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NodeJS : how to use arguments like req, res, result in functions?

I'm fairly new to JS especially Node and Express. I am following some tutorials on how to build an API and at the same time learning about JS special features such as let/const/var, arrow functions etc.
In many tutorials I have seen things likes this :
somecode.then((result) => {someothercode})
With: "somecode" being for example a get request
Is "result" the name of the returned value or is it a convention that JS developper use?
By that I mean, does this for example work?
somecode.then((foo) => {someothercode})
Also for req, res variables what does this mean?
app.get("/users/:userId", [
usersController.getById
]);
Here is the getById function (using once again the "result"):
exports.getById = (req, res) => {
userModel.findById(req.params.userId).then((result) => {
res.status(200).send(result);
});
};
the getById method defined in the controller needs (req, res), does that mean, when i call it like the code above, the req and res arguments are implicitly used?
Also it needs a parameter :
req.params.userId
which is in the url of the route, how does it pass to another file?
I have a route.js file that uses a controller.js file that uses a model.js. How does the param go from route to controller?
And it won't work if I change the param name right? for example:
req.params.id
Sorry for long post, I'm trying to understand JS logic to get some good habits and write clean code.
Thanks!

Is "result" the name of the returned value or is it a convention that JS developper use? By that I mean, does this for example work?
From my experience, yes - result is often used. Often times you'll see thing like value, response, but ultimately it can be whatever you define. I would recommend sticking to convention, and also check out the MDN Promise tutorial if you are starting out with understanding NodeJS asynchronous operations.
Also for req, res variables what does this mean?
app.get("/users/:userId", [
usersController.getById
]);
That is a middleware chain. Check out the Express docs for more information.
the getById method defined in the controller needs (req, res), does that mean, when i call it like the code above, the req and res arguments are implicitly used? Also it needs a parameter :
req.params.userId
which is in the url It won't work if I change the param name right? for example:
req.params.id
Yes, that is using a named parameter. Without the full router code, it is hard to know how the getById method is linked to the defined route. The Express routing documentation will likely be a good start on that.

Is "result" the name of the returned value or is it a convention that JS developper use?
result is the name of a new variable you are creating to represent the value passed in from the Promise resolution. Yes, your foo example will work.
(req, res) => {} is the same (mostly) as a function that looks like this:
function getById(req, res) {...}
req, and res are just representational of the values that will be passed to this function. They could just as easily have been called (foo, bar).

It looks like you're struggling with understanding callback functions. Consider the following code then please crack open the source code for the packages that you are using. and it looks like you are using express.js
function something(callback) {
var x = 5;
var y = 'anything';
callback(x, y);
}
something(function(req, res) {
console.log(req);
console.log(res);
});
the something function is created and inside of that function scope, var x and y are created with any type. then when we invoke or use something function we are passing a function as a variable that gets passed in as variable callback then it can be used since it is a function so we call callback with x and y which can literally be any value and for effect, I am passing back a number and a string as req and res.

It's just a convention. Note that the code:
somecode.then((result) => {someothercode});
Is actually:
somecode.then(myFunction);
Since somecode is a Promise, your function may be called with zero or one argument. It is up to you to name this argument:
function myFunction (foo) {
// use foo here
}
somecode.then(myFunction);
Of course, unlike some other languages, javascript does not force you to name your function. You can just use a nameless (anonymous) function:
somecode.then(function(mango) { /* use mango here */ })
Arrow functions is a new syntax allowing you to write anonymous functions in a shorter style (it also behaves slightly differently with regards to scope and the value of this)
Express.js and http.Server
In node's http.Server library and Express.js framework, each server request will call a function you define and pass it two arguments: the request object and the response object. The variables req and res are just conventions people use when writing their own request handler functions. You can name them anything you like. For example you may prefer to use request and response instead or rx and tx:
app.get('/say/hello', (rx, tx) => tx.send('Hello'));
How many arguments do I write a callback function with??
The best way to know is to read the documentation of the module you are using. It is not the only way to know - you can of course read the source code instead. But it is often easier to read the documentation. Because of this, javascript modules tend to have really good documentation (otherwise they would be unusable and ignored by the community).
Express.js will actually pass three arguments to your callback (not two!!) - request, response and next where next is a function you can call if you want Express to continue processing instead of replying to the request. One interesting feature of javascript is that you are allowed to call functions with fewer or more arguments and it is not considered a syntax error:
function example (x) {}
example(); // not an error
example(1); // not an error
example(1,2,3,4); // also not an error
Express uses this feature by always calling your callback with three arguments while allowing you to declare said callback with only two arguments if you don't need the third, next argument.

javascript: module calls a function in the file that requires the module

My first time writing a js library. The library is intended to execute, at specific times, functions in the file that required the library. Kind of like Angular executes user implemented hooks such as $onInit, except that, in my case, user can define an arbitrary number of functions to be called by my library. How can I implement that?
One way I have in mind is to define a registerFunction(name, function) method, which maps function names to implementations. But can user just give me an array of names and I automatically register the corresponding functions for them?

Unless you have a specific requirement that it do so, your module does not need to know the names of the functions it is provided. When your module invokes those functions, it will do so by acting on direct references to them rather than by using their names.
For example:
// my-module.js
module.exports = function callMyFunctions( functionList ) {
functionList.forEach( fn => fn() )
}
// main application
const myFunc1 = () => console.log('Function 1 executing')
const myFunc2 = () => console.log('Function 2 executing')
const moduleThatInvokesMyFunctions = require('./my-module.js')
// instruct the module to invoke my 2 cool functions
moduleThatInvokesMyFunctions([ myFunc1, myFunc2 ])
//> Function 1 executing
//> Function 2 executing
See that the caller provides direct function references to the module, which the module then uses -- without caring or even knowing what those functions are called. (Yes, you can obtain their names by inspecting the function references, but why bother?)
If you want a more in-depth answer or explanation, it would help to know more about your situation. What environment does your library target: browsers? nodejs? Electron? react-native?
The library is intended to execute, at specific times, functions in the file that required the library
The "at specific times" suggests to me something that is loosely event-based. So, depending on what platform you're targeting, you could actually use a real EventEmitter. In that case, you'd invent unique names for each of the times that a function should be invoked, and your module would then export a singleton emitter. Callers would then assign event handlers for each of the events they care about. For callers, that might look like this:
const lifecycleManager = require('./your-module.js')
lifecycleManager.on( 'boot', myBootHandler )
lifecycleManager.on( 'config-available', myConfigHandler )
// etc.
A cruder way to handle this would be for callers to provide a dictionary of functions:
const orchestrateJobs = require('./your-module.js')
orchestrateJobs({
'boot': myBootHandler,
'config-available': myConfigHandler
})
If you're not comfortable working with EventEmitters, this may be appealing. But going this route requires that you consider how to support other scenarios like callers wanting to remove a function, and late registration.
Quick sketch showing how to use apply with each function:
// my-module.js
module.exports = function callMyFunctions( functionList ) {
functionList.forEach( fn => fn.apply( thisValue, arrayOfArguments ) )
}
Note that this module still has no idea what names the caller has assigned to these functions. Within this scope, each routine bears the moniker "fn."
I get the sense you have some misconceptions about how execution works, and that's led you to believe that the parts of the program need to know the names of other parts of the program. But that's not how continuation-passing style works.
Since you're firing caller functions based on specific times, it's possible the event model might be a good fit. Here's a sketch of what that might look like:
// caller
const AlarmClock = require('./your-module.js')
function doRoosterCall( exactTime ) {
console.log('I am a rooster! Cock-a-doodle-doo!')
}
function soundCarHorn( exactTime ) {
console.log('Honk! Honk!')
}
AlarmClock.on('sunrise', doRoosterCall)
AlarmClock.on('leave-for-work', soundCarHorn)
// etc
To accomplish that, you might do something like...
// your-module.js
const EventEmitter = require('events')
const singletonClock = new EventEmitter()
function checkForEvents() {
const currentTime = new Date()
// check for sunrise, which we'll define as 6:00am +/- 10 seconds
if(nowIs('6:00am', 10 * 1000)) {
singletonClock.emit('sunrise', currentTime)
}
// check for "leave-for-work": 8:30am +/- 1 minute
if(nowIs('8:30am', 60 * 1000)) {
singletonClock.emit('leave-for-work', currentTime)
}
}
setInterval( checkForEvents, 1000 )
module.exports = singletonClock
(nowIs is some handwaving for time-comparisons. When doing cron-like work, you should assume your heartbeat function will almost never be fired when the time value is an exact match, and so you'll need something to provide "close enough" comparisons. I didn't provide an impl because (1) it seems like a peripheral concern here, and (2) I'm sure Momentjs, date-fns, or some other package provides something great so you won't need to implement it yourself.

JavaScript: Is the nesting of constructor instances inside a constructed 'wrapper' problematic?

Hopefully this question won't be flagged as too subjective but I'm newish to OOP and struggling a bit when it come to sharing data between parts of my code that I think should be separated to some extent.
I'm building a (non-geo) map thing (using leaflet.js which is superduper) which has a map (duh) and a sidebar that basically contains a UI (toggling markers both individually and en masse, searching said marker toggles as well as other standard UI behaviour). Slightly confused about organisation too (how modular is too modular but I can stumble through that myself I guess). I am using a simple JSON file for my settings for the time being.
I started with static methods stored in objects which is essentially unusable or rather un-reusable so I went for nested constructors (kinda) so I could pass the parent scope around for easier access to my settings and states properties:
function MainThing(settings) {
this.settings = options;
this.states = {};
}
function SubthingMaker(parent) {
this.parent = parent;
}
SubthingMaker.prototype.method = function() {
var data = this.parent.settings.optionOne;
console.log(data);
this.parent.states.isVisible = true;
};
MainThing.prototype.init = function() {
this.subthing = new SubthingMaker(this);
// and some other fun stuff
};
And then I could just create and instance of MainThing and run MainThing.init() and it should all work lovely. Like so:
var options = {
"optionOne": "Hello",
"optionTwo": "Goodbye"
}
var test = new MainThing(options);
test.init();
test.subthing.method();
Should I really be nesting in this manner or will it cause me problems in some way? If this is indeed okay, should I keep going deeper if needed (maybe the search part of my ui wants its own section, maybe the map controls should be separate from DOM manipulation, I dunno) or should I stay at this depth? Should I just have separate constructors and store them in an object when I create an instance of them? Will that make it difficult to share/reference data stored elsewhere?
As regards my data storage, is this an okay way to handle it or should I be creating a controller for my data and sending requests and submissions to it when necessary, even if that data is then tucked away in simple JSON format? this.parent does really start to get annoying after a while, I suppose I should really be binding if I want to change my scope but it just doesn't seem to be an elegant way to access the overall state data of the application especially since the UI needs to check the state for almost everything it does.
Hope you can help and I hope I don't come across as a complete idiot, thanks!
P.S. I think the code I posted works but if it doesn't, its the general idea I was hoping to capture not this specific example. I created a much simpler version of my actual code because I don't want incur the wrath of the SO gods with my first post. (Yes, I did just use a postscript.)

An object may contain as many other objects as are appropriate for doing it's job. For example, an object may contain an Array as part of its instance data. Or, it may contain some other custom object. This is normal and common.
You can create/initialize these other objects that are part of your instance data in either your constructor or in some other method such as a .init() method whichever is more appropriate for your usage and design.
For example, you might have a Queue object:
function Queue() {
this.q = [];
}
Queue.prototype.add = function(item) {
this.q.push(item);
return this;
}
Queue.prototype.next = function() {
return this.q.shift();
}
var q = new Queue();
q.add(1);
q.add(2);
console.log(q.next()); // 1
This creates an Array object as part of its constructor and then uses that Array object in the performance of its function. There is no difference here whether this creates a built-in Array object or it calls new on some custom constructor. It's just another Javascript object that is being used by the host object to perform its function. This is normal and common.

One note is that what you are doing with your MainThing and SubthingMaker violates OOP principles, because they are too tightly coupled and have too wide access to each other internals:
SubthingMaker.prototype.method = function() {
// it reads something from parent's settings
var data = this.parent.settings.optionOne;
console.log(data);
// it changes parent state directly
this.parent.states.isVisible = true;
};
While better idea could be to make them less dependent.
It is probably OK for the MainThing to have several "subthings" as your main thing looks like a top-level object which will coordinate smaller things.
But it would be better to isolate these smaller things, ideally they should work even there is no MainThing or if you have some different main thing:
function SubthingMaker(options) {
// no 'parent' here, it just receives own options
this.options = options;
}
SubthingMaker.prototype.method = function() {
// use own options, instead of reading then through the MainThing
var data = this.options.optionOne;
console.log(data);
// return the data from the method instead of
// directly modifying something in MainThing
return true;
this.parent.states.isVisible = true;
};
MainThing.prototype.doSomething = function() {
// MainThing calls the subthing and modifies own data
this.parent.states.isVisible = this.subthing.method();
// and some other fun stuff
};
Also to avoid confusion, it is better not to use parent / child terms in this case. What you have here is aggregation or composition of objects, while parent / child are usually used to describe the inheritance.

Express - better pattern for passing data between middleware functions

I just brought up this issue with Express and I am interested in what StackOverflow thinks about it:
https://github.com/strongloop/express/issues/2831
my question is why Express opts to disallow the developer from passing data directly between middleware functions and basically forces you to assign temporary data to the request object in what I have always considered to be a quite awkward assignment.
to be more specific:
passing data between middleware functions usually involves doing this
req.specialData = {}
next();
however, it would be perhaps much easier and more performant (!) if this were possible
next(null,data);
or
function mySpecialMiddleWare(req,res,next,data){}
//now, call the above function
mySpecialMiddleWare(req,res,next, {some:data});
the problem is that Express uses a, what I believe to be, stupid way of determining if the call is to a normal middleware function or the next error first middleware function, by checking to see if the function.length > 3 or function.length === 4...
is there any chance what I am saying makes any sense?
wouldn't it be better/easier/faster/stronger to allow direct passing of data via middleware functions instead of assigning data awkwardly to req ??
perhaps Express already has this capability and I am just misinformed?

my question is why Express opts to disallow the developer from passing data directly between middleware functions and basically forces you to assign temporary data to the request object in what I have always considered to be a quite awkward assignment.
So I think the API is the way it is to encourage most middleware to be modular, re-usable, and loosely coupled. That means the generally should do something without being concerned too much about what other middleware might run before them or after them. In order to achieve this and create an ecosystem of loosely-compatible middleware functions, express keeps the API fairly generic. There are pros and cons to this. But as a direct answer to your first question, I would say to keep the interface simple, consistent, and flexible as well as try to avoid strict ordering dependencies.
As in your case, there might be implicit dependencies between middlewares. Another common example would be typically your session middleware has an implicit dependency that your cookie middleware runs before it. Having these all be implicit can be argued to create bugs and unnecessary troubleshooting. On the other hand, it enables application developers to more easily mix and match middlewares that might otherwise have no awareness of one another.
In hindsight I think both I as well as some of the express maintainers would agree that using function arity (number of expected arguments) for semantic API reasons was an odd and poor choice on TJ's part. I think if the project were to be rewritten a more explicit API would be defined for error handling.
wouldn't it be better/easier/faster/stronger to allow direct passing of data via middleware functions instead of assigning data awkwardly to req ??
Better - this is highly arguable and opinion based. There is a lot to be said for the simplicity of it and the evidence is the huge ecosystem and usage. There are alternatives available such as hapi, restify, etc, though so you might consider them.
Easier - probably not. What's there is pretty easy.
Faster - probably not in any meaningful way. Not sure why you think your version would be faster, but best to bring metrics when you make such claims.
Stronger - If by "stronger" you mean more explicit that's probably true but there's a reason some people still prefer JavaScript even though TypeScript all the way up through Haskell exist and are definitely "stronger" in some sense of the word.

There might be advantages to what Express is doing, however, it is worth noting that Express allows passing data between middlewares using res.locals. With that said, your question inspired me to make this library. Expressjs-plus.
With it, you will be able to pass data between your middlewares easily without having to use the request nor the response object.
You can use regular javascript functions e.g. function(varInResLocals, varInReqParams, callback) instead of express middleware signature.
Here is a working example for you.
var express = require('express');
var ExpressPlus = require('expressjs-plus').ExpressPlus;
var app = express();
// simple handler example
var userHandler = function(param, paramsArray, req, res){
if(param !== 'user') return false;
paramsArray.push("USER WAS FOUND!");
return true;
};
// this handler allows you to pass res.locals properties between your middlewares seemingly,
// it the parameter was found in locals, it attaches it to paramsArray.
var resLocalsHandler = function(param, paramsArray, req, res){
if(param in res.locals){
paramsArray.push(res.locals[param]);
return true;
}else return false;
};
var appPlus = new ExpressPlus(app, [userHandler, resLocalsHandler], []);
var regularFunction = function(user, id, cb){
return cb(null, { response: {user: user, id: id}, status: 200, resLocalsVar: "passVar" });
};
// resLocalsVar was passed in a previous method
var regularFunction2 = function(resLocalsVar, user, id, cb){
// now you can have access to it
console.log(resLocalsVar);
return cb(null);
};
// the responder at the end will use res.locals.status and res.locals.response to issue an HTTP response
app.use(appPlus.GMV(regularFunction), appPlus.GMV(regularFunction2), appPlus.responder);
// adds error handlers, it will add a default error handler along with the list of error handlers passed
// in this case, no error handlers were passed
appPlus.setErrorHandlers();
app.listen(3001, function(){
console.log('Listening!');
});

Deps autorun in Meteor JS

Decided to test out Meteor JS today to see if I would be interested in building my next project with it and decided to start out with the Deps library.
To get something up extremely quick to test this feature out, I am using the 500px API to simulate changes. After reading through the docs quickly, I thought I would have a working example of it on my local box.
The function seems to only autorun once which is not how it is suppose to be working based on my initial understanding of this feature in Meteor.
Any advice would be greatly appreciated. Thanks in advance.
if (Meteor.isClient) {
var Api500px = {
dep: new Deps.Dependency,
get: function () {
this.dep.depend();
return Session.get('photos');
},
set: function (res) {
Session.set('photos', res.data.photos);
this.dep.changed();
}
};
Deps.autorun(function () {
Api500px.get();
Meteor.call('fetchPhotos', function (err, res) {
if (!err) Api500px.set(res);
else console.log(err);
});
});
Template.photos.photos = function () {
return Api500px.get();
};
}
if (Meteor.isServer) {
Meteor.methods({
fetchPhotos: function () {
var url = 'https://api.500px.com/v1/photos';
return HTTP.call('GET', url, {
params: {
consumer_key: 'my_consumer_key_here',
feature: 'fresh_today',
image_size: 2,
rpp: 24
}
});
}
});
}

Welcome to Meteor! A couple of things to point out before the actual answer...
Session variables have reactivity built in, so you don't need to use the Deps package to add Deps.Dependency properties when you're using them. This isn't to suggest you shouldn't roll your own reactive objects like this, but if you do so then its get and set functions should return and update a normal javascript property of the object (like value, for example), rather than a Session variable, with the reactivity being provided by the depend and changed methods of the dep property. The alternative would be to just use the Session variables directly and not bother with the Api500px object at all.
It's not clear to me what you're trying to achieve reactively here - apologies if it should be. Are you intending to repeatedly run fetchPhotos in an infinite loop, such that every time a result is returned the function gets called again? If so, it's really not the best way to do things - it would be much better to subscribe to a server publication (using Meteor.subscribe and Meteor.publish), get this publication function to run the API call with whatever the required regularity, and then publish the results to the client. That would dramatically reduce client-server communication with the same net result.
Having said all that, why would it only be running once? The two possible explanations that spring to mind would be that an error is being returned (and thus Api500px.set is never called), or the fact that a Session.set call doesn't actually fire a dependency changed event if the new value is the same as the existing value. However, in the latter case I would still expect your function to run repeatedly as you have your own depend and changed structure surrounding the Session variable, which does not implement that self-limiting logic, so having Api500px.get in the autorun should mean that it reruns when Api500px.set returns even if the Session.set inside it isn't actually doing anything. If it's not the former diagnosis then I'd just log everything in sight and the answer should present itself.