Exactly how Javascript works

Question

I wanted to know how it works exactly, where it is necessary to use within some scope as if etc.

Answer 1

As this one says documentation, the return:

• Sets the value returned by a function. When there is no specified value, undefined will be returned.
• Stop running the current function.

Return with value

function retornaValor() {
    return 1;
}
console.log(retornaValor());

The code above prints 1 on the console.

Worthless return

function retornoVazio() {
    return;
}
console.log(retornoVazio());

The code above prints undefined on the console.

Return out of function (overall)

On the other hand, if you try to use the return out of a function, in a code executed in a browser, you will receive an error. For example, in Chrome, I got:

Uncaught Syntaxerror: Illegal Return statement

Block return {}

Even if used within a code block, the return will terminate the current function or cause the error as described above. There is no concept of returning a block. Example

function f(val) {
    if (val) {
        return 1;
    } else {
        return 2
    }
}

The above code will return 1 or 2 to the function, regardless of the keys.

How much and how to use the return

Some people believe that each method/function should have a single output point. Consider the following example:

function f(a, cond) {
    if (cond) {
        return -2;
    } else {
        for (var i = 0; i < a.length; i++) {
            if (a[i] == 1) return i;
        }
    }
    return -1;
}

This function is small, but soon can arise dozens conditions and returns, making the code difficult to understand.

An alternative would be the following:

function f(a, cond) {
    val retorno = cond ? -2 : -1;
    if (!cond) {
        for (var i = 0; i < a.length; i++) {
            if (a[i] == 1) {
                retorno = i;
                break;
            }
        }
    }
    return retorno;
}

Obviously it is a matter of personal opinion. Particularly, I believe that the problem lies in creating a clear logic and not in the amount of returns.

See examples in jsfiddle

Answer 2

You can use the return for 2 types of situations.

1. Return a value.
2. Stop the execution flow of the current function.

1. The usage situation is when you need to validate, calculate or modulate the code.

function Calculadora(valor1, valor2, operacao) {
   var resultado = 0;

   if (operacao == "+")
       resultado = Soma(valor1, valor2);
   // ...   

   return resultado;
}

function Soma(valor1, valor2) {
   return parseInt(valor1) + parseInt(valor2);
}

2. The usage situation is when you no longer need to continue running the current function, the result of it is already determined.

function ExibeValor(valor) // código apenas didático, sim é feio.
{
   if (parseInt(valor) <= 10) {
      alert('O valor é menor/igual a 10.');
      return;
   }

   if (parseInt(valor) > 10 && parseInt(valor) <= 20) {
      alert('O valor é maior que 10 e menor/igual a 20.');
      return;
   }

   if (parseInt(valor) > 20 && parseInt(valor) <= 30) {
      alert('O valor é maior que 20 e menor/igual a 30.');
      return;
   }

   if (parseInt(valor) > 30 && parseInt(valor) <= 40) {
      alert('O valor é maior que 30 e menor/igual a 40.');
      return;
   }
}

Let’s suppose you call the function by passing the value 5, you will enter the 1° if and ready you don’t need to run all other if to know the return of the function. This improves the execution time and makes the processing less expensive.

Answer 3

The Return in javascript is similar to the Return in Java and C#, which are the languages I know, already in Delphi the Return for the little I know works differently while going through a javascript Return to Function (or the current scope) is aborted by not going through the stream, and returning the value to its invocator, this behavior can also be perceived using a browser debug like the Firebug, or the native Google Chrome

function TesteRetorno(param){
if(param > 50){
    return "Maior que 50";
}
if(param > 10){
    return "Maior que 10";
}
return "Menor que 10";
}
alert(TesteRetorno(2));

Example