Introducción a eventos

Los eventos son acciones u ocurrencias que suceden en el sistema que está programando y que el sistema le informa para que pueda responder de alguna manera si lo desea. Por ejemplo, si el usuario hace clic en un botón en una página web, es posible que desee responder a esa acción mostrando un cuadro de información. En este artículo, discutiremos algunos conceptos importantes que rodean los eventos y veremos cómo funcionan en los navegadores. Este no será un estudio exhaustivo; solo lo que necesitas saber en esta etapa.

Prerrequisitos: Conocimientos básicos de informática, entendimiento básico de HTML y CSS, Primeros pasos con JavaScript.
Objetivo: Comprender la teoría fundamental de los eventos, cómo funcionan en los navegadores y cómo los eventos pueden diferir en distintos entornos de programación.

Una seria de eventos afortunados

Como se mencionó anteriormente, los eventos son acciones u ocurrencias que suceden en el sistema que está programando — el sistema disparará una señal de algún tipo cuando un evento ocurra y también proporcionará un mecanismo por el cual se puede tomar algún tipo de acción automáticamente (p.e., ejecutando algún código) cuando se produce el evento. Por ejemplo, en un aeropuerto cuando la pista está despejada para que despegue un avión, se comunica una señal al piloto y, como resultado, comienzan a pilotar el avión.

En el caso de la Web, los eventos se desencadenan dentro de la ventana del navegador y tienden a estar unidos a un elemento específico que reside en ella — podría ser un solo elemento, un conjunto de elementos, el documento HTML cargado en la pestaña actual o toda la ventana del navegador. Hay muchos tipos diferentes de eventos que pueden ocurrir, por ejemplo:

  • El usuario hace clic con el mouse sobre un elemento determinado o coloca el cursor sobre un elemento determinado.
  • El usuario presiona una tecla en el teclado.
  • El usuario cambia el tamaño o cierra la ventana del navegador.
  • Una página web termina de cargar.
  • Un formulario se envía
  • Un video se reproduce, pausa o finaliza la reproducción.
  • Un error ocurre.

Se deducirá de esto (y echar un vistazo a MDN Referencia de eventos) que hay muchos eventos a los que se puede responder.

Cada evento disponible tiene un controlador de eventos, que es un bloque de código (generalmente una función JavaScript definida por el usuario) que se ejecutará cuando se active el evento. Cuando dicho bloque de código se define para ejecutarse en respuesta a un disparo de evento, decimos que estamos registrando un controlador de eventos. Tenga en cuenta que los controladores de eventos a veces se llaman oyentes de eventos — son bastante intercambiables para nuestros propósitos, aunque estrictamente hablando, trabajan juntos. El oyente escucha si ocurre el evento y el controlador es el código que se ejecuta en respuesta a que ocurra.

Nota: Es útil tener en cuenta que los eventos web no son parte del lenguaje central de JavaScript: se definen como parte de las API integradas en el navegador.

Un ejemplo simple

Veamos un ejemplo simple para explicar lo que queremos decir aquí. Ya has visto eventos y controladores de eventos en muchos de los ejemplos de este curso, pero vamos a recapitular solo para consolidar nuestro conocimiento. En el siguiente ejemplo, tenemos un solo <button>, que cuando se presiona, hará que el fondo cambie a un color aleatorio:

<button>Cambiar color</button>

El JavaScript se ve así:

var btn = document.querySelector('button');

function random(number) {
  return Math.floor(Math.random()*(number+1));
}

btn.onclick = function() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
}

En este código, almacenamos una referencia al botón dentro de una variable llamada btn, usando la función Document.querySelector (). También definimos una función que devuelve un número aleatorio. La tercera parte del código es el controlador de eventos. La variable btn apunta a un elemento <button>, y este tipo de objeto tiene una serie de eventos que pueden activarse y, por lo tanto, los controladores de eventos están disponibles. Estamos escuchando el disparo del evento "click", estableciendo la propiedad del controlador de eventos onclick para que sea igual a una función anónima que contiene código que generó un color RGB aleatorio y establece el <body> color de fondo igual a este.

Este código ahora se ejecutará cada vez que se active el evento "click" en el elemento <button>, es decir, cada vez que un usuario haga clic en él.

El resultado de ejemplo es el siguiente:

No son solo páginas web

Otra cosa que vale la pena mencionar en este punto es que los eventos no son particulares de JavaScript — la mayoría de los lenguajes de programación tienen algún tipo de modelo de eventos, y la forma en que funciona a menudo diferirá de la forma en que funciona en JavaScript. De hecho, el modelo de eventos en JavaScript para páginas web difiere del modelo de eventos para JavaScript, ya que se utiliza en otros entornos.

Por ejemplo, Node.js es un entorno en tiempo de ejecución de JavaScript muy popular que permite a los desarrolladores usar JavaScript para crear aplicaciones de red y del lado del servidor. El modelo de eventos de Node.js se basa en que los oyentes (listeners) escuchen eventos y los emisores (emitters) emitan eventos periódicamente — no suena tan diferentes, pero el código es bastante diferente, haciendo uso de funciones como on() para registrar un oyente de eventos, y once() para registrar un oyente de eventos que anula el registro después de que se haya ejecutado una vez. The documentos de eventos de conexión HTTP proporcionan un buen ejemplo de uso.

Como otro ejemplo, ahora también puede usar JavaScript para crear complementos de navegadores — mejoras de funcionalidad del navegador — utilizando una tecnología llamada WebExtensions. El modelo de eventos es similar al modelo de eventos web, pero un poco diferente — las propiedades de los oyentes de eventos se escriben en camel-case (ej. onMessage en lugar de onmessage), y deben combinarse con la función addListener. Consulte la página runtime.onMessage para ver un ejemplo.

No necesita comprender nada sobre otros entornos en esta etapa de su aprendizaje; solo queríamos dejar en claro que los eventos pueden diferir en diferentes entornos de programación.

Ways of using web events

There are a number of different ways in which you can add event listener code to web pages so that it will be run when the associated event fires. In this section, we will review the different mechanisms and discuss which ones you should use.

Event handler properties

These are the properties that exist to contain event handler code that we have seen most frequently during the course. Returning to the above example:

var btn = document.querySelector('button');

btn.onclick = function() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
}

The onclick property is the event handler property being used in this situation. It is essentially a property like any other available on the button (e.g. btn.textContent, or btn.style), but it is a special type — when you set it to be equal to some code, that code will be run when the event fires on the button.

You could also set the handler property to be equal to a named function name (like we saw in Build your own function). The following would work just the same:

var btn = document.querySelector('button');

function bgChange() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
}

btn.onclick = bgChange;

There are many different event handler properties available. Let's do an experiment.

First of all, make a local copy of random-color-eventhandlerproperty.html, and open it in your browser. It's just a copy of the simple random color example we've been playing with already in this article. Now try changing btn.onclick to the following different values in turn, and observing the results in the example:

  • btn.onfocus and btn.onblur — The color will change when the button is focused and unfocused (try pressing tab to tab on to the button and off again). These are often used to display information about how to fill in form fields when they are focused, or display an error message if a form field has just been filled in with an incorrect value.
  • btn.ondblclick — The color will change only when it is double-clicked.
  • window.onkeypress, window.onkeydown, window.onkeyup — The color will change when a key is pressed on the keyboard. keypress refers to a general press (button down and then up), while keydown and keyup refer to just the key down and key up parts of the keystroke, respectively. Note that it doesn't work if you try to register this event handler on the button itself — we've had to register it on the window object, which represents the entire browser window.
  • btn.onmouseover and btn.onmouseout — The color will change when the mouse pointer is moved so it begins hovering over the button, or when it stops hovering over the button and moves off of it, respectively.

Some events are very general and available nearly anywhere (for example an onclick handler can be registered on nearly any element), whereas some are more specific and only useful in certain situations (for example it makes sense to use onplay only on specific elements, such as <video>).

Inline event handlers — don't use these

You might also see a pattern like this in your code:

<button onclick="bgChange()">Press me</button>
function bgChange() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
}

Note: You can find the full source code for this example on GitHub (also see it running live).

The earliest method of registering event handlers found on the Web involved event handler HTML attributes (aka inline event handlers) like the one shown above — the attribute value is literally the JavaScript code you want to run when the event occurs. The above example invokes a function defined inside a <script> element on the same page, but you could also insert JavaScript directly inside the attribute, for example:

<button onclick="alert('Hello, this is my old-fashioned event handler!');">Press me</button>

You'll find HTML attribute equivalents for many of the event handler properties; however, you shouldn't use these — they are considered bad practice. It might seem easy to use an event handler attribute if you are just doing something really quick, but they very quickly become unmanageable and inefficient.

For a start, it is not a good idea to mix up your HTML and your JavaScript, as it becomes hard to parse — keeping your JavaScript all in one place is better; if it is in a separate file you can apply it to multiple HTML documents.

Even in a single file, inline event handlers are not a good idea. One button is OK, but what if you had 100 buttons? You'd have to add 100 attributes to the file; it would very quickly turn into a maintenance nightmare. With JavaScript, you could easily add an event handler function to all the buttons on the page no matter how many there were, using something like this:

var buttons = document.querySelectorAll('button');

for (var i = 0; i < buttons.length; i++) {
  buttons[i].onclick = bgChange;
}

Note that another option here would be to use the forEach() built-in method available on all Array objects:

buttons.forEach(function(button) {
  button.onclick = bgChange;
});

Note: Separating your programming logic from your content also makes your site more friendly to search engines.

addEventListener() and removeEventListener()

The newest type of event mechanism is defined in the Document Object Model (DOM) Level 2 Events Specification, which provides browsers with a new function — addEventListener(). This functions in a similar way to the event handler properties, but the syntax is obviously different. We could rewrite our random color example to look like this:

var btn = document.querySelector('button');

function bgChange() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
}   

btn.addEventListener('click', bgChange);

Note: You can find the full source code for this example on GitHub (also see it running live).

Inside the addEventListener() function, we specify two parameters — the name of the event we want to register this handler for, and the code that comprises the handler function we want to run in response to it. Note that it is perfectly appropriate to put all the code inside the addEventListener() function, in an anonymous function, like this:

btn.addEventListener('click', function() {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  document.body.style.backgroundColor = rndCol;
});

This mechanism has some advantages over the older mechanisms discussed earlier. For a start, there is a counterpart function, removeEventListener(), which removes a previously added listener. For example, this would remove the listener set in the first code block in this section:

btn.removeEventListener('click', bgChange);

This isn't significant for simple, small programs, but for larger, more complex programs it can improve efficiency to clean up old unused event handlers. Plus, for example, this allows you to have the same button performing different actions in different circumstances — all you've got to do is add/remove event handlers as appropriate.

Second, you can also register multiple handlers for the same listener. The following two handlers would not be applied:

myElement.onclick = functionA;
myElement.onclick = functionB;

As the second line would overwrite the value of onclick set by the first. This would work, however:

myElement.addEventListener('click', functionA);
myElement.addEventListener('click', functionB);

Both functions would now run when the element is clicked.

In addition, there are a number of other powerful features and options available with this event mechanism. These are a little out of scope for this article, but if you want to read up on them, have a look at the addEventListener() and removeEventListener() reference pages.

What mechanism should I use?

Of the three mechanisms, you definitely shouldn't use the HTML event handler attributes — these are outdated, and bad practice, as mentioned above.

The other two are relatively interchangeable, at least for simple uses:

  • Event handler properties have less power and options, but better cross-browser compatibility (being supported as far back as Internet Explorer 8). You should probably start with these as you are learning.
  • DOM Level 2 Events (addEventListener(), etc.) are more powerful, but can also become more complex and are less well supported (supported as far back as Internet Explorer 9). You should also experiment with these, and aim to use them where possible.

The main advantages of the third mechanism are that you can remove event handler code if needed, using removeEventListener(), and you can add multiple listeners of the same type to elements if required. For example, you can call addEventListener('click', function() { ... }) on an element multiple times, with different functions specified in the second argument. This is impossible with event handler properties because any subsequent attempts to set a property will overwrite earlier ones, e.g.:

element.onclick = function1;
element.onclick = function2;
etc.

Note: If you are called upon to support browsers older than Internet Explorer 8 in your work, you may run into difficulties, as such ancient browsers use different event models from newer browsers. But never fear, most JavaScript libraries (for example jQuery) have built-in functions that abstract away cross-browser differences. Don't worry about this too much at this stage in your learning journey.

Other event concepts

In this section, we will briefly cover some advanced concepts that are relevant to events. It is not important to understand these fully at this point, but it might serve to explain some code patterns you'll likely come across from time to time.

Event objects

Sometimes inside an event handler function, you might see a parameter specified with a name such as event, evt, or simply e. This is called the event object, and it is automatically passed to event handlers to provide extra features and information. For example, let's rewrite our random color example again slightly:

function bgChange(e) {
  var rndCol = 'rgb(' + random(255) + ',' + random(255) + ',' + random(255) + ')';
  e.target.style.backgroundColor = rndCol;
  console.log(e);
}  

btn.addEventListener('click', bgChange);

Note: You can find the full source code for this example on GitHub (also see it running live).

Here you can see that we are including an event object, e, in the function, and in the function setting a background color style on e.target — which is the button itself. The target property of the event object is always a reference to the element that the event has just occurred upon. So in this example, we are setting a random background color on the button, not the page.

Note: You can use any name you like for the event object — you just need to choose a name that you can then use to reference it inside the event handler function. e/evt/event are most commonly used by developers because they are short and easy to remember. It's always good to stick to a standard.

e.target is incredibly useful when you want to set the same event handler on multiple elements and do something to all of them when an event occurs on them. You might, for example, have a set of 16 tiles that disappear when they are clicked on. It is useful to always be able to just set the thing to disappear as e.target, rather than having to select it in some more difficult way. In the following example (see useful-eventtarget.html for the full source code; also see it running live here), we create 16 <div> elements using JavaScript. We then select all of them using document.querySelectorAll(), then loop through each one, adding an onclick handler to each that makes it so that a random color is applied to each one when clicked:

var divs = document.querySelectorAll('div');

for (var i = 0; i < divs.length; i++) {
  divs[i].onclick = function(e) {
    e.target.style.backgroundColor = bgChange();
  }
}

The output is as follows (try clicking around on it — have fun):

Most event handlers you'll encounter just have a standard set of properties and functions (methods) available on the event object (see the Event object reference for a full list). Some more advanced handlers, however, add specialist properties containing extra data that they need to function. The Media Recorder API, for example, has a dataavailable event, which fires when some audio or video has been recorded and is available for doing something with (for example saving it, or playing it back). The corresponding ondataavailable handler's event object has a data property available containing the recorded audio or video data to allow you to access it and do something with it.

Preventing default behavior

Sometimes, you'll come across a situation where you want to stop an event doing what it does by default. The most common example is that of a web form, for example, a custom registration form. When you fill in the details and press the submit button, the natural behaviour is for the data to be submitted to a specified page on the server for processing, and the browser to be redirected to a "success message" page of some kind (or the same page, if another is not specified.)

The trouble comes when the user has not submitted the data correctly — as a developer, you'll want to stop the submission to the server and give them an error message telling them what's wrong and what needs to be done to put things right. Some browsers support automatic form data validation features, but since many don't, you are advised to not rely on those and implement your own validation checks. Let's look at a simple example.

First, a simple HTML form that requires you to enter your first and last name:

<form>
  <div>
    <label for="fname">First name: </label>
    <input id="fname" type="text">
  </div>
  <div>
    <label for="lname">Last name: </label>
    <input id="lname" type="text">
  </div>
  <div>
     <input id="submit" type="submit">
  </div>
</form>
<p></p>

Now some JavaScript — here we implement a very simple check inside an onsubmit event handler (the submit event is fired on a form when it is submitted) that tests whether the text fields are empty. If they are, we call the preventDefault() function on the event object — which stops the form submission — and then display an error message in the paragraph below our form to tell the user what's wrong:

var form = document.querySelector('form');
var fname = document.getElementById('fname');
var lname = document.getElementById('lname');
var submit = document.getElementById('submit');
var para = document.querySelector('p');

form.onsubmit = function(e) {
  if (fname.value === '' || lname.value === '') {
    e.preventDefault();
    para.textContent = 'You need to fill in both names!';
  }
}

Obviously, this is pretty weak form validation — it wouldn't stop the user validating the form with spaces or numbers entered into the fields, for example — but it is ok for example purposes. The output is as follows:

Note: for the full source code, see preventdefault-validation.html (also see it running live here.)

Event bubbling and capture

The final subject to cover here is something that you'll not come across often, but it can be a real pain if you don't understand it. Event bubbling and capture are two mechanisms that describe what happens when two handlers of the same event type are activated on one element. Let's look at an example to make this easier — open up the show-video-box.html example in a new tab (and the source code in another tab.) It is also available live below:

This is a pretty simple example that shows and hides a <div> with a <video> element inside it:

<button>Display video</button>

<div class="hidden">
  <video>
    <source src="rabbit320.mp4" type="video/mp4">
    <source src="rabbit320.webm" type="video/webm">
    <p>Your browser doesn't support HTML5 video. Here is a <a href="rabbit320.mp4">link to the video</a> instead.</p>
  </video>
</div>

When the <button> is clicked, the video is displayed, by changing the class attribute on the <div> from hidden to showing (the example's CSS contains these two classes, which position the box off the screen and on the screen, respectively):

btn.onclick = function() {
  videoBox.setAttribute('class', 'showing');
}

We then add a couple more onclick event handlers — the first one to the <div> and the second one to the <video>. The idea is that when the area of the <div> outside the video is clicked, the box should be hidden again; when the video itself is clicked, the video should start to play.

videoBox.onclick = function() {
  videoBox.setAttribute('class', 'hidden');
};

video.onclick = function() {
  video.play();
};

But there's a problem — currently, when you click the video it starts to play, but it causes the <div> to also be hidden at the same time. This is because the video is inside the <div> — it is part of it — so clicking on the video actually runs both the above event handlers.

Bubbling and capturing explained

When an event is fired on an element that has parent elements (e.g. the <video> in our case), modern browsers run two different phases — the capturing phase and the bubbling phase.

In the capturing phase:

  • The browser checks to see if the element's outer-most ancestor (<html>) has an onclick event handler registered on it in the capturing phase, and runs it if so.
  • Then it moves on to the next element inside <html> and does the same thing, then the next one, and so on until it reaches the element that was actually clicked on.

In the bubbling phase, the exact opposite occurs:

  • The browser checks to see if the element that was actually clicked on has an onclick event handler registered on it in the bubbling phase, and runs it if so.
  • Then it moves on to the next immediate ancestor element and does the same thing, then the next one, and so on until it reaches the <html> element.

(Click on image for bigger diagram)

In modern browsers, by default, all event handlers are registered in the bubbling phase. So in our current example, when you click the video, the click event bubbles from the <video> element outwards to the <html> element. Along the way:

  • It finds the video.onclick... handler and runs it, so the video first starts playing.
  • It then finds the videoBox.onclick... handler and runs it, so the video is hidden as well.

Fixing the problem with stopPropagation()

This is annoying behavior, but there is a way to fix it! The standard event object has a function available on it called stopPropagation(), which when invoked on a handler's event object makes it so that handler is run, but the event doesn't bubble any further up the chain, so no more handlers will be run.

We can, therefore, fix our current problem by changing the second handler function in the previous code block to this:

video.onclick = function(e) {
  e.stopPropagation();
  video.play();
};

You can try making a local copy of the show-video-box.html source code and having a go at fixing it yourself, or looking at the fixed result in show-video-box-fixed.html (also see the source code here).

Note: Why bother with both capturing and bubbling? Well, in the bad old days when browsers were much less cross-compatible than they are now, Netscape only used event capturing, and Internet Explorer used only event bubbling. When the W3C decided to try to standardize the behavior and reach a consensus, they ended up with this system that included both, which is the one modern browsers implemented.

Note: As mentioned above, by default all event handlers are registered in the bubbling phase, and this makes more sense most of the time. If you really want to register an event in the capturing phase instead, you can do so by registering your handler using addEventListener(), and setting the optional third property to true.

Event delegation

Bubbling also allows us to take advantage of event delegation — this concept relies on the fact that if you want some code to run when you click on any one of a large number of child elements, you can set the event listener on their parent and have events that happen on them bubble up to their parent rather than having to set the event listener on every child individually. Remember earlier that we said bubbling involves checking the element the event is fired on for an event handler first, then moving up to the element's parent, etc.?

A good example is a series of list items — if you want each one of them to pop up a message when clicked, you can set the click event listener on the parent <ul>, and events will bubble from the list items to the <ul>.

This concept is explained further on David Walsh's blog, with multiple examples — see How JavaScript Event Delegation Works.

Conclusion

You should now know all you need to know about web events at this early stage. As mentioned above, events are not really part of the core JavaScript — they are defined in browser Web APIs.

Also, it is important to understand that the different contexts in which JavaScript is used tend to have different event models — from Web APIs to other areas such as browser WebExtensions and Node.js (server-side JavaScript). We are not expecting you to understand all these areas now, but it certainly helps to understand the basics of events as you forge ahead with learning web development.

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

See also

  • Event order (discussion of capturing and bubbling) — an excellently detailed piece by Peter-Paul Koch.
  • Event accessing (discussion of the event object) — another excellently detailed piece by Peter-Paul Koch.
  • Event reference

In this module