Node js Best Practice Exception Handling

Question

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

User · Answer

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.

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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:

https://github.com/petkaantonov/bluebird

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.

User · Answer

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

User · Answer

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/

User · Answer

If you want use Services in Ubuntu Upstart   Node as a service in Ubuntu 11 04 with upstart  monit and forever js

[node.js] Node.js Best Practice Exception Handling

The answer is

Best practices of Node.JS error handling

Number1: Use promises for async error handling

Number2: Use only the built-in Error object

Number3: Distinguish operational vs programmer errors

Number4: Handle errors centrally, through but not within middleware

Number5: Document API errors using Swagger

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

Number7: Use a mature logger to increase errors visibility

Number8: Discover errors and downtime using APM products

Examples related to node.js

Examples related to exception-handling

Examples related to serverside-javascript

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