I am making two axios calls like this
axios
.all([call1(), call2()])
.then(axios.spread(function(Resp1, Resp2) {}).catch(onError)
If one of the calls fails with a 500 server error, flow goes to catch block. I want it to go to then block even if one call fails and only go to catch if both fail. How can I do this?
It is not possible with .all of axios. But you could do something else. Create a method which gets a promise as param, and executes it, and in case of failure, just return null or whatever. This way you will be able to control the error case.
For that I recommend you read this article:
https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html
More precicly subtitle Rookie mistake #5: using side effects instead of returning
Related
I am a bit new to JavaScript web dev, and so am still getting my head around the flow of asynchronous functions, which can be a bit unexpected to the uninitiated. In my particular use case, I want execute a routine on the list of available databases before moving into the main code. Specifically, in order to ensure that a test environment is always properly initialized, I am dropping a database if it already exists, and then building it from configuration files.
The basic flow I have looks like this:
let dbAdmin = client.db("admin").admin();
dbAdmin.listDatabases(function(err, dbs){/*Loop through DBs and drop relevant one if present.*/});
return await buildRelevantDB();
By peppering some console.log() items throughout, I have determined that the listDatabases() call basically puts the callback into a queue of sorts. I actually enter buildRelevantDB() before entering the callback passed to listDatabases. In this particular example, it seems to work anyway, I think because the call that reads the configuration file is also asynchronous and so puts items into the same queue but later, but I find this to be brittle and sloppy. There must be some way to ensure that the listDatabases portion resolves before moving forward.
The closest solution I found is here, but I still don't know how to get the callback I pass to listDatabases to be like a then as in that solution.
Mixing callbacks and promises is a bit more advanced technique, so if you are new to javascript try to avoid it. In fact, try to avoid it even if you already learned everything and became a js ninja.
Dcumentation for listDatabases says it is async, so you can just await it without messing up with callbacks:
const dbs = await dbAdmin.listDatabases();
/*Loop through DBs and drop relevant one if present.*/
The next thing, there is no need to await before return. If you can await within a function, it is async and returns a promise anyway, so just return the promise from buildRelevantDB:
return buildRelevantDB();
Finally, you can drop database directly. No need to iterate over all databases to pick one you want to drop:
await client.db(<db name to drop>).dropDatabase();
I am still quite new to Node.js and can't seem to find anything to help me around this.
I am having an issue of getting the query from my last record and adding it to my variable.
If I do it like below: -
let lastRecord = Application.find().sort({$natural:-1}).limit(1).then((result) => { result });
Then I get the value of the variable showing in console.log as : -
Promise { <pending> }
What would I need to do to output this correctly to my full data?
Here is it fixed:
Application.findOne().sort({$natural:-1}).exec().then((lastRecord) => {
console.log(lastRecord); // "lastRecord" is the result. You must use it here.
}, (err) => {
console.log(err); // This only runs if there was an error. "err" contains the data about the error.
});
Several things:
You are only getting one record, not many records, so you just use findOne instead of find. As a result you also don't need limit(1) anymore.
You need to call .exec() to actually run the query.
The result is returned to you inside the callback function, it must be used here.
exec() returns a Promise. A promise in JavaScript is basically just a container that holds a task that will be completed at some point in the future. It has the method then, which allows you to bind functions for it to call when it is complete.
Any time you go out to another server to get some data using JavaScript, the code does not stop and wait for the data. It actually continues executing onward without waiting. This is called "asynchronisity". Then it comes back to run the functions given by then when the data comes back.
Asynchronous is simply a word used to describe a function that will BEGIN executing when you call it, but the code will continue running onward without waiting for it to complete. This is why we need to provide some kind of function for it to come back and execute later when the data is back. This is called a "callback function".
This is a lot to explain from here, but please go do some research on JavaScript Promises and asynchronisity and this will make a lot more sense.
Edit:
If this is inside a function you can do this:
async function someFunc() {
let lastRecord = await Application.findOne().sort({$natural:-1}).exec();
}
Note the word async before the function. This must me there in order for await to work. However this method is a bit tricky to understand if you don't understand promises already. I'd recommend you start with my first suggestion and work your way up to the async/await syntax once you fully understand promises.
Instead of using .then(), you'll want to await the record. For example:
let lastRecord = await Application.find().sort({$natural:-1}).limit(1);
You can learn more about awaiting promises in the MDN entry for await, but the basics are that to use a response from a promise, you either use await or you put your logic into the .then statement.
I am using promise in bluebird to send mysql queries and manage the control flow & errors. Here is what it looks like:
sendQuery1(..)
.then(sendQuery2(..))
.then(function(results from last query){
if(rain){
res.render(...)
}else{
/*
I need to send additional 2 queries here
*/
}
}).catch(errors);
promise chain is very convenient, but I found out the error handling will be a messy if there are multiple subchains inside.
For here I probably need to write the following inside the /* */
return sendQuery3(..)
.then(sendQuery4(..))
.then(function(..){
res.render(".....")
}).catch(error2);
Are there any better ways to handle this type of problems?
I don't exactly see your problem here. When you have a chain inside a chain (which you normally don't, you should check your architecture right there), you can just catch the errors like shown.
I'd advise you use a global (or local) error handler function, and pass that to the catch-function. Therefore, even when you have multiple catches, you can use the same error-handler.
The in my opinion best solution would be to create a "promise chain bypass", therefore using catch to skip certain parts based on your condition. If this is not what you are looking for, please specify your problem.
You will catch the error, there's no problem in your code. Yup it gets messy but you can go with the following approach to keep the code clean and still implement your logic:
sendQuery1(..)
.then(sendQuery2(..))
.then(function(results from last query){
if(rain){
res.render(...)
throw new Error('breakChain'); //intentionally throwing error to skip the remaining chain
}
return; //will act like 'else'
})
.then(sendQuery3(..))
.then(sendQuery4(..))
.catch(function (e) {
if(e.message != 'breakChain') //act on error if it was other than 'breakChain'
throw e;
});
What you are discussing here is branching of the chain based on some logic condition. It is generally better to actually branch the chain rather than throw an error just to abort the rest of the chain. This keeps errors as errors rather than the scheme that Shaharyar proposed of synthesizing an error that isn't really an error.
You can branch the chain by returning a new promise chain from within a .then() handler like this:
sendQuery1(..).then(function(r1) {
return sendQuery2(...);
}).then(function(r2){
if (rain){
// processing is done, so just render
res.render(...)
} else {
// return promise here to attach this new branch to the original chain
return sendQuery3(..).then(sendQuery4).then(function() {
// process last query
});
}
}).catch(errors);
FYI, since you've only posted pseudo-code, not real code and we can't see which functions need access to which prior results in order to do their work, we can't fully optimize the code. This is to show an example of how branching works inside a promise chain. That's the principle you probably want to use. If you show your real code, we can offer a much more specific and optimized answer.
I am trying to understand how to build my error handling system for my api.
Let's say I have a the following line in a controller method :
var age = json.info.age;
with
json = {"id":1, "name":"John", info": {"age":27, "sex":"m"}}
Let's say that the object doesn't contain an info field, I'll get the following error TypeError: Cannot read property 'info' of undefined and my server will crash.
Is there a way to make a higher level abstraction and catch all the potential errors that I could have? Or should I have a try/catch system for each of the methods of my controllers?
BEWARE OF THE CODE BELOW, IT WILL BITE YOU WHENEVER IT CAN!
Don't use the code snippet below if you do not understand its
implications, please read the whole answer.
You can use the node way for uncaught errors. Add this in your config/bootstrap.js
Updated the snippet below to add what was said in the comments, also added a warning about using a global to respond to the user.
process.on('uncaughtException', function (err) {
// Handle your errors here
// global.__current__ is added via middleware
// Be aware that this is a bad practice,
// global.__current__ being a global, can change
// without advice, so you might end responding with
// serverError() to a different request than the one
// that originated the error if this one happened async
global.__current__.res.serverError();
})
Now, can doesn't mean should. It really depends on your needs, but do not try to catch BUGS in your code, try to catch at a controller level the issues that might not happen every time but are somehow expected, like a third-party service that responded with empty data, you should handle that in your controller. The uncaughtException is mainly for logging purposes, its better to let your app crash if there is a bug. Or you can do something more complicated (that might be better IMHO), which is to stop receiving requests, respond to the error 500 (or a custom one) to user that requested the faulty endpoint, and try to complete the other requests that do not relate to that controller, then log and shutdown the server. You will need several instances of sails running to avoid zero downtime, but that is material for another question. What you asked is how to get uncaught exceptions at a higher lvl than the controllers.
I suggest you read the node guide for error handling
Also read about domains, even thought they are deprecated you can use them, but you would have to deal with them per controller action, since sails does not provide any help with that.
I hope it helps.
You can check this way if you want to:
if (object != null && object.response != null && object.response.docs != null){
//Do your stuff here with your document
}
I don't really get what is your "object" variable in the first place, so i don't know if you can check it at a different level, is it a sails parameter to your controller ?
So that's how I did it, thanks to Zagen's answer.
module.exports.bootstrap = function(cb) {
process.on('uncaughtException', function (err) {
//Handle your errors here
logger.fatal(err);
global.__current__.res.serverError();
})
cb();
};
I send a generic error 500 to the user if any uncaught exception is thrown, and I log the error to the fatal level. On that way, my server is still accessible 24/7 and I can monitor the logs at another level and trigger an alarm on a fatal error. I can then fix the exception that was thrown.
I'm having trouble understanding how to handle something that it seems like would be a pretty basic aspect of express. If I have some code that throws an exception in an async callback, there is no way I can catch that exception because the try/catch block is no longer in scope by the time the callback is running. In these scenarios the browser will hang until it eventually give up stating that the server is unresponsive. This is a very bad user experience. I would much rather be able to immediately return a 500 error to the client. The default express error handler apparently does not handle this situation. Here is some sample code:
var express = require("express");
var app = express();
app.use(app.router);
//express error handler (never called)
app.use(function(err, req, res, next) {
console.log(err);
res.send(500);
});
app.get("/test", function(req, res, next) {
require("fs").readFile("/some/file", function(err, data) {
a.b(); //blow up
});
});
app.listen(8888);
In the above code, the line a.b() throws a "ReferenceError: a is not defined" exception. The defined error handler is never called. Notice that the err object returned by fs.readFile() is null in this case because the file was correctly read. The bug is the code inside the async handler.
I have read this post about using node's uncaughtExpception even, but the documentation says not use that method. Even if I did use it, how would I then send the 500 response back to the user? The express response object is no longer around for me to use.
So how do you handle this scenario?
OK, I'm just going to post an entirely different answer since my first answer has some valuable information but is off topic in hindsight.
The short answer: the correct thing to do is what already happens: your program should print a stack trace and exit with an error.
The underlying thinking:
So I think you need to think about errors in different categories. My first answer deals with data-related errors that a well-written program can and should handle cleanly. What you are describing is a CRASH. If you read the node.js documentation you linked to, it is correct. The only useful thing your program can do at this point is exit with a stack trace and allow a process supervisor to restart it and attain an understood state. Once your program has crashed, it is essentially unrecoverable due to the extremely wide range of errors that could be the root cause of an exception getting to the top of the stack. In your specific example, this error is just going to continue happening every time until the source code bug is fixed and the application is redeployed. If you are worried that untested and buggy code is going to get into your application, adding more untested and buggy error handling code isn't really addressing the right problem.
But in short, no, there's no way to get a reference to the HTTP request object that caused this exception so AFAIK you cannot change the way this is perceived by the end user in the browser, outside of handling this at an intermediate reverse proxy layer where you could configure a crude timeout and send a more friendly error page (which of course would be useless for any request that isn't for a full HTML document).
The Bible of Error Handling in Node
Error Handling in Node.js by Dave Pacheco is the definitive work on this topic in my opinion. It is comprehensive, extensive, and thorough. I recommend reading and re-reading periodically.
To address #asparagino's comments, if an unhandled exception is easily reproducible or happening with high frequency, it's not an edge case, it's a bug. The correct thing to do is improve your code to not generate uncaught exceptions in face of this situation. Actually handle the condition, thus converting a programmer error into an operational error, and your program can continue without a restart and without an uncaught exception.
You should use express's error handling middleware via app.use(error, req, res, next). Express maintains a separate middleware stack that it uses when the normal middleware stack throws an uncaught exception. (Side note that express just looks at the callback's arity (number of expected arguments) to categorize it as regular middleware or error handling middleware, which is a bit magical, so just keep in mind you must declare the arguments as above for express to understand this is an error handling middleware).
And based on your question and comments, just understand that exceptions aren't all that useful in node.js because every async call gets a new stack, which is why callbacks are used everywhere and the 1st argument is an error universally. Your try/catch block in the example is only going to catch an exception thrown directly by findById (like if id were undefined, as it is in your snippet), but once an actual call to the database is made, that's it, the call stack is over and no further exceptions can happen until a totally different call stack starts when node invokes the async IO callback.
Thanks for the answer, but this only works if I put the try/catch inside the async callback and have the catch do next(exp). I'd like to avoid having separate try/catch blocks in every single async callback.
Nope, that's not true. You don't have to manually call next(exp). Express will catch the error and trigger the error handling middleware for you (that's how express does it's developer-friendly exception report pages in dev mode anyway). And async libraries don't throw exceptions even under "normal" error conditions. They pass an error to the callback, so in general you don't have to bother with try/catch that much in node. Just never ignore an error parameter passed to a callback function, and you're fine.
You don't see this boilerplate in node:
someDb.query(someCriteria, function (error, result) {
try {
//some code to deal with result
} catch (exception) {
callback(exception);
}
});
You do see this though:
someDb.query(someCriteria, function (error, result) {
if (error) {
callback(error);
return;
}
//some code to deal with result
});
Node handles IO differently, which means the call stack works differently, which means exceptions work differently, which means error handling works differently. You can write a stable node/express app that handles errors without crashing without writing a single try/catch. (express has one that handles uncaught errors that bubble all the way to the top). It's not a functional limitation, it's just a consquence of async IO that means you have to write your code differently and handle errors with callbacks instead of exceptions. Thinking of it as a "limitation" as opposed to the "way it is" is putting a negative connotation on something that is really just a technical reality. There are clean and robust patterns for exception handling in both a synchronous and asynchronous paradigm.