Nodejs function, functioning

Question

app.get('/', function(req, res){
    res.send('Ola Mundo');
  });

In the case of this simple code just to illustrate we have a function in nodejs with 2 parameters the request and the answer, I was wondering if someone can explain me better how this works in full, after all my concept of function in other languages and frameworks, when you have 2 parameters, it means that the function should receive such parameters.

Like in a case where:

function area(a, b) {
 c = a.b
 return c
}

On receiving a and b we have the final result returned c, but in Node that is not what happens at least from my perspective, the REQ parameter is actually a client request, but the RES parameter is usually manipulated within the function as a return parameter and not input, it’s as if the requisitioner asks for something and already gives the answer, and we just treat it and return what he already has.

Is the doubt clear? It’s good conceptual I hope someone understands and can take away this doubt that is pressing my mind...

Answer 1

I start by saying that app.get is a normal function that takes two parameters: A path in the form of string and a callback. Soon you can view this function so:

app.get(caminho, callback);

In your example the way is '/' and the callback is function(req, res){res.send('Ola Mundo');}.

After this first point, callbacks is probably a topic that you will want to delve into, and that will be a big part of the confusion. The reason you receive a callback is because the function is asynchronous and will only be called later when a request HTTP is made.

I take this opportunity to make a small counterexample of your area function with callbacks, but not without first correcting:

function area(a, b) {
  let c = a * b;
  return c;
}

This same function could be written with a callback though it didn’t make much sense because it’s not asynchronous. But it would look like this:

function area(a, b, callback){
  let c = a * b;
  callback(c);
};

area(10, 5, function(resultado) {
  console.log(resultado);
});

In this example the callback is a function that receives the result and does anything with it, and would make sense if it were asynchronous.

Let’s then modify this example by including a setTimeout:

function area(a, b, callback){
  let c = a * b;
  setTimeout(() => callback(c), 2000); //só processa o resultado passado 2 segundos
};

area(10, 5, function(resultado) {
  console.log(resultado);
});

console.log("Calculo leva 2 segundos a ser feito");

Note that in both examples there is no return because the function is asynchronous.

Returning now to Node and Express, the function(req, res){ is actually the callback, or the function with the code you want it to execute when that route is called. It can be called by several methods, such as get, post, patch, delete, among others and this is coded as string in the first parameter. callback is called with two parameters that are objects and usually take the name of req and res respectively, although they may have any other name they want.

the actual REQ parameter is a client request

The parameter req is an object with several of the order information the customer has made, such as:

• body - contains key order value pairs
• params - contains the order parameters that are the dynamic part of the address
• ip - the IP address from which the request originated

And the res is also an object that represents the response and has several methods to control it, such as:

• status - change the response http code
• send - send the reply with its body
• redirect - to redirect to another route

I also advise to take a look and explore a little express documentation to stay more inside than represent these objects and all supported features