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

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'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.

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.