Node programmers conventionally use a paradigm like this:
let callback = function(err, data) {
if (err) { /* do something if there was an error */ }
/* other logic here */
};
Why not simplify the function to accept only a single parameter, which is either an error, or the response?
let callback = function(data) {
if (isError(data)) { /* do something if there was an error */ }
/* other logic here */
};
Seems simpler. The only downside I can see is that functions can't return errors as their actual intended return value - but I believe that is an incredibly insignificant use-case.
Why is the error-first pattern considered standard?
EDIT: Implementation of isError:
let isError = obj => obj instanceof Error;
ANOTHER EDIT: Is it possible that my alternate method is somewhat more convenient than node convention, because callbacks which only accept one parameter are more likely to be reusable for non-callback use-cases as well?
(See "Update" below for an npm module to use the callback convention from the question.)
This is just a convention. Node could use the convention that you suggest as well - with the exception that you wouldn't be able to return an error object as an intended value on success as you noticed, which may or may not be a problem, depending on your particular requirements.
The thing with the current Node convention is that sometimes the callbacks may not expect any data and the err is the only parameter that they take, and sometimes the functions expect more than one value on success - for example see
request(url, (err, res, data) => {
if (err) {
// you have error
} else {
// you have both res and data
}
});
See this answer for a full example of the above code.
But you might as well make the first parameter to be an error even in functions that take more than one parameter, I don't see any issue with your style even then.
The error-first Node-style callbacks is what was originally used by Ryan Dahl and it is now pretty universal and expected for any asynchronous functions that take callbacks. Not that this convention is better than what you suggest or worse, but having a convention - whatever it is - make the composition of callbacks and callback taking functions possible, and modules like async rely on that.
In fact, I see one way in which your idea is superior to the classical Node convention - it's impossible to call the callback with both error and the first non-error argument defined, which is possible for Node style callbacks and sometimes can happen. Both conventions could potentially have the callback called twice though - which is a problem.
But there is another widely used convention in JavaScript in general and Node in particular, where it's impossible to define both error and data and additionally it's impossible to call the callback twice - instead of taking a callback you return a promise and instead of explicitly checking the error value in if as is the case in Node-style callbacks or your style callbacks, you can separately add success and failure callbacks that only get relevant data.
All of those styles are pretty much equivalent in what they can do:
nodeStyle(params, function (err, data) {
if (err) {
// error
} else {
// success
}
};
yourStyle(params, function (data) {
if (isError(data)) {
// error
} else {
// success
}
};
promiseStyle(params)
.then(function (data) {
// success
})
.catch(function (err) {
// error
});
Promises may be more convenient for your needs and those are already widely supported with a lot of tools to use them, like Bluebird and others.
You can see some other answers where I explain the difference between callbacks and promises and how to use the together in more detail, which may be helpful to you in this case:
A detailed explanation on how to use callbacks and promises
Explanation on how to use promises in complex request handlers
An explanation of what a promise really is, on the example of AJAX requests
Examples of mixing callbacks with promises
Of course I see no reason why you couldn't write a module that converts Node-style callbacks into your style callbacks or vice versa, and the same with promises, much like promisify and asCallback work in Bluebird. It certainly seems doable if working with your callback style is more convenient for you.
Update
I just published a module on npm that you can use to have your preferred style of callbacks:
https://www.npmjs.com/package/errc
You can install it and use in your project with:
npm install errc --save
It allows you to have a code like this:
var errc = require('errc');
var fs = require('fs');
var isError = function(obj) {
try { return obj instanceof Error; } catch(e) {}
return false;
};
var callback = function(data) {
if (isError(data)) {
console.log('Error:', data.message);
} else {
console.log('Success:', data);
}
};
fs.readFile('example.txt', errc(callback));
For more examples see:
https://github.com/rsp/node-errc-example
I wrote this module as an example of how to manipulate functions and callbacks to suit your needs, but I released it under the MIT license and published on npm so you can use it in real projects if you want.
This demonstrates the flexibility of Node, its callback model and the possibility to write higher-order functions to create your own APIs that suit your needs. I publish it in hope that it may be useful as an example to understand the Node callback style.
Because without this convention, developers would have to maintain different signatures and APIs, without knowing where to place the error in the arguments array.
In most cases, there can be many arguments, but only one error - and you know where to find it.
Joyent even wrote about this at the time they were more involved:
Callbacks are the most basic way of delivering an event
asynchronously. The user passes you a function (the callback), and you
invoke it sometime later when the asynchronous operation completes.
The usual pattern is that the callback is invoked as callback(err,
result), where only one of err and result is non-null, depending on
whether the operation succeeded or failed.
Yeah we can develop code style as you said. But there would be some problems.If we maintain code style what we want , different signatures of API increases and of course there would be dilemma between developers. They create their layers( error and success stages for example) again. Common conventions play an important role in spreading best practices among developers.
Ingenral, the error-first Node-style callbacks is what was originally used by Ryan Dahl and it is now pretty universal and expected for any asynchronous functions that take callbacks. Not that this convention is better than what you suggest or worse, but having a convention - whatever it is - make the composition of callbacks and callback taking functions possible, and modules like async rely on that.
Related
I would like to store a variable that is shared between all stack frames (top down) in a call chain. Much like ThreadLocal in Java or C#.
I have found https://github.com/othiym23/node-continuation-local-storage but it keeps loosing context for all my use cases and it seems that you have to patch the libraries you are using to make it local-storage-aware which is more or less impossible for our code base.
Are there really not any other options available in Node? Could domains, stacktraces or something like that be used to get a handle (id) to the current call chain. If this is possible I can write my own thread-local implementation.
Yes, it is possible. Thomas Watson has spoken about it at NodeConf Oslo 2016 in his Instrumenting Node.js in Production (alt.link).
It uses Node.js tracing - AsyncWrap (which should eventually become a well-established part of the public Node API). You can see an example in the open-source Opbeat Node agent or, perhaps even better, check out the talk slides and example code.
Now that more than a year has passed since I originally asked this question, it finally looks like we have a working solution in the form of Async Hooks in Node.js 8.
https://nodejs.org/api/async_hooks.html
The API is still experimental, but even then it looks like there is already a fork of Continuation-Local-Storage that uses this new API internally.
https://www.npmjs.com/package/cls-hooked
TLS is used in some places where ordinary, single-threaded programs would use global variables but where this would be inappropriate in multithreaded cases.
Since javascript does not have exposed threads, global variable is the simplest answer to your question, but using one is a bad practice.
You should instead use a closure: just wrap all your asynchronous calls into a function and define your variable there.
Functions and callbacks created within closure
(function() (
var visibleToAll=0;
functionWithCallback( params, function(err,result) {
visibleToAll++;
// ...
anotherFunctionWithCallback( params, function(err,result) {
visibleToAll++
// ...
});
});
functionReturningPromise(params).then(function(result) {
visibleToAll++;
// ...
}).then(function(result) {
visibleToAll++;
// ...
});
))();
Functions created outside of closure
Should you require your variable to be visible inside functions not defined within request scope, you can create a context object instead and pass it to functions:
(function c() (
var ctx = { visibleToAll: 0 };
functionWithCallback( params, ctx, function(err,result) {
ctx.visibleToAll++;
// ...
anotherFunctionWithCallback( params, ctx, function(err,result) {
ctx.visibleToAll++
// ...
});
});
functionReturningPromise(params,ctx).then(function(result) {
ctx.visibleToAll++;
// ...
}).then(function(result) {
ctx.visibleToAll++;
// ...
});
))();
Using approach above all of your functions called inside c() get reference to same ctx object, but different calls to c() have their own contexts. In typical use case, c() would be your request handler.
Binding context to this
You could bind your context object to this in called functions by invoking them via Function.prototype.call:
functionWithCallback.call(ctx, ...)
...creating new function instance with Function.prototype.bind:
var boundFunctionWithCallback = functionWithCallback.bind(ctx)
...or using promise utility function like bluebird's .bind
Promise.bind(ctx, functionReturningPromise(data) ).then( ... )
Any of these would make ctx available inside your function as this:
this.visibleToAll ++;
...however it has no real advantage over passing context around - your function still has to be aware of context passed via this, and you could accidentally pollute global object should you ever call function without context.
I am trying to create a Node module (using harmony) that upon loading by another module/application, has to be yielded to so that things in it's construct can be executed and loaded before any of it's exposed functions can be called.
The issue I am having is that I cannot seem to yield to the internal function that is being executed, using module.exports. An example would help.
module.exports = function*(s_id){
console.log('loading the module lets it execute up till here');
if (!(this instanceof Tester)) return yield new Tester();
}
function* Tester(){
console.log('but we never execute this generator function');
}
Tester.prototype = {
model : function*(){
// other functions
}
}
It's been stumping me for hours now! I feel like the solution is super simple but I cannot seem to wrap my head around it. I have tried to simply make the Tester() function the export, but am still having the same issue. Why can't I seem to yield to the Tester() function?
Also, what may an alternative be to this approach? I want to maintain the Object nature of the module so that the module can be loaded with different inputs, such as the s_id variable/object in the example above.
a Node module (using harmony) that upon loading by another module/application, has to be yielded to so that things in it's construct can be executed and loaded before any of it's exposed functions can be called
Don't do that. Generators are not made for asynchrony. yield doesn't do what you want here. A module is not "yielded" to await something in it to load. yield is magic, but not async magic.
If you must use an asynchronous module loading process, export a promise for your actual module. That is a standard interface for something to await, and can be consumed using a standardized approach that does not rely on internals of your module.
You still can use yield syntax for constructing that promise, just use your favorite coroutine library.
return yield new Tester();
…
function* Tester(){…}
Uh oh. Well yes, apparently it is possible to call generator functions as constructors. But believe me, it is not what you want. A constructor for an arbitrary object should return that object, instead of an iterator (just like it shouldn't return a promise). If some of your object methods are generator methods, that's fine, but your constructor should be not.
If you really want to use a generator function in your code like that (and it's not meant to be a constructor), you
will need to yield* the iterator you've created (tester()) instead of yielding it
must not overwrite its .prototype property like you did, as that causes the iterator to malfunction. (Of course you should never do that at all, even though most of the time it works)
If you want to publish a module that has sequenced IO, is it ever OK to write,
./sequenced_actions.js
module.exports = function * () {}
Thereby permitting something like,
co( function * {
yield require('./sequenced_actions');
} )();
if you want your modules to reach the largest audience possible, just write them in promises. hopefully node v0.12 will have native promises, so this will make things easier
Yes, it's okay to do that.
Generator function is just an ordinary function under the hood. And since node.js allows an arbitrary value to be exports object of a module, you can export whatever you want there.
I wrote a library with a number of async functions.
A SYNCHRONOUS helper function throws an error if one of the parameters is plain wrong:
proto.makeParameters= function( filters ){
default:
throw( new Error("Field type unknown: " + fieldObject.type ) );
break;
}
In my async functions, when I use it, I have:
proto.someAsyncFunction = function( cb ){
// Run the query
try {
var parameters = this.makeParameters( filters );
} catch( e ){
return cb( e );
}
}
So:
Is it good practice that asyncfunctions should never ever throw? (Like I did)
Right now, I am catching ALL errors. Shall I e more choosy? Maybe make up an error type and just check for that? If so, what should I do in either case?
Your assumptions on the async code are correct. See this post by Isaac Schlueter himself on the topic:
The pattern in node is that sync methods throw, and async methods pass
the error as the first argument to the callback. If the first
argument to your callback is falsey (usually null or undefined), then
all is well with the world.
http://groups.google.com/forum/#!msg/nodejs/W9UVJCKcJ7Q/rzseRbourCUJ
Is it good practice that async functions should never ever throw? (Like I did)
async functions, of course, will throw exceptions whenever we like it not, simply because of the software imperfection. So throwing custom exception is completely fine but what is important is how to correctly catch them.
The problem is that differently from sync code the stack of the async exception may not be available. So when the exception occurs, it is not always possible to say where to return the control and where is the handler. node.js has two methods of specifying what to do when the exception in asynchronous code occurs: process uncaughtException and domains.
As you see, dealing of the exceptions in async code is tricky so throwing an exception should be considered as a last option. If the function just returns status of the operation it is not an exception.
It seems for me that in the provided code fragment the exception is thrown correctly because it indicates that the method was called improperly and cannot do its job. In other words, the error is permanent. This indicates serious flaw in the application that should be fixed. But if the function cannot create a parameter for some temporary reason that can be fixed without modifying the application, then returning the status is more appropriate choice.
What is the best practice for injecting dependencies into models? And especially, what if their getter are asynchronous, as with mongodb.getCollection()?
The point is to inject dependencies once with
var model = require('./model')({dep1: foo, dep2: bar});
and call all member methods without having to pass them as arguments. Neither do I want to have each method to begin with a waterfall of async getters.
I ended up with a dedicated exports wrapper that proxies all calls and passes the async dependencies.
However, this creates a lot of overhead, it's repetitive a lot and I generally do not like it.
var Entity = require('./entity');
function findById(id, callback, collection) {
// ...
// callback(null, Entity(...));
};
module.exports = function(di) {
function getCollection(callback) {
di.database.collection('users', callback);
};
return {
findById: function(id, callback) {
getCollection(function(err, collection) {
findById(id, callback, collection);
});
},
// ... more methods, all expecting `collection`
};
};
What is the best practice for injecting dependencies, especially those with async getters?
If your need is to support unit testing, dependency injection in a dynamic language like javascript is probably more trouble than it's worth. Note that just about none of the modules you require from others are likely to use the patterns for DI you see in Java, .NET, and with other statically compiled languages.
If you want to mock out behavior in order to isolate specific units of code for testing, see the 'sinon' module http://sinonjs.org/. It allows you to dynamically swap in/out interceptors that can either spy on method calls or replace them altogether. In practice, you would write a mocha test where you require your module, then require a module that's leveraged in your code. Use sinon to spy or stub a method on that module and as a result, you can isolate your code.
There is one scenario where I've not been able to completely isolate 3rd party code with sinon, and this is when the act of require()ing a module executes some behavior that you don't want to run in your test. For that scenario, I made a super simple module called 'mockrequire' https://github.com/mateodelnorte/mockrequire that allows you to provide an inline mock to be required instead of the actual module. You can provide a mock that uses spy or stub from sinon and have the same syntax and patterns as all the rest of your tests.
Hopefully this answers the underlying question from your post. ;)
In very simple situations, you could simply export a function that modifies objects in your file scope and returns your actual exports object, but if you want to inject more variably (i.e. for more than one use from your app) it's generally better to create a wrapper object like you have done.
You can reduce some overhead and indentation in some situations by using a wrapper class instead of a function returning an object.
For instance
function findById(id, callback, collection) {
// ...
// callback(null, Entity(...));
};
function Wrapper(di) {
this.di = di;
}
module.exports = Wrapper; // or do 'new' usage in a function if preferred
Wrapper.prototype.findById = function (id, callback) {
// use this.di to call findById and getCollection
}, // etc
Other than that, it's not a whole lot you can do to improve things. I like this approach though. Keeps the state di explicit and separate from the function body of findById and by using a class you reduce the nesting of indentation a little bit at least.