exception handling, thrown errors, within promises

exception handling, thrown errors, within promises - javascript

I am running external code as a 3rd party extension to a node.js service. The API methods return promises. A resolved promise means the action was carried out successfully, a failed promise means there was some problem carrying out the operation.
Now here's where I'm having trouble.
Since the 3rd party code is unknown, there could be bugs, syntax errors, type issues, any number of things that could cause node.js to throw an exception.
However, since all the code is wrapped up in promises, these thrown exceptions are actually coming back as failed promises.
I tried to put the function call within a try/catch block, but it's never triggered:
// worker process
var mod = require('./3rdparty/module.js');
try {
mod.run().then(function (data) {
sendToClient(true, data);
}, function (err) {
sendToClient(false, err);
});
} catch (e) {
// unrecoverable error inside of module
// ... send signal to restart this worker process ...
});
In the above psuedo-code example, when an error is thrown it turns up in the failed promise function, and not in the catch.
From what I read, this is a feature, not an issue, with promises. However I'm having trouble wrapping my head around why you'd always want to treat exceptions and expected rejections exactly the same.
One case is about actual bugs in the code, possibly irrecoverable -- the other is just possible missing configuration information, or a parameter, or something recoverable.
Thanks for any help!

Crashing and restarting a process is not a valid strategy to deal with errors, not even bugs. It would be fine in Erlang, where a process is cheap and does one isolated thing, like serving a single client. That doesn't apply in node, where a process costs orders of magnitude more and serves thousands of clients at once
Lets say that you have 200 requests per second being served by your service. If 1% of those hit a throwing path in your code, you would get 20 process shutdowns per second, roughly one every 50ms. If you have 4 cores with 1 process per core, you would lose them in 200ms. So if a process takes more than 200ms to start and prepare to serve requests (minimum cost is around 50ms for a node process that doesn't load any modules), we now have a successful total denial of service. Not to mention that users hitting an error tend to do things like e.g. repeatedly refresh the page, thereby compounding the problem.
Domains don't solve the issue because they cannot ensure that resources are not leaked.
Read more at issues #5114 and #5149.
Now you can try to be "smart" about this and have a process recycling policy of some sort based on a certain number of errors, but whatever strategy you approach it will severely change the scalability profile of node. We're talking several dozen requests per second per process, instead of several thousands.
However, promises catch all exceptions and then propagate them in a manner very similar to how synchronous exceptions propagate up the stack. Additionally, they often provide a method finally which is meant to be an equivalent of try...finally Thanks to those two features, we can encapsulate that clean-up logic by building "context-managers" (similar to with in python, using in C# or try-with-resources in Java) that always clean up resources.
Lets assume our resources are represented as objects with acquire and dispose methods, both of which return promises. No connections are being made when the function is called, we only return a resource object. This object will be handled by using later on:
function connect(url) {
return {acquire: cb => pg.connect(url), dispose: conn => conn.dispose()}
}
We want the API to work like this:
using(connect(process.env.DATABASE_URL), async (conn) => {
await conn.query(...);
do other things
return some result;
});
We can easily achieve this API:
function using(resource, fn) {
return Promise.resolve()
.then(() => resource.acquire())
.then(item =>
Promise.resolve(item).then(fn).finally(() =>
// bail if disposing fails, for any reason (sync or async)
Promise.resolve()
.then(() => resource.dispose(item))
.catch(terminate)
)
);
}
The resources will always be disposed of after the promise chain returned within using's fn argument completes. Even if an error was thrown within that function (e.g. from JSON.parse) or its inner .then closures (like the second JSON.parse), or if a promise in the chain was rejected (equivalent to callbacks calling with an error). This is why its so important for promises to catch errors and propagate them.
If however disposing the resource really fails, that is indeed a good reason to terminate. Its extremely likely that we've leaked a resource in this case, and its a good idea to start winding down that process. But now our chances of crashing are isolated to a much smaller part of our code - the part that actually deals with leakable resources!
Note: terminate is basically throwing out-of-band so that promises cannot catch it, e.g. process.nextTick(() => { throw e });. What implementation makes sense might depend on your setup - a nextTick based one works similar to how callbacks bail.
How about using callback based libraries? They could potentially be unsafe. Lets look at an example to see where those errors could come from and which ones could cause problems:
function unwrapped(arg1, arg2, done) {
var resource = allocateResource();
mayThrowError1();
resource.doesntThrow(arg1, (err, res) => {
mayThrowError2(arg2);
done(err, res);
});
}
mayThrowError2() is within an inner callback and will still crash the process if it throws, even if unwrapped is called within another promise's .then. These kinds of errors aren't caught by typical promisify wrappers and will continue to cause a process crash as per usual.
However, mayThrowError1() will be caught by the promise if called within .then, and the inner allocated resource might leak.
We can write a paranoid version of promisify that makes sure that any thrown errors are unrecoverable and crash the process:
function paranoidPromisify(fn) {
return function(...args) {
return new Promise((resolve, reject) =>
try {
fn(...args, (err, res) => err != null ? reject(err) : resolve(res));
} catch (e) {
process.nextTick(() => { throw e; });
}
}
}
}
Using the promisified function within another promise's .then callback now results with a process crash if unwrapped throws, falling back to the throw-crash paradigm.
Its the general hope that as you use more and more promise based libraries, they would use the context manager pattern to manage their resources and therefore you would have less need to let the process crash.
None of these solutions are bulletproof - not even crashing on thrown errors. Its very easy to accidentally write code that leaks resources despite not throwing. For example, this node style function will leak resources even though it doesn't throw:
function unwrapped(arg1, arg2, done) {
var resource = allocateResource();
resource.doSomething(arg1, function(err, res) {
if (err) return done(err);
resource.doSomethingElse(res, function(err, res) {
resource.dispose();
done(err, res);
});
});
}
Why? Because when doSomething's callback receives an error, the code forgets to dispose of the resource.
This sort of problem doesn't happen with context-managers. You cannot forget to call dispose: you don't have to, since using does it for you!
References: why I am switching to promises, context managers and transactions

It is almost the most important feature of promises. If it wasn't there, you might as well use callbacks:
var fs = require("fs");
fs.readFile("myfile.json", function(err, contents) {
if( err ) {
console.error("Cannot read file");
}
else {
try {
var result = JSON.parse(contents);
console.log(result);
}
catch(e) {
console.error("Invalid json");
}
}
});
(Before you say that JSON.parse is the only thing that throws in js, did you know that even coercing a variable to a number e.g. +a can throw a TypeError?
However, the above code can be expressed much more clearly with promises because there is just one exception channel instead of 2:
var Promise = require("bluebird");
var readFile = Promise.promisify(require("fs").readFile);
readFile("myfile.json").then(JSON.parse).then(function(result){
console.log(result);
}).catch(SyntaxError, function(e){
console.error("Invalid json");
}).catch(function(e){
console.error("Cannot read file");
});
Note that catch is sugar for .then(null, fn). If you understand how the exception flow works you will see it is kinda of an anti-pattern to generally use .then(fnSuccess, fnFail).
The point is not at all to do .then(success, fail) over , function(fail, success) (I.E. it is not an alternative way to attach your callbacks) but make written code look almost the same as it would look when writing synchronous code:
try {
var result = JSON.parse(readFileSync("myjson.json"));
console.log(result);
}
catch(SyntaxError e) {
console.error("Invalid json");
}
catch(Error e) {
console.error("Cannot read file");
}
(The sync code will actually be uglier in reality because javascript doesn't have typed catches)

Promise rejection is simply a from of failure abstraction. So are node-style callbacks (err, res) and exceptions. Since promises are asynchronous you can't use try-catch to actually catch anything, because errors a likely to happen not in the same tick of event loop.
A quick example:
function test(callback){
throw 'error';
callback(null);
}
try {
test(function () {});
} catch (e) {
console.log('Caught: ' + e);
}
Here we can catch an error, as function is synchronous (though callback-based). Another:
function test(callback){
process.nextTick(function () {
throw 'error';
callback(null);
});
}
try {
test(function () {});
} catch (e) {
console.log('Caught: ' + e);
}
Now we can't catch the error! The only option is to pass it in the callback:
function test(callback){
process.nextTick(function () {
callback('error', null);
});
}
test(function (err, res) {
if (err) return console.log('Caught: ' + err);
});
Now it's working just like in the first example.The same applies to promises: you can't use try-catch, so you use rejections for error-handling.

Related

Can this piece of code theoretically stop a Node JS app from crashing? [duplicate]

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

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

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

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).

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.

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).

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.

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).

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.

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/

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

getCountryRegionData: (countryName, stateName) => {
let countryData, stateData
try {
countryData = countries.find(
country => country.countryName === countryName
)
} catch (error) {
console.log(error.message)
return error.message
}
try {
stateData = countryData.regions.find(state => state.name === stateName)
} catch (error) {
console.log(error.message)
return error.message
}
return {
countryName: countryData.countryName,
countryCode: countryData.countryShortCode,
stateName: stateData.name,
stateCode: stateData.shortCode,
}
},

Is there any way to break out of a function by running another function in JS

I am working on a simple project and I would like to create a simple helper function that checks for a error in a callback. If there is a error then it should break the whole function that called it. Code example:
//Makes call to database and tries to insert element
db.collection("data").insertOne(
{
key: 'some-data'
}, (error, result) => {
//Return error if something goes wrong - else error is empty
checkError(error, "Unable to load database");
console.log("Succes item added")
}
);
Note: Yes this is node.js but this whole principle could be repeated in js with other callbacks - very simple repeatable error principle.
So in the insertOne function the first argument is some data I am adding to the database. The second argument is the callback function that is called after this async operation is finished. It returns a error which I could just handle by adding this if statement to the callback:
if (error) {
console.error(error);
return;
}
Buuut thats disrespecting the dry principle (bc I write the exact same if statement everywhere with no syntax being changed except the message) and is also distracting when reading the callback function. Now my issue is in the function checkError() even tho I can just print the error with the message or throw the error, I dont actually have a way to break the original callback so that it doesnt cause any more havoc in my database. I will go on to promisify this callback which is a solution. BUT I want to know if there is a way to this in the way I presented it here. Note: I dont want to use the try catch block bc thats replacing a if statement with another two blocks.
My checkError function:
const checkError = function (error, msg = "Something went wrong") {
if (error) console.error(`${msg}: error`);
//Break original block somehow ¯\_(ツ)_/¯
};
If I were to compress my question it would be: how to break a function with another function. Is there any way to achieve this?

I don't think this is possible. But you could achieve something similar with this:
function checkError (error, msg = "Something went wrong") {
if (!error) return false;
console.error(`${msg}: error`);
return true;
};
db.collection("data").insertOne(
{
key: 'some-data'
}, (error, result) => {
//Return error if something goes wrong - else error is empty
if (checkError(error, "Unable to load database")) return;
console.log("Succes item added")
}
);

Things become easier when you use promises.
Often asynchronous APIs provide a promise interface, and this is also the case for mongodb/mongoose, where you can chain a .exec() call to execute the database query and get a promise in return. This gives you access to the power of JavaScript's async/await syntax. So you can then do like this:
async function main() {
// Connect to database
// ...
// Other db transactions
// ...
let result = await db.collection("data").insertOne({ key: 'some-data'}).exec();
console.log("Item added successfully");
// Any other database actions can follow here using the same pattern
// ...
}
main().catch(err => {
console.log(err);
});
The idea here is that await will throw an exception if the promise returned by .exec() eventually rejects. You can either put a standard try...catch construct around it to deal with that error, or you can just let it happen. In the latter case the promise returned by the wrapping async function will reject. So you can deal with the error at a higher level (like done above).
This way of working also removes the need for numerous nested callbacks. Often you can keep the nesting to just one of two levels by using promises.

How does Javascript know if there has been an error when executing a function (callbacks)

So I am reading about callbacks because I´m learning backend development on Node.js, and in several webs they say this good practice about writing callbacks with error argument as its first argument:
For example:
fs.readFile('/foo.txt', function(err, data) {
// If an error occurred, handle it (throw, propagate, etc)
if(err) {
console.log('Unknown Error');
return;
}
// Otherwise, log the file contents
console.log(data);
});
Ok sure, I think I understand it clearly what is happening. If once the module fs finishes reading the file "foo.text" there is an error, then the callback function executes console.log("Uknown error") but how Javascript / Node knows that the variable err corresponds to an error in the code??
Because If i name it error instead of err , I imagine it also works right? And what If put it in the second argument? I imagine then it wouldn´t work. Is that it? If its why it is called a good practice if there is no other way to put the error argument but in the first place.

but how Javascript / Node knows that the variable err corresponds to an error in the code??
By convention. The way readFile (and other Node.js callback-style functions) are written, they call their callback with the error as the first argument, or null as the first argument. The name of that parameter in the function signature is irrelevant (you can call it anything you like; err, e, and error are all common). It's the fact it's the first parameter that matters, because it will receive the first argument when called.
In these modern times, though, things are moving away from Node.js callback-style APIs and toward APIs using Promises, which make the error and success paths much more distinct. Then async/await syntax is layered on top of promises to make it possible to write asynchronous code using the standard logical flow control structures.
Node.js callback style (like your code):
const fs = require("fs");
// ...
fs.readFile('/foo.txt', function(err, data) {
// If an error occurred, handle it (throw, propagate, etc)
if (err) {
// It failed
console.log(err);
return;
}
// It worked
console.log(data);
});
With promises via the fs.promises API:
const fsp = require("fs").promises;
// ...
fsp.readFile('/foo.txt')
.then(data => {
// It worked
console.log(data);
})
.catch(err => {
console.log(err);
});
Of course, you may not handle errors at that level; you might instead return the result of calling then so that the caller can chain off it and handle errors (or pass it off to its caller, etc.).
With async/await (this must be inside an async function, although in modules top-level await is coming):
const fsp = require("fs").promises;
// ...inside an `async` function:
try {
const data = await fsp.readFile('/foo.txt');
} catch (err) {
console.log(err);
}
And again, you might not handle errors at that level; you might let them propagate and have the caller handle them (or the caller might let them propgate to its caller to handle them, etc.).
Not all of Node.js's API has promises yet. You can wrap a single callback-style API function with promises via util.promisify; you can wrap an entire API via various npm modules like promisify.

When using promises, should we use throw or reject when dealing with errors relating to incorrect arguments [duplicate]

I'm writing a JavaScript function that makes an HTTP request and returns a promise for the result (but this question applies equally for a callback-based implementation).
If I know immediately that the arguments supplied for the function are invalid, should the function throw synchronously, or should it return a rejected promise (or, if you prefer, invoke callback with an Error instance)?
How important is it that an async function should always behave in an async manner, particularly for error conditions? Is it OK to throw if you know that the program is not in a suitable state for the async operation to proceed?
e.g:
function getUserById(userId, cb) {
if (userId !== parseInt(userId)) {
throw new Error('userId is not valid')
}
// make async call
}
// OR...
function getUserById(userId, cb) {
if (userId !== parseInt(userId)) {
return cb(new Error('userId is not valid'))
}
// make async call
}

Ultimately the decision to synchronously throw or not is up to you, and you will likely find people who argue either side. The important thing is to document the behavior and maintain consistency in the behavior.
My opinion on the matter is that your second option - passing the error into the callback - seems more elegant. Otherwise you end up with code that looks like this:
try {
getUserById(7, function (response) {
if (response.isSuccess) {
//Success case
} else {
//Failure case
}
});
} catch (error) {
//Other failure case
}
The control flow here is slightly confusing.
It seems like it would be better to have a single if / else if / else structure in the callback and forgo the surrounding try / catch.

This is largely a matter of opinion. Whatever you do, do it consistently, and document it clearly.
One objective piece of information I can give you is that this was the subject of much discussion in the design of JavaScript's async functions, which as you may know implicitly return promises for their work. You may also know that the part of an async function prior to the first await or return is synchronous; it only becomes asynchronous at the point it awaits or returns.
TC39 decided in the end that even errors thrown in the synchronous part of an async function should reject its promise rather than raising a synchronous error. For example:
async function someAsyncStuff() {
return 21;
}
async function example() {
console.log("synchronous part of function");
throw new Error("failed");
const x = await someAsyncStuff();
return x * 2;
}
try {
console.log("before call");
example().catch(e => { console.log("asynchronous:", e.message); });
console.log("after call");
} catch (e) {
console.log("synchronous:", e.message);
}
There you can see that even though throw new Error("failed") is in the synchronous part of the function, it rejects the promise rather than raising a synchronous error.
That's true even for things that happen before the first statement in the function body, such as determining the default value for a missing function parameter:
async function someAsyncStuff() {
return 21;
}
async function example(p = blah()) {
console.log("synchronous part of function");
throw new Error("failed");
const x = await Promise.resolve(42);
return x;
}
try {
console.log("before call");
example().catch(e => { console.log("asynchronous:", e.message); });
console.log("after call");
} catch (e) {
console.log("synchronous:", e.message);
}
That fails because it tries to call blah, which doesn't exist, when it runs the code to get the default value for the p parameter I didn't supply in the call. As you can see, even that rejects the promise rather than throwing a synchronous error.
TC39 could have gone the other way, and had the synchronous part raise a synchronous error, like this non-async function does:
async function someAsyncStuff() {
return 21;
}
function example() {
console.log("synchronous part of function");
throw new Error("failed");
return someAsyncStuff().then(x => x * 2);
}
try {
console.log("before call");
example().catch(e => { console.log("asynchronous:", e.message); });
console.log("after call");
} catch (e) {
console.log("synchronous:", e.message);
}
But they decided, after discussion, on consistent promise rejection instead.
So that's one concrete piece of information to consider in your decision about how you should handle this in your own non-async functions that do asynchronous work.

How important is it that an async function should always behave in an async manner, particularly for error conditions?
Very important.
Is it OK to throw if you know that the program is not in a suitable state for the async operation to proceed?
Yes, I personally think it is OK when that is a very different error from any asynchronously produced ones, and needs to be handled separately anyway.
If some userids are known to be invalid because they're not numeric, and some are will be rejected on the server (eg because they're already taken) you should consistently make an (async!) callback for both cases. If the async errors would only arise from network problems etc, you might signal them differently.
You always may throw when an "unexpected" error arises. If you demand valid userids, you might throw on invalid ones. If you want to anticipate invalid ones and expect the caller to handle them, you should use a "unified" error route which would be the callback/rejected promise for an async function.
And to repeat #Timothy: You should always document the behavior and maintain consistency in the behavior.

Callback APIs ideally shouldn't throw but they do throw because it's very hard to avoid since you have to have try catch literally everywhere. Remember that throwing error explicitly by throw is not required for a function to throw. Another thing that adds to this is that the user callback can easily throw too, for example calling JSON.parse without try catch.
So this is what the code would look like that behaves according to these ideals:
readFile("file.json", function(err, val) {
if (err) {
console.error("unable to read file");
}
else {
try {
val = JSON.parse(val);
console.log(val.success);
}
catch(e) {
console.error("invalid json in file");
}
}
});
Having to use 2 different error handling mechanisms is really inconvenient, so if you don't want your program to be a fragile house of cards (by not writing any try catch ever) you should use promises which unify all exception handling under a single mechanism:
readFile("file.json").then(JSON.parse).then(function(val) {
console.log(val.success);
})
.catch(SyntaxError, function(e) {
console.error("invalid json in file");
})
.catch(function(e){
console.error("unable to read file")
})

Ideally you would have a multi-layer architecture like controllers, services, etc. If you do validations in services, throw immediately and have a catch block in your controller to catch the error format it and send an appropriate http error code. This way you can centralize all bad request handling logic. If you handle each case youll end up writing more code. But thats just how I would do it. Depends on your use case

How to wrangle Node.JS async

I am struggling with getting my head around how to overcome and handle the async nature of Node.JS. I have done quite a bit of reading on it and tried to make Node do what I want by either using a message passing solution or callback functions.
My problem is I have a object where I want to constructor to load a file and populate an array. Then I want all calls to this function use that loaded data. So I need the original call to wait for the file to be loaded and all subsequent calls to use the already loaded private member.
My issue is that the function to load load the data and get the data is being executed async even if it return a function with a callback.
Anyways, is there something simple I am missing? Or is there an easier pattern I could use here? This function should return part of the loaded file but returns undefined. I have checked that the file is actually being loaded, and works correctly.
function Song() {
this.verses = undefined;
this.loadVerses = function(verseNum, callback) {
if (this.verses === undefined) {
var fs = require('fs'),
filename = 'README.md';
fs.readFile(filename, 'utf8', function(err, data) {
if (err) {
console.log('error throw opening file: %s, err: %s', filename, err);
throw err;
} else {
this.verses = data;
return callback(verseNum);
}
});
} else {
return callback(verseNum);
}
}
this.getVerse = function(verseNum) {
return this.verses[verseNum + 1];
}
}
Song.prototype = {
verse: function(input) {
return this.loadVerses(input, this.getVerse);
}
}
module.exports = new Song();
Update:
This is how I am using the song module from another module
var song = require('./song');
return song.verse(1);

"My issue is that the function to load the data and get the data is being executed async even if it return a function with a callback."
#AlbertoZaccagni what I mean by that scentence is that this line return this.loadVerses(input, this.getVerse); returns before the file is loaded when I expect it to wait for the callback.
That is how node works, I will try to clarify it with an example.
function readFile(path, callback) {
console.log('about to read...');
fs.readFile(path, 'utf8', function(err, data) {
callback();
});
}
console.log('start');
readFile('/path/to/the/file', function() {
console.log('...read!');
});
console.log('end');
You are reading a file and in the console you will likely have
start
about to read...
end
...read!
You can try that separately to see it in action and tweak it to understand the point. What's important to notice here is that your code will keep on running skipping the execution of the callback, until the file is read.
Just because you declared a callback does not mean that the execution will halt until the callback is called and then resumed.
So this is how I would change that code:
function Song() {
this.verses = undefined;
this.loadVerses = function(verseNum, callback) {
if (this.verses === undefined) {
var fs = require('fs'),
filename = 'README.md';
fs.readFile(filename, 'utf8', function(err, data) {
if (err) {
console.log('error throw opening file: %s, err: %s', filename, err);
throw err;
} else {
this.verses = data;
return callback(verseNum);
}
});
} else {
return callback(verseNum);
}
}
}
Song.prototype = {
verse: function(input, callback) {
// I've removed returns here
// I think they were confusing you, feel free to add them back in
// but they are not actually returning your value, which is instead an
// argument of the callback function
this.loadVerses(input, function(verseNum) {
callback(this.verses[verseNum + 1]);
});
}
}
module.exports = new Song();
To use it:
var song = require('./song');
song.verse(1, function(verse) {
console.log(verse);
});
I've ignored
the fact that we're not treating the error as first argument of the callback
the fact that calling this fast enough will create racing conditions, but I believe this is another question

[Collected into an answer and expanded from my previous comments]
TL;DR You need to structure your code such that the result of any operation is only used inside that operation's callback, since you do not have access to it anywhere else.
And while assigning it to an external global variable will certainly work as expected, do so will only occur after the callback has fired, which happens at a time you cannot predict.
Commentary
Callbacks do not return values because by their very nature, they are executed sometime in the future.
Once you pass a callback function into a controlling asynchronous function, it will be executed when the surrounding function decides to call it. You do not control this, and so waiting for a returned result won't work.
Your example code, song.verse(1); cannot be expected to return anything useful because it is called immediately and since the callback hasn't yet fired, will simply return the only value it can: null.
I'm afraid this reliance on asynchronous functions with passed callbacks is an irremovable feature of how NodeJS operates; it is at the very core of it.
Don't be disheartened though. A quick survey of all the NodeJS questions here shows quite clearly that this idea that one must work with the results of async operations only in their callbacks is the single greatest impediment to anyone understanding how to program in NodeJS.
For a truly excellent explanation/tutorial on the various ways to correctly structure NodeJS code, see Managing Node.js Callback Hell with Promises, Generators and Other Approaches.
I believe it clearly and succinctly describes the problem you face and provides several ways to refactor your code correctly.
Two of the features mentioned there, Promises and Generators, are programming features/concepts, the understanding of which would I believe be of great use to you.
Promises (or as some call them, Futures) is/are a programming abstraction that allows one to write code a little more linearly in a if this then that style, like
fs.readFileAsync(path).then(function(data){
/* do something with data here */
return result;
}).catch(function(err){
/* deal with errors from readFileAsync here */
}).then(function(result_of_last_operation){
/* do something with result_of_last_operation here */
if(there_is_a_problem) throw new Error('there is a problem');
return final_result;
})
.catch(function(err){
/* deal with errors when there_is_a_problem here */
}).done(function(final_result){
/* do something with the final result */
});
In reality, Promises are simply a means of marshaling the standard callback pyramid in a more linear fashion. (Personally I believe they need a new name, since the idea of "a promise of some value that might appear in the future" is not an easy one to wrap one's head around, at least it wasn't for me.)
Promises do this by (behind the scenes) restructuring "callback hell" such that:
asyncFunc(args,function callback(err,result){
if(err) throw err;
/* do something with the result here*/
});
becomes something more akin to:
var p=function(){
return new Promise(function(resolve,reject){
asyncFunc(args,function callback(err,result){
if(err) reject(err)
resolve(result);
});
});
});
p();
where any value you provide to resolve() becomes the only argument to the next "then-able" callback and any error is passed via rejected(), so it can be caught by any .catch(function(err){ ... }) handlers you define.
Promises also do all the things you'd expect from the (somewhat standard) async module, like running callbacks in series or in parallel and operating over the elements of an array, returning their collected results to a callback once all the results have been gathered.
But you will note that Promises don't quite do what you want, because everything is still in callbacks.
(See bluebird for what I believe is the simplest and thus, best Promises package to learn first.)
(And note that fs.readFileAsync is not a typo. One useful feature of bluebird is that it can be made to add this and other Promises-based versions of fs's existing functions to the standard fs object. It also understands how to "promisify" other modules such as request and mkdirp).
Generators are the other feature described in the tutorial above, but are only available in the new, updated but not yet officially released version of JavaScript (codenamed "Harmony").
Using generators would also allow you to write code in a more linear manner, since one of the features it provides is the ability of waiting on the results of an asynchronous operation in a way that doesn't wreak havoc with the JavaScript event-loop. (But as I said, it's not a feature in general use yet.)
You can however use generators in the current release of node if you'd like, simply add "--harmony" to the node command line to tell it to turn on the newest features of the next version of JavaScript.

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JavaScript is the programming language of the Web.

exception handling, thrown errors, within promises - javascript

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