Wednesday, April 09, 2014

Bootstrap's Alert.js Plugin, line by line

At work I recently needed to write a jQuery plugin that uses Bootstrap styling, so I looked at how alert.js is implemented, since it is a fairly small plugin. In this post I'll go over alert.js line by line, and do my best to explain what is going on. You should have a decent understanding of JavaScript and some familiarity with jQuery, but I'll assume no knowledge of how to write a jQuery plugin or anything specific to Bootstrap.


Let's start with an overview of how the Alert plugin can be used. You can find the Bootstrap Alert plugin documentation here. Essentially there are two ways to use Alerts, and this applies to other Bootstrap plugins as well

  1. Use the jQuery plugin function directly. For Alerts this looks like this

    // To close the alert:
  2. Data API: use data-* attributes to configure in a more declarative style. For Alerts, the primary data attribute is data-dismiss

    <button type="button" class="close" data-dismiss="alert"

We'll see how the code in alert.js supports both of these styles.

What's this +function business?

Okay, let's start at the top. The first line of code is this

+function ($) {

We have an unnamed (anonymous) function, and a + in front of it. What's going on there? For the moment let's ignore the body of this JavaScript function definition and look at how this function gets invoked:

+function ($) {

    // ...


This is an example of the Immediately Invoked Function Expression (IIFE) pattern. The intent of an IIFE is to introduce stronger lexical scoping in JavaScript and to allow for some functions defined within the IIFE to remain private. There are several variations of this pattern. One way to do it is to have the IIFE return an object with references to public functions while leaving some functions private. In this case, nothing is returned. Instead, the plugin is added to the jQuery object. Because nothing is returned by the IIFE, we can use a trick to get the JavaScript interpreter to call our anonymous function immediately, and that trick is to put an unary operator in front of the function definition.

The sole argument passed to the function is jQuery for the $ parameter. Why passing jQuery as the $ parameter, why not just use $ from the global scope like so?

+function () {

    $('#foo').hide(); // etc...

We could do that, but that would only work as long as $ refers to the jQuery function. There are other libraries that define the $ reference, so in order to interoperate with them we'll assume only that there is a jQuery reference in the global scope. But it's nice that we can essentially alias jQuery to $ within our plugin definition in a lexically scoped and tidy way.

use strict

The next line of alert.js is a String, 'use strict':

+function ($) {
    'use strict';

According to the Strict mode article on the Mozilla Developer Network

ECMAScript 5's strict mode is a way to opt in to a restricted variant of JavaScript. Strict mode isn't just a subset: it intentionally has different semantics from normal code.

Perl has something similar. 'use strict' is a way of running your JavaScript code through the interpreter in a less loosy-goosy manner. Here are some benefits of running in strict mode:

  • Impossible to accidentally create global variables (for example, mistyped variable names)
  • Severals classes of silent errors become thrown exceptions
  • Simplifies variable naming which allows JavaScript interpreters to better optimize code (for example, eval doesn't introduce new variables into the surrounding scope)
  • Several security enhancing restrictions are added

See the linked article for more information.

Alert class definition

Now for the Alert class definition. Our Alert class has a constructor and one method, close. First, the dismiss variable is defined. This is a jQuery selector that is used here in the constructor to bind to the click event from a DOM element that matches this selector. The way this works is that any DOM element that is el or a descendant of el that has a data-dismiss attribute with value alert that fires a click event will be handled by this.close:

var dismiss = '[data-dismiss="alert"]'
var Alert   = function (el) {
    $(el).on('click', dismiss, this.close)

Alert close function

The close method is defined for the Alert class by adding it to Alert's prototype. This method does the following:

  1. Figure out the $parent element (i.e., the alert div) that should be closed. There are a few different ways to determine the $parent and we'll look at those shortly.
  2. Trigger a custom event before closing the $parent and one after it is closed.
  3. Use a CSS3 transition to fade the $parent and then remove it.
Finding the $parent to close

First, figure out the $parent element. Here's the first part of close:

Alert.prototype.close = function (e) {
    var $this    = $(this)
    var selector = $this.attr('data-target')

    if (!selector) {
        selector = $this.attr('href')
        selector = selector && selector.replace(/.*(?=#[^\s]*$)/, '') // strip for ie7

    var $parent = $(selector)

    if (e) e.preventDefault()

    if (!$parent.length) {
        $parent = $this.hasClass('alert') ? $this : $this.parent()
// ...

In the first line of close, this refers to the element that is the target of click event. So in the standard case where you have a close <button> inside a <div class="alert"> element, this is the <button>. However, close could be called through the plugin function, as we'll see later, so this might refer to the alert itself (<div class="alert">). For now, we'll consider the case that this refers to the close button.

The first option to check is a data-target attribute on the close button. The value is expected to be a selector. You would use it like this:

<div id="myalert">
    <button class="close" aria-hidden="true"
        data-dismiss="alert" type="button">×</button>
    <strong>Holy guacamole!</strong>
    Best check yo self, you're not looking too good.

Click on the button should cause <div id="myalert"> to be closed.

The second option to check is the href attribute. This can be used likewise.

This line then checks to see if anything matches those options:

var $parent = $(selector)

If we haven't set data-target or href on our close button, or if we have but they don't match anything, then $parent will have a length of 0. That's what is checked for next. If we still haven't found $parent there are a couple more things to try.

So the third option is to check to see if $this has class alert. If it does then we assume that this is the element that should be faded and removed.

The fourth option, the default if none of the other checks matched anything, is to set $parent to the parent of $this.

Like I said earlier, it's possible for the close method to be called directly in which case $this is the alert itself. In that case, it is the third option that would typically be used for figuring out the $parent.

Custom events for before and after closing

Before actually closing the alert, close dispatches a event on the $parent element.

$parent.trigger(e = $.Event(''))

if (e.isDefaultPrevented()) return

The first line creates the custom event, using jQuery's $.Event and also sets it to the variable e. The name of the event, "", utilizes jQuery event namespaces. This just makes it possible to listen for the close event just from Bootstrap's Alert plugin by binding to the event. It also makes it easier in Bootstrap to remove event handlers. For example, all of the following would work to remove an event handler bound to


For removing, only one of the namespaces need match. I put together a JSFiddle to demonstrate jQuery event namespaces that I think helps demonstrate how they work:

$(document).on('', function() {
    console.log(" handler");

$(document).on('close.alert', function() {
    console.log("close.alert handler");

$(document).on('', function() {
    console.log(" handler");

$(document).on('close', function() {
    console.log("close handler");

console.log("triggering close...");
$(document).trigger('close'); // triggers '', 'close.alert',
                              // '' and 'close'

$(document).trigger(''); // triggers '' and ''

console.log("triggering close.alert...");
$(document).trigger('close.alert'); // triggers '' and 
                                    // 'close.alert'

$(document).trigger(''); // triggers ''

console.log("Remove close.alert handlers and triggering close...");
$(document).off('close.alert'); // removes and close.alert
//$(document).off('.alert'); // removes and close.alert as well
$(document).trigger('close');   // triggers '' and 'close'

So that's event namespacing and triggering the close event. The next line is to return if the close event had preventDefault called on it. The idea is that a listener for the close event could call e.preventDefault() to prevent the alert from actually closing.

The other custom event that the Alert plugin dispatches is the closed event which is named It is dispatched when the Alert is actually removed from the DOM. We'll see the code to trigger this event in the next section where we'll look at how the Alert is faded out and removed from the DOM.

Removing and fading out the Alert

Now we'll look at how the Alert is faded out and removed from the DOM.


function removeElement() {

$.support.transition && $parent.hasClass('fade') ?
    .one($.support.transition.end, removeElement)
    .emulateTransitionEnd(150) :

Fading out the Alert is accomplished by using a CSS3 transition. To understand how it works, let's look at the CSS:

.fade {
    opacity: 0;
    transition: opacity 0.15s linear 0s;
} {
    opacity: 1;

When an element has the fade and in classes, its opacity is 1 and when it only has the fade class its opacity is 0. The transition CSS property defines how quickly and in what way the opacity property changes from 0 to 1 when in is added to an element that already has fade. In this case however, the close method is removing the in class which causes the opacity to transition, linearly, from 1 to 0, causing it to fade out of view and become invisible.

Making the Alert invisible is not enough however. We want to actually remove it from the DOM. If we don't then it will still take up space on the web page, it just won't be visible. To accomplish this we need to know when the transition finishes. Once the transition finishes we can then remove the Alert from the DOM.

The removeElement function is defined so as to trigger the and then remove the Alert from the DOM. In an ideal world, we'd just bind the transitionend event to the removeElement handler. However there are a few complications:

  1. It's possible that the Alert doesn't have the fade class, in other words, it is not desired that it actually fade out. That's fine, the transition is optional. That's what is checked for with $parent.hasClass('fade').
  2. It's possible that this browser doesn't support CSS transitions. That's what $.support.transition is checking for. More on this later.
  3. It's possible that even if the browser supports CSS transitions that it doesn't call the transition end event transitionend. The $.support.transition.end property holds the name of the transition end event for that browser.
  4. Finally, it's possible that the transition end event won't be called. So we need to programmatically dispatch the event if it isn't called. That's what emulateTransitionEnd(150) is doing.

You can see the tests Bootstrap does to determine the transition end event name in transition.js. Also defined in transition.js is the emulateTransitionEnd function.

So let's go line by line. First, we'll remove the in class which will start the transition from opacity 1 to 0 over the course of 0.15 seconds. Next, define a function, removeElement, that will be called when the transition finishes. We'll bind removeElement to the transition end event.

removeElement will trigger the event, the second custom event, and it will remove the Alert from the DOM.

So next we have a ternary expression. If the browser supports transitions and the alert (the $parent) has class fade, then bind removeElement to the transition end event exactly once ($ The call to emulateTransitionEnd(150) will set a 150 millisecond timeout and dispatch the transition end event if it hasn't already been dispatched (again, see transition.js for details). If the browser doesn't support Transitions or $parent doesn't have the fade class, then just call removeElement.

Alert jQuery Plugin Definition

Ok, a brief recap. We've defined the Alert class and it's single close method, which does almost all of the heavy lifting for this plugin. Now it's time to actually define the Alert class as a jQuery plugin. Here's the code:

var old = $.fn.alert

$.fn.alert = function (option) {
    return this.each(function () {
        var $this = $(this)
        var data  = $'bs.alert')

        if (!data) $'bs.alert', (data = new Alert(this)))
        if (typeof option == 'string') data[option].call($this)

$.fn.alert.Constructor = Alert

First thing here is to capture the current value of $.fn.alert into the old variable. This is used later in noConflict, and it just a way to prevent this alert plugin from interfering with another alert plugin with the same name.

The $.fn.alert function is defined next. The way this function will be used is like so


So this refers to the result of a jQuery selector, which means it is an Array of DOM elements. So for each matching DOM element we'll run this function:

function() {
    var $this = $(this)
    var data  = $'bs.alert')

    if (!data) $'bs.alert', (data = new Alert(this)))
    if (typeof option == 'string') data[option].call($this)

Within the scope of this function, this refers to the current DOM element. $this is a jQuery wrapped reference to the DOM element.

When this function creates an Alert instance for this DOM element it will store it as a reference associated with the DOM element. var data = $'bs.alert') is checking if we've already created an Alert instance for this DOM element. The third line of the function creates the Alert instance (data = new Alert(this)) if it hasn't already been created and it will be saved to the bs.alert key:

if (!data) $'bs.alert', (data = new Alert(this)))

Finally, you may have noticed that the plugin function accepts an options argument. This is a common jQuery plugin pattern. There aren't any actual options for the Alert class, but options can also be a string with the name of a method that you want to invoke on the Alert instance. That's what is being done here. data, remember, is a reference to the Alert instance. data[option] therefore refers to a function on the Alert instance. It is called with $this as the context (that is, within the method invoked by call, this will refer to $this).

Also, as a jQuery plugin best practice, the result of calling this.each is returned to enable chaining of jQuery function calls.

The other thing we haven't talked about yet is this line:

$.fn.alert.Constructor = Alert

See this StackOverflow answer for a long explanation. but, basically, if we don't do this then the Alert constructor remains private to the closure in which we defined it. By creating the Constructor reference, other code can directly instantiate an Alert instance without needing to invoke it indirectly through the jQuery API.

Alert noConflict

Now we're at the noConflict function:

$.fn.alert.noConflict = function () {
    $.fn.alert = old
    return this

This is a simple convention for handling the situation that there is another jQuery plugin also called alert. This function sets $.fn.alert to its old value and returns this which refers to the $.fn.alert function defined above.

Alert Data API

Finally we get to the Data API. This allows for purely declarative use of the Alert plugin, no JavaScript calls necessary.

$(document).on('', dismiss, Alert.prototype.close)

This binds the 'click' event for any DOM element in the page matching the dismiss selector to Alert's close method. Recall that the dismiss selector was defined as [data-dismiss="alert"]. So any clickable element in the page that has the attribute data-dismiss with value alert will have its click event handled by Alert's close method.

The difference with the Data API is that there is no actual Alert instance created. Instead Alert.prototype.close method is used directly, as a pure function. The way close will be invoked in this case is basically identical to how it will be invoked via the binding we see in the Alert constructor:

$(el).on('click', dismiss, this.close)

Additional Resources


Peter said...

This is a great breakdown Marcus. Exactly what I was looking for while I try to add custom functionality to a site following the same format and syntax of the existing Bootstrap plugins. Thanks!

Leszek Pietrzak said...

Nice work, thx!

Seán Hanson said...

Super useful! I found myself looking at Alert for the same reasons, and this answered the last lingering questions before I finished up my plugin. I wish I'd seen this before I'd started =)