Una función retorna valores

Los valores de retorno son exactamente como suenan: los valores devueltos por la función cuando se completa. Ya has alcanzado los valores de retorno varias veces, aunque es posible que no hayas pensado en ellos explícitamente. Volvamos a un código familiar:

var myText = "I am a string";
var newString = myText.replace("string", "sausage");
console.log(newString);
// la función de cadena replace () toma una cadena,
// sustituyendo una subcadena con otra y devoviendo
// una cadena nueva con la sustitución realizada

Vimos exactamente este bloque de código en nuestro primer artículo de función. Estamos invocando la función replace () en la cadena myText, y le pasamos dos parámetros: la subcadena a encontrar y la subcadena con la que reemplazarla. Cuando esta función se completa (termina de ejecutarse), devuelve un valor, que es una nueva cadena con el reemplazo realizado. En el código anterior, estamos guardando este valor de retorno como el valor de la variable newString.

Si observa la página de referencia MDN de la función de reemplazo, verá una sección llamada Valor de retorno. Es muy útil conocer y comprender qué valores devuelven las funciones, por lo que tratamos de incluir esta información siempre que sea posible.

Algunas funciones no devuelven un valor de retorno como tal (en nuestras páginas de referencia, el valor de retorno aparece como void o undefined en tales casos). Por ejemplo, en la función displayMessage () que creamos en el artículo anterior, no se devuelve ningún valor específico como resultado de la función que se invoca. Simplemente hace que aparezca un cuadro en algún lugar de la pantalla, ¡eso es todo!

Generalmente, se usa un valor de retorno donde la función es un paso intermedio en un cálculo de algún tipo. Quieres llegar a un resultado final, que involucra algunos valores. Esos valores deben ser calculados por una función, que luego devuelve los resultados para que puedan usarse en la siguiente etapa del cálculo.

To return a value from a custom function, you need to use ... wait for it ... the return keyword. We saw this in action recently in our random-canvas-circles.html example. Our draw() function draws 100 random circles somewhere on an HTML <canvas>:

function draw() {
  ctx.clearRect(0, 0, WIDTH, HEIGHT);
  for (var i = 0; i < 100; i++) {
    ctx.beginPath();
    ctx.fillStyle = "rgba(255,0,0,0.5)";
    ctx.arc(random(WIDTH), random(HEIGHT), random(50), 0, 2 * Math.PI);
    ctx.fill();
  }
}

Inside each loop iteration, three calls are made to the random() function, to generate a random value for the current circle's x coordinate, y coordinate, and radius, respectively. The random() function takes one parameter — a whole number — and it returns a whole random number between 0 and that number. It looks like this:

function randomNumber(number) {
  return Math.floor(Math.random() * number);
}

This could be written as follows:

function randomNumber(number) {
  var result = Math.floor(Math.random() * number);
  return result;
}

But the first version is quicker to write, and more compact.

We are returning the result of the calculation Math.floor(Math.random()*number) each time the function is called. This return value appears at the point the function was called, and the code continues. So for example, if we ran the following line:

ctx.arc(random(WIDTH), random(HEIGHT), random(50), 0, 2 * Math.PI);

and the three random() calls returned the values 500, 200, and 35, respectively, the line would actually be run as if it were this:

ctx.arc(500, 200, 35, 0, 2 * Math.PI);

The function calls on the line are run first and their return values substituted for the function calls, before the line itself is then executed.

Let's have a go at writing our own functions featuring return values.

1. First of all, make a local copy of the function-library.html file from GitHub. This is a simple HTML page containing a text <input> field and a paragraph. There's also a <script> element in which we have stored a reference to both HTML elements in two variables. This little page will allow you to enter a number into the text box, and display different numbers related to it in the paragraph below.

2. Let's add some useful functions to this <script> element. Below the existing two lines of JavaScript, add the following function definitions:

   js

   function squared(num) {
     return num * num;
   }
   
   function cubed(num) {
     return num * num * num;
   }
   
   function factorial(num) {
     var x = num;
     while (x > 1) {
       num *= x - 1;
       x--;
     }
     return num;
   }
   

   The squared() and cubed() functions are fairly obvious — they return the square or cube of the number given as a parameter. The factorial() function returns the factorial of the given number.

3. Next, we're going to include a way to print out information about the number entered into the text input. Enter the following event handler below the existing functions:

   js

   input.addEventListener("change", () => {
     const num = parseFloat(input.value);
     if (isNaN(num)) {
       para.textContent = "You need to enter a number!";
     } else {
       para.textContent = `${num} squared is ${squared(num)}. `;
       para.textContent += `${num} cubed is ${cubed(num)}. `;
       para.textContent += `${num} factorial is ${factorial(num)}. `;
     }
   });
   

   Here we are creating an onchange event handler that runs whenever the change event fires on the text input — that is, when a new value is entered into the text input, and submitted (enter a value then press tab for example). When this anonymous function runs, the existing value entered into the input is stored in the num variable.
   Next, we do a conditional test — if the entered value is not a number, we print an error message into the paragraph. The test looks at whether the expression isNaN(num) returns true. We use the isNaN() function to test whether the num value is not a number — if so, it returns true, and if not, false.
   If the test returns false, the num value is a number, so we print out a sentence inside the paragraph element stating what the square, cube, and factorial of the number are. The sentence calls the squared(), cubed(), and factorial() functions to get the required values.

4. Save your code, load it in a browser, and try it out.

At this point, we'd like you to have a go at writing out a couple of functions of your own and adding them to the library. How about the square or cube root of the number, or the circumference of a circle with a radius of length num?

This exercise has brought up a couple of important points besides being a study on how to use the return statement. In addition, we have:

• Looked at another example of writing error handling into our functions. It is generally a good idea to check that any necessary parameters have been provided, and in the right datatype, and if they are optional, that some kind of default value is provided to allow for that. This way, your program will be less likely to throw errors.
• Thought about the idea of creating a function library. As you go further into your programming career, you'll start to do the same kinds of things over and over again. It is a good idea to start keeping your own library of utility functions that you use very often — you can then copy them over to your new code, or even just apply it to any HTML pages where you need it.

So there we have it — functions are fun, very useful and, although there's a lot to talk about in regards to their syntax and functionality, fairly understandable given the right articles to study.

If there is anything you didn't understand, feel free to read through the article again, or contact us to ask for help.

• Functions in-depth — a detailed guide covering more advanced functions-related information.
• Callback functions in JavaScript — a common JavaScript pattern is to pass a function into another function as an argument, which is then called inside the first function. This is a little beyond the scope of this course, but worth studying before too long.

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