except propagate the error to their caller" (From the blog Joyent, ranked 1 for the keywords “Node.JS error handling”)

"…You may end up handling the same error at several levels of the stack. This happens when lower levels can’t do anything useful except propagate the error to their caller, which propagates the error to its caller, and so on. Often, only the top-level caller knows what the appropriate response is, whether that’s to retry the operation, report an error to the user, or something else. But that doesn’t mean you should try to report all errors to a single top-level callback, because that callback itself can’t know in what context the error occurred"


Number5: Document API errors using Swagger

TL;DR: Let your API callers know which errors might come in return so they can handle these thoughtfully without crashing. This is usually done with REST API documentation frameworks like Swagger

Otherwise: An API client might decide to crash and restart only because he received back an error he couldn’t understand. Note: the caller of your API might be you (very typical in a microservices environment)

Blog quote: "You have to tell your callers what errors can happen" (From the blog Joyent, ranked 1 for the keywords “Node.JS logging”)

…We’ve talked about how to handle errors, but when you’re writing a new function, how do you deliver errors to the code that called your function? …If you don’t know what errors can happen or don’t know what they mean, then your program cannot be correct except by accident. So if you’re writing a new function, you have to tell your callers what errors can happen and what they mea


Number6: Shut the process gracefully when a stranger comes to town

TL;DR: When an unknown error occurs (a developer error, see best practice number #3)- there is uncertainty about the application healthiness. A common practice suggests restarting the process carefully using a ‘restarter’ tool like Forever and PM2

Otherwise: When an unfamiliar exception is caught, some object might be in a faulty state (e.g an event emitter which is used globally and not firing events anymore due to some internal failure) and all future requests might fail or behave crazily

Code example - deciding whether to crash

//deciding whether to crash when an uncaught exception arrives
//Assuming developers mark known operational errors with error.isOperational=true, read best practice #3
process.on('uncaughtException', function(error) {
 errorManagement.handler.handleError(error);
 if(!errorManagement.handler.isTrustedError(error))
 process.exit(1)
});


//centralized error handler encapsulates error-handling related logic 
function errorHandler(){
 this.handleError = function (error) {
 return logger.logError(err).then(sendMailToAdminIfCritical).then(saveInOpsQueueIfCritical).then(determineIfOperationalError);
 }

 this.isTrustedError = function(error)
 {
 return error.isOperational;
 }

Blog quote: "There are three schools of thoughts on error handling" (From the blog jsrecipes)

…There are primarily three schools of thoughts on error handling: 1. Let the application crash and restart it. 2. Handle all possible errors and never crash. 3. Balanced approach between the two


Number7: Use a mature logger to increase errors visibility

TL;DR: A set of mature logging tools like Winston, Bunyan or Log4J, will speed-up error discovery and understanding. So forget about console.log.

Otherwise: Skimming through console.logs or manually through messy text file without querying tools or a decent log viewer might keep you busy at work until late

Code example - Winston logger in action

//your centralized logger object
var logger = new winston.Logger({
 level: 'info',
 transports: [
 new (winston.transports.Console)(),
 new (winston.transports.File)({ filename: 'somefile.log' })
 ]
 });

//custom code somewhere using the logger
logger.log('info', 'Test Log Message with some parameter %s', 'some parameter', { anything: 'This is metadata' });

Blog quote: "Lets identify a few requirements (for a logger):" (From the blog strongblog)

…Lets identify a few requirements (for a logger): 1. Time stamp each log line. This one is pretty self explanatory – you should be able to tell when each log entry occured. 2. Logging format should be easily digestible by humans as well as machines. 3. Allows for multiple configurable destination streams. For example, you might be writing trace logs to one file but when an error is encountered, write to the same file, then into error file and send an email at the same time…


Number8: Discover errors and downtime using APM products

TL;DR: Monitoring and performance products (a.k.a APM) proactively gauge your codebase or API so they can auto-magically highlight errors, crashes and slow parts that you were missing

Otherwise: You might spend great effort on measuring API performance and downtimes, probably you’ll never be aware which are your slowest code parts under real world scenario and how these affects the UX

Blog quote: "APM products segments" (From the blog Yoni Goldberg)

"…APM products constitutes 3 major segments:1. Website or API monitoring – external services that constantly monitor uptime and performance via HTTP requests. Can be setup in few minutes. Following are few selected contenders: Pingdom, Uptime Robot, and New Relic 2. Code instrumentation – products family which require to embed an agent within the application to benefit feature slow code detection, exceptions statistics, performance monitoring and many more. Following are few selected contenders: New Relic, App Dynamics 3. Operational intelligence dashboard – these line of products are focused on facilitating the ops team with metrics and curated content that helps to easily stay on top of application performance. This is usually involves aggregating multiple sources of information (application logs, DB logs, servers log, etc) and upfront dashboard design work. Following are few selected contenders: Datadog, Splunk"


The above is a shortened version - see here more best practices and examples

@K. Craven 2015-09-25 01:36:59

Catching errors has been very well discussed here, but it's worth remembering to log the errors out somewhere so you can view them and fix stuff up.

​Bunyan is a popular logging framework for NodeJS - it supporst writing out to a bunch of different output places which makes it useful for local debugging, as long as you avoid console.log. ​ In your domain's error handler you could spit the error out to a log file.

var log = bunyan.createLogger({
  name: 'myapp',
  streams: [
    {
      level: 'error',
      path: '/var/tmp/myapp-error.log'  // log ERROR to this file
    }
  ]
});

This can get time consuming if you have lots of errors and/or servers to check, so it could be worth looking into a tool like Raygun (disclaimer, I work at Raygun) to group errors together - or use them both together. ​ If you decided to use Raygun as a tool, it's pretty easy to setup too

var raygunClient = new raygun.Client().init({ apiKey: 'your API key' });
raygunClient.send(theError);

​ Crossed with using a tool like PM2 or forever, your app should be able to crash, log out what happened and reboot without any major issues.

@B T 2013-04-10 19:45:46

nodejs domains is the most up to date way of handling errors in nodejs. Domains can capture both error/other events as well as traditionally thrown objects. Domains also provide functionality for handling callbacks with an error passed as the first argument via the intercept method.

As with normal try/catch-style error handling, is is usually best to throw errors when they occur, and block out areas where you want to isolate errors from affecting the rest of the code. The way to "block out" these areas are to call domain.run with a function as a block of isolated code.

In synchronous code, the above is enough - when an error happens you either let it be thrown through, or you catch it and handle there, reverting any data you need to revert.

try {  
  //something
} catch(e) {
  // handle data reversion
  // probably log too
}

When the error happens in an asynchronous callback, you either need to be able to fully handle the rollback of data (shared state, external data like databases, etc). OR you have to set something to indicate that an exception has happened - where ever you care about that flag, you have to wait for the callback to complete.

var err = null;
var d = require('domain').create();
d.on('error', function(e) {
  err = e;
  // any additional error handling
}
d.run(function() { Fiber(function() {
  // do stuff
  var future = somethingAsynchronous();
  // more stuff

  future.wait(); // here we care about the error
  if(err != null) {
    // handle data reversion
    // probably log too
  }

})});

Some of that above code is ugly, but you can create patterns for yourself to make it prettier, eg:

var specialDomain = specialDomain(function() {
  // do stuff
  var future = somethingAsynchronous();
  // more stuff

  future.wait(); // here we care about the error
  if(specialDomain.error()) {
    // handle data reversion
    // probably log too
  } 
}, function() { // "catch"
  // any additional error handling
});

UPDATE (2013-09):

Above, I use a future that implies fibers semantics, which allow you to wait on futures in-line. This actually allows you to use traditional try-catch blocks for everything - which I find to be the best way to go. However, you can't always do this (ie in the browser)...

There are also futures that don't require fibers semantics (which then work with normal, browsery JavaScript). These can be called futures, promises, or deferreds (I'll just refer to futures from here on). Plain-old-JavaScript futures libraries allow errors to be propagated between futures. Only some of these libraries allow any thrown future to be correctly handled, so beware.

An example:

returnsAFuture().then(function() {
  console.log('1')
  return doSomething() // also returns a future

}).then(function() {
  console.log('2')
  throw Error("oops an error was thrown")

}).then(function() {
  console.log('3')

}).catch(function(exception) {
  console.log('handler')
  // handle the exception
}).done()

This mimics a normal try-catch, even though the pieces are asynchronous. It would print:

1
2
handler

Note that it doesn't print '3' because an exception was thrown that interrupts that flow.

Take a look at bluebird promises:

Note that I haven't found many other libraries other than these that properly handle thrown exceptions. jQuery's deferred, for example, don't - the "fail" handler would never get the exception thrown an a 'then' handler, which in my opinion is a deal breaker.

@Esailija 2013-10-28 08:37:09

The proper promises specification in Javascript is known as Promises/A+. You may see a list of implementations here: github.com/promises-aplus/promises-spec/blob/master/…. Note that a bare Promises/A+ is unusable in practice - Promises/A+ still leaves a lot of practical problems for libraries to solve themselves. However absolutely essential things like the error propagation you show, deterministic execution order and safety from stack overflow are guaranteed.

@UpTheCreek 2015-10-08 13:22:17

@Sudsy 2013-09-10 04:15:57

After reading this post some time ago I was wondering if it was safe to use domains for exception handling on an api / function level. I wanted to use them to simplify exception handling code in each async function I wrote. My concern was that using a new domain for each function would introduce significant overhead. My homework seems to indicate that there is minimal overhead and that performance is actually better with domains than with try catch in some situations.

http://www.lighthouselogic.com/#/using-a-new-domain-for-each-async-function-in-node/

@Dan Dascalescu 2019-09-13 18:37:18

@Cap 2012-09-06 13:01:20

If you want use Services in Ubuntu(Upstart): Node as a service in Ubuntu 11.04 with upstart, monit and forever.js

@Simon Maynard 2013-01-10 00:16:11

I wrote about this recently at http://snmaynard.com/2012/12/21/node-error-handling/. A new feature of node in version 0.8 is domains and allow you to combine all the forms of error handling into one easier manage form. You can read about them in my post.

You can also use something like Bugsnag to track your uncaught exceptions and be notified via email, chatroom or have a ticket created for an uncaught exception (I am the co-founder of Bugsnag).

@MattSidor 2016-04-02 00:01:13

The domain module is now officially deprecated. nodejs.org/api/domain.html

@Michael Yagudaev 2012-09-11 14:26:39

One instance where using a try-catch might be appropriate is when using a forEach loop. It is synchronous but at the same time you cannot just use a return statement in the inner scope. Instead a try and catch approach can be used to return an Error object in the appropriate scope. Consider:

function processArray() {
    try { 
       [1, 2, 3].forEach(function() { throw new Error('exception'); }); 
    } catch (e) { 
       return e; 
    }
}

It is a combination of the approaches described by @balupton above.

@joeytwiddle 2016-06-19 04:14:29

Instead of throwing errors, some developers recommend using the Result concept from Rust to return either an OK or a Fail, when failure is a known possibility. This keeps failures separate from unexpected errors. One JS implementation of this is r-result.

@joeytwiddle 2016-06-19 04:14:42

It's an app-wide design decision. I think your concept of returning errors is roughly equivalent, and simple to get started with (no extra dependencies), but less explicit (Result makes you painfully aware when failures may need to be handled) and less efficient in those cases when a stack is built unnecessarily.

@Michael Yagudaev 2012-06-12 16:35:07

I would just like to add that Step.js library helps you handle exceptions by always passing it to the next step function. Therefore you can have as a last step a function that check for any errors in any of the previous steps. This approach can greatly simplify your error handling.

Below is a quote from the github page:

any exceptions thrown are caught and passed as the first argument to the next function. As long as you don't nest callback functions inline your main functions this prevents there from ever being any uncaught exceptions. This is very important for long running node.JS servers since a single uncaught exception can bring the whole server down.

Furthermore, you can use Step to control execution of scripts to have a clean up section as the last step. For example if you want to write a build script in Node and report how long it took to write, the last step can do that (rather than trying to dig out the last callback).

@nponeccop 2011-09-05 20:11:51

You can catch uncaught exceptions, but it's of limited use. See http://debuggable.com/posts/node-js-dealing-with-uncaught-exceptions:4c933d54-1428-443c-928d-4e1ecbdd56cb

monit, forever or upstart can be used to restart node process when it crashes. A graceful shutdown is best you can hope for (e.g. save all in-memory data in uncaught exception handler).

@momo 2011-09-05 20:43:45

+1 The link is useful, thanks. I am still looking for the best practice and the meaning of "graceful restart" in the context of node.js

@Ilkka 2011-12-29 18:26:27

My understanding of "graceful restart" in this context would be essentially what nponeccop suggests: let the process die, and let whatever is running it in the first place restart it.

@SatheeshJM 2012-09-13 03:38:05

Thanks a lot for that link! Really useful!

@wprl 2015-02-04 22:23:47

This is a great answer. I would disagree about returning an Error in your first example however. Returning an Error makes the return value polymorphic which muddles the semantics of the function unnecessarily. Furthermore, diving by 0 is already handled in JavaScript by giving Infinity, -Infinity, or NaN, values where typeof === 'number'. They can be checked for with !isFinite(value). In general I would recommend never returning an Error from a function. Better in terms of code legibility and maintenance to throw or return a special non-polymorphic value w/ consistent semantics.

@Kev 2018-05-14 20:46:21

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