Building Your Own Widget Library with YUI

By YUI TeamJune 24th, 2008

The Yahoo! User Interface Library (YUI) has an ample assortment of
components. Nevertheless, there will be always some functionality you want that a library like YUI hasn’t anticipated or hasn’t built yet.
Sometimes you just want a subset of the many options a component might provide; in other cases, you may have a default configuration that you’d like to bake into a component to facilitate consistent use across your site. The flexibility built into YUI provides intrinsic support for extending, customizing, or creating wholly new components. If you find yourself in one of these situations, then building your own library of YUI-based components may make good sense.

This lengthy article is meant to get you started creating your own custom components using the tools available to you within YUI, including the Element Utility and the Event Utility. Understanding these tools can save you lots of time and make it easy to create API-driven components that expose powerful hooks to implementers making use of your work.

Customizing Existing YUI Components

Let’s begin by looking at how you would go about creating your own custom version of an existing YUI component.

For example, a server-side transaction may take quite a while and you may want
to signal to the user that it is in progress. The YUI Panel component is
a handy way to produce a floating pop-up window. Panel serves many purposes, though,
so it is not specifically tailored for this "loading message" implementation. You will need to implement something like
the following to have your "loading-panel" pop-up displayed:

if (!loadingPanel) {
    loadingPanel = new YAHOO.widget.Panel('loadingPanel',{
        fixedcenter: true, 
        constraintoviewport: true, 
    loadingPanel.setHeader("Loading ...");
    loadingPanel.setBody('<img src="loading.gif" />');

Variable loadingPanel holds a reference to the Panel instance; once
this instance is declared, later popups just reuse the existing instance and are therefore faster and less resource-intensive. But including implementation code like this inline in every page you need is
a waste of time (because any change needs to be made in multiple places) and, for your users, a waste of bandwidth. Let’s trim this down by turning it into a custom version of YUI’s Panel.

In the first sample, you can see this same code
both as inline code and as a reusable custom component. The custom component would normally
reside in a separate file that would be included in all pages. I defined
it in the page in this sample to make it easier to compare the inline syntax with the library-style syntax. The page uses
the YUI Loader to load

The YUI Library takes care to avoid using global variables that might collide
with other libraries. In fact, the whole library is packed in the
properties of a single global object called YAHOO; thus, as long as
everybody keeps away from the YAHOO global variable, there should be
little risk of collision. We can do the same for our own libraries because,
though we control our own code, in the hopefully long life of that code
other features might be added. Mashups, ad providers and others
might use the global namespace carelessly and inadvertently break our application.
Thus, the first step is to call the YAHOO.namespace static function to create an object space for our own library.


This will create a property SATYAM under YAHOO. YUI uses only a
very few names under YAHOO; you can keep safe by simply avoiding any
of those few. None of those names uses anything but lowercase characters
so any or all uppercase letters will keep you even safer.

You could create your own namespace by simply assigning an empty object to YAHOO directly,
but the namespace function first checks to see if the namespace exists and preserves it if
it does exist. If you have several library files, you will likely want to load more than one of them in any page. Each of those library files will want to make sure
the new namespace exists but while preserving it if it has already been
created by another of your library files.

Once you have your own branch under the YAHOO namespace, you define
the constructor for your custom loading panel:

YAHOO.SATYAM.LoadingPanel = function(id) {
        id || YAHOO.util.Dom.generateId() , 
            width: "100px", 
            fixedcenter: true, 
            constraintoviewport: true, 
            underlay: "shadow", 
            close: false, 
            visible: false, 
            draggable: true

    this.setHeader("Loading ...");
    this.setBody('<img src="loading.gif" />');
    YAHOO.util.Dom.setStyle(this.body, 'textAlign', 'center');

Using YAHOO.lang.extend

Compare this code with the previous code box; you’ll note that it’s quite similar. You
might be wondering what superclass is. That is part of the
inheritance mechanism provided by YUI’s extend method. After you declare the constructor
for your new object, you call extend:

YAHOO.lang.extend(YAHOO.SATYAM.LoadingPanel, YAHOO.widget.Panel);

The sequence of first defining the constructor for the new object and then
extending it with the base object may seem counterintuitive when read in a linear fashion. When you think about the sequence of execution,
though, it makes perfect sense: The new object is defined by what will become its constructor, but the
constructor itself does not get executed immediately. The extend function does get executed immediately, so by the time a new LoadingPanel object is
instanced, our new object has already been extended.

After extending the new object, this will refer to both the new object
and the base object. Our new object inherits from Panel methods such as setHeader,
setBody, render as well as properties like body that can
then be accessed via this.

Sometimes we might want to override a method of the base object. Indeed,
we have already done that — by declaring the constructor of our custom
object, we are overriding the constructor of the base class. The extend
function places all original methods under a property named superclass, so
you still have access to all the functionality of the original method. The first
thing we do within the constructor of the new object is to call the constructor of
the base (superclass) object and, since that is a static function, we need to
adjust the scope of the superclass constructor so that it runs in the scope of our Loading Panel’s constructor. We use method call of JavaScript native Function
object, which takes the first argument as the execution scope of the function
called and passes it the rest of the arguments.  The YUI Librarydocumentation often uses the term "class" or "superclass" as a carry over from classical programming
languages. JavaScript doesn’t actually have classes (objects are derived from
one another and inheritance is prototypal instead of classical)  but the word "class" is loosely descriptive of the mechanism.

Calling the base class constructor does not need to be the first thing to do
in the constructor of the new class; in fact, it might not be done at all, depending on
the object you are defining. You are free to do it if and when you choose.
The sequence of first defining the new constructor and then extending it before
trying to create an instance of that object is mandatory.

Using the object we just created is easy:

if (!loadingPanel2) {
    loadingPanel2 = new YAHOO.SATYAM.LoadingPanel();

We simply create a new instance if none exists, and then we show it. We
don’t use the optional argument, a string that would become the id of the HTML
element that our widget will create. Since we don’t care to,
we leave it out and let our function make one up via YUI
‘s method generateId.

Creating and Using Shortcuts for Common YUI References

When looking at the example you might find that
most of the names of the basic YUI components are not spelled out in full.
That is because, close to the top of the source file, I have declared several
shortcuts — that is, local variables that hold references to those library
components. This has several advantages:

  • since the shortcuts are local variables inside a function they don’t trash
    the global namespace;
  • it is shorter to write;
  • it is easier on the interpreter since instead of, for example,
    looking for property Dom in object util within the object YAHOO,
    which is global and, thus, the last scope in which the interpreter will search for a
    symbol when all other local spaces fail, it simply looks for the shortcut,
    which is in the local scope, the first place it searches;
  • compression utilities such as the YUI
    are free to minify variables declared locally, something it
    cannot do with global ones because they could be referenced in other
    included files that might not be compressed.

So far, we’ve seen the mechanism YUI provides to make inheritance easy to
handle but we haven’t yet produced an actual library file. We’ll do it
YUI-style. The file LoadingPanel.js contains
a single component — basically the code we’ve seen so far. It first
declares the namespace to use:


Then, it puts all the code within an anonymous function, which gets
immediately executed (notice the empty parenthesis at the end). This
allows us to declare the shortcuts for the YUI components we will use within the
library without trashing the global namespace:

    var Dom = YAHOO.util.Dom,
        Event = YAHOO.util.Event,
        Panel = YAHOO.widget.Panel;

    // here goes the library contents itself


Right in the middle of that block we put the library code exactly as we’ve
seen it in the previous sample.

Registering a Component with YUI

Finally, at the very end we add the
following line:

YAHOO.register('SATYAM.LoadingPanel', YAHOO.SATYAM.LoadingPanel, {version: "0.99", build: '11'});

Method register stores the provided information in a well known place
for the entire YUI Library to access. This serves several purposes. Some YUI Library components can use optional components, usually to provide some fancy
behavior such as animated transitions, so knowing where to look for currently loaded components makes it easier to see whether your optional components are present.

Most important, though, is that if you use the YUI
, the register
function is the one it uses to check whether the load it has initiated finally
succeeds. The call to the register function should be the last thing you do in each source file; that line effectively tells YUI that the module you’re registering is now ready for use. The YUI Loader depends on this method invocation to know when your module is safe to use.

We use that library file in our second example
file. To load the library we use the following code:

var loader = new YAHOO.util.YUILoader();
    name: 'SATYAM.LoadingPanel',
    type: 'js',
    requires: ['container'],
    fullpath: 'LoadingPanel.js'
loader.require('reset', 'grids', 'base', 'SATYAM.LoadingPanel');

After we declare the instance of the YUI Loader that we will use, we call
method addModule providing the information about our library file.
We have to use exactly the same name we declared when we called method register
at the end of the library itself, we tell it it is a JavaScript file (the YUI
Loader can also load CSS files), we tell the dependencies it has and finally we
point to its location. Perhaps fullpath was not a good choice of a
name for the property since usually we will provide a relative path. What
it actually means is that it won’t be concatenated with the base path declared
elsewhere, which defaults to Yahoo’s servers.

Once the library information is added to the instance of the YUI
, we
can request it just as we would with standard YUI components. We just list it in
the array of modules we require for our application using the same name we’ve
given the module all along. Keep in mind that adding the module metadata via addModule does not actually load the module; it simply lets YUI Loader know it exists. It is the require method that
tells YUI Loader which modules we actually need in this page. When we
finally call method insert, the YUI Loader will go through all the
modules requested, find their dependencies, calculate the load order based on the dependency tree, start loading them and wait for the loaded modules
to call the register function to signal they are loaded and ready.

This same technique could be used to create your login boxes or other
standard popups or your own equivalents to the
browser’s native "accept" and "confirm" dialogs inheriting from Dialog and
SimpleDialog in
the YUI Container component. You can also use it to customize many other
standard objects: Rich Text Editors with your own set of toolbar tools and
functionality, a DataSource with your server communication options already set
up, your own application-wide menu and so on. These are all based on
existing YUI components that we simply customize to suit the environment of our

Developing Wholly New Widgets with YUI

So far we’ve been looking at how to customize an existing component using extend. What if we want to develop a completely new component?

Let’s say we want to develop a full set of input fields so we can build input
forms via JavaScript. Our basic input component will be an HTML <label>
coupled with an <input> element, plus an error indicator.

The YUI Element Utility as a Foundation for Component Development

Most YUI visual components (widgets) use a common foundation, the Element
. Element provides several features to help manage HTML elements; additional features are inherited
from AttributeProvider and EventProvider (for managing configuration attributes and custom events).
One of the main design goals of the YUI Library is that it should work the same in
all supported browsers (those in the A grade
). By wrapping an HTML
element with Element and using its methods instead of the raw DOM methods, which
can behave differently in each browser, you automatically achieve a degree of
cross-browser compatibility.

You can see the results in the file Fields.js.
By now you should be familiar with the envelope: We declare the SATYAM
namespace, enclose everything in an anonymous function that gets immediately
executed, we end it with a call to register, and within the anonymous function we
define the usual shortcuts. Though constants don’t exist in JavaScript, we
also declare a prefix for all the classNames of the HTML elements we will generate
and we call it CSS_PREFIX, all in uppercase, following the coding
convention of other languages which do support constants (which helps to make our intent more plain). We will be generous in giving CSS classNames to many
of the elements we create so as to allow the visual designer granular control
over the presentation without having to modify the program itself.

We have also included a CSS
. The stylesheet is indispensable for design purposes;
calling the sample page ‘plain’ when you don’t use
the stylesheet is being generous. You will notice in the sample page that
there are two fieldsets, and one is the mirror image of the other. The JavaScript and HTML used to produce those two forms are exactly the same; they differ only in the CSS class applied. Achieving this kind of presentation independence with plain <table> elements (a common fallback used to build form layouts before CSS became ubiquitous) would have
been impossible because the code and markup would have needed to be unique for each form.

There is very little to be done in the constructor:

var CSS_PREFIX = 'satyam_field_';
var TextField = function(oConfigs) {
   , document.createElement('div') , oConfigs);


YAHOO.SATYAM.TextField = TextField;

call the constructor of the object we inherit from: Element. (Note: We haven’t called extend yet in the code above; we’ll add that method in a moment.) As
we have seen before, YAHOO.lang.extend places all prototype members of the base
object under the superclass property, including its constructor. We call the constructor,
adjusting the scope to the object itself. Element requires a reference to
the HTML element for which it will provide a wrapper and an object containing any necessary configuration attributes.
We simply create an HTML <div> element on the spot when calling the base
constructor and we pass on the same configuration attributes we have
received to our own constructor.

Managing DOM Events and Custom Events with Element

Element already provides wrappers for many standard DOM events. By
inheriting from Element we
are already wrapping click, dblclick,
keydown, keypress, keyup, mousedown, mousemove,
mouseout, mouseover, mouseup, focus, blur and
submit on the div that we create for each TextField
instance. These events can be produced either by the DOM element wrapped in
Element or they may bubble
from the DOM elements that are descendants of the wrapped element. All of them can be
listened to by subscribing through the on method:

myFieldInstance.on("click", someFunction);

In this case, we will add a "custom event" of our own called updateEvent. Element inherits from EventProvider, which provides a standard mechanism to create, fire
and listen to custom events. Thus, handling of our
will be just like handling all actual DOM events; EventProvider will
make them all look alike. Actually, the call to createEvent is
optional. EventProvider is capable of creating events on demand when they are subscribed to. When the library fires the event, if the data structure
hasn’t been created, it means nobody is listening to the event. Here’s how you would subscribe to the updateEvent:

myFieldInstance.on("updateEvent", someFunction);

Note that the syntax is just the same as subscribing to the DOM-based click event as we saw above.

The TextField constructor function we just created is contained within the anonymous
function so it is invisible outside of that function — there is no way (yet) to access this constructor globally. It is handy to have
such a short name to refer to it and it is faster for the interpreter as well,
but we do want to make it public. We do that by copying the reference to that
object to YAHOO.SATYAM.TextField, which then becomes the public name for the
object under the namespace we created before.

A Closer Look at extend

Now, let’s take a look at how we extend our TextField object with Element:

Lang.extend(TextField, YAHOO.util.Element, {
    initAttributes: function (oConfigs) { /* ... */ },
    render: function (parentEl) { /* ... */ },
    destroy: function () { /* ... */ },
    getValue: function () { /* ... */ },
    setValue: function (newValue) { /* ... */ },
    _renderInputEl: function (containerEl) { /* ... */ }

As I noted above, the mechanism for inheritance can seem counterintuitive in that we
have to declare the inheritor constructor first and only then can we extend it
with the properties and methods of the base object. We do this via the YAHOO.lang.extend
method (Lang is the shortcut we declared for YAHOO.lang). We tell extend which object inherits from which. A third, optional, argument to extend is an object containing properties and methods that will also become part of the new object prototype (and will override members of the same name
inherited from the superclass, if present). Here, we are declaring several additional methods, which I’ve stubbed out for now — we’ll look at some of these more closely in a moment, and all of them appear in full in the functioning example. (If you want to learn a bit more about how extend works, take a look at the dedicated example and tutorial provided for this method and at its API doc reference.)

Element and AttributeProvider

Element (via AttributeProvider) provides a standard mechanism for dealing with managed configuration
attributes. It provides get and set methods to access them
and reads initial values automatically via the constructor’s oConfig
argument as we saw above. Element
already has a few attributes of
its own, but when we create an object based on it we will usually add some custom
attributes. Element creates and initializes its own in method initAttributes, we do the same with ours.

initAttributes: function (oConfigs) {
       , oConfigs);

    var container = this.get('element');

    this.setAttributeConfig('labelEl', {
        readOnly: true,
        value: container.appendChild(document.createElement('label'))
    this.setAttributeConfig('inputEl', {
        readOnly: true,
        value: container.appendChild(document.createElement('div'))

    this.setAttributeConfig('name', {
        writeOnce: true,
        validator: Lang.isString
    this.setAttributeConfig('id', {
        writeOnce: true,
        validator: function (value) {
            return /^[a-zA-Z][\w0-9\-_.:]*$/.test(value);
        value: Dom.generateId(),
        method: function (value) {
            this.get('inputEl').id = value;

    this.setAttributeConfig('label', {
        validator: Lang.isString,
        method: function (value) {
            this.get('labelEl').innerHTML = value;
        value: ''

    this.setAttributeConfig('value', {
        value: null

        validator: Lang.isFunction,
        value: function (value) {
            return false;

Our method initAttributes overrides Element’s own method; nevertheless,
the original initAttributes is not lost because extend has
copied all of the original members of Element’s prototype to Field’s superclass property. Within our initAttributes method, we first call
the original initAttributes, adjusting the scope to that of our own
object, and then we start defining our own attributes. We use method setAttributeConfig
to do so. It requires a name for the attribute and then a series of
optional arguments, some of which are shown in the codeblock above. An attribute can be readOnly,
writeOnce or, if not stated otherwise, readable and writable. It can
have a validator function that should return true if the value set is
valid. We use some of YUI’s own functions from YAHOO.lang, such as isString,
isBoolean or isFunction to validate some of
the attributes but we use our own function for the id attribute, since
the DOM
property cannot be just any string. Some of the attributes have initial values whereas
some will be left undefined. For id, the initial value will be produced by
method YAHOO.util.Dom.generateId which ensures that you get a valid and
unique id.

The final setAttribute property we want to look at is the method property. This property contains a function that will be executed when the configuration attribute is set. This, of course,
is nothing more than what a normal property setter method should do but, since
JavaScript does not provide for a standard and brief way to handle setters,
YUI’s AttributeProvider does so.

When we set an attribute to a new value (myComponent.set('someAttribute,'new value')),
if the attribute is not readOnly or if it is writeOnce it hasn’t
been written yet, the function in the validator property will be called, if present. If that returns true, the function in the method property will
then be called. All this doesn’t mean much to the end user who won’t see a
thing, but, if you load the -debug versions of Event and Element, YUI will
output log messages at each step in a Logger Control console or in the browser console.

In the sample above, we are using Dom.generateId() to produce a default id for inputEl
and we have a function set in the method property that assigns the id to inputEl.
However, the method function is not called for initial, default values, inputEl won’t get the
default value if not explicitly set.
We can ensure the method function gets called by using
AttributeProvider‘s refresh method sometime
after we have called Element‘s constructor:


The first argument is an array containing the names of the attributes to be refreshed, the second tells whether it
will trigger the notification events (more on them in the next paragraph), which you usually don’t want fired the first time
around so you set the silent argument to true.
AttributeProvider will then go through the
attributes you listed and call the method function for each of them.
Beware that if the attribute has been explicitly set in that oConfig argument we used above when creating the object,
the method function would have been called once
and calling refresh will call it once again so when using refresh be aware of this possible repetition

That, however, is not the whole story. The developer using your library
might want to respond to the setting of a configuration attribute. Though
possible, letting the developer using your library to override the method
or validator properties is risky.
AttributeProvider provides a couple of events for each attribute
the library user can listen to.
After the new attribute value has been validated and right before calling the
function assigned to the method property, AttributeProvider will fire a
beforeXXXXChange event, where the XXXX is the name of your attribute with its
first letter changed to uppercase. Thus, myComponent.set('something','whatever')
will fire the beforeSomethingChange event. If the function
listening to this event returns exactly false (not a falsy value
such as not returning anything), the setting will be cancelled, the value of the
attribute will remain unchanged and the function in the method property
will not be called. Thus, a function listening to the beforeXXXXChange
event may act as a validator function. If all listeners return a value that is not false, the validator returns true, and
the value is finally set,
will fire the XXXXChange event. Here the case of the
attribute name in the XXXX part is preserved; the string
‘Change’ is simply appended to it. The event listener would then act
pretty much like the function in the attribute’s method property.

There is one more feature of AttributeProvider that’s worth highlighting. When initializing an
object just created, quite often there are dependencies in between its settings (ie,
you can’t set one until you set some other). When you create an
object derived from Element, you will usually provide a set of attributes all at
once. If these attributes are not set in the right order, the object will
not initialize correctly. In AttributeProvider
the attributes
will be set in the order in which setAttributeConfig was called, not the
order in which the attributes appear in the call to the constructor. In the example
above, attributes name, id and label will be set in that
order, regardless of the order they appear on the object creation options (the
ones above them are either readOnly or are set as writeOnce and have been written
to). In this way you have
a predictable initialization sequence, regardless of the order in which configuration options are listed by your component’s user.

Adding a render Method

Calling Element‘s constructor and overriding initAttributes
are the only things we need to do to have all the benefits of inheriting from Element. From now on, we can do as we please. However, to keep some
consistency with the rest of the YUI Library we will provide a couple of more or
less standard methods, render and destroy (some of the older YUI components lack these methods, but going forward they are de rigueur). Method render
will create all the HTML elements that will make the component and will append
them to the document.

render: function (parentEl) {
    parentEl = Dom.get(parentEl);

    if (!parentEl) {
        YAHOO.log('Missing mandatory argument in YAHOO.SATYAM.TextField.render:  parentEl','error','Field');
        return null;
    var containerEl = this.get('element');
    this.addClass(CSS_PREFIX + 'container');
    var inputEl = this.get('inputEl');
    Dom.addClass(inputEl,CSS_PREFIX + 'input');

    var labelEl = this.get('labelEl');
    Dom.addClass(labelEl,CSS_PREFIX + 'label');


    this.on('updateEvent', function (oArgs) {
        var errMsg = this.get('validator').call(this, this.get('value'));
        if (errMsg) {
            Dom.addClass(inputEl,CSS_PREFIX + 'error');
            inputEl.title = errMsg;
        } else {
            Dom.removeClass(inputEl,CSS_PREFIX + 'error');
            inputEl.title = '';

Method render needs to know which DOM element to append our component
to. As with all YUI widgets, we accept either an actual DOM reference or the id
attribute of an HTML element. YUI Dom’s get method ensures
that we end up having a valid DOM reference. If parentEl is not a valid object
(possibly it doesn’t exist) then we return after sending an error message to the
YUI Logger. This is a good
way to provide errors to the developer without troubling the end user who can do
little about it anyway. If the Logger component is loaded and active, YAHOO.log
will write into it, otherwise, like in a production environment, it will do
nothing, which is in line with what browsers normally do in most cases (fail quietly and get
on with whatever happens next). For this example, I’ve made this call to YAHOO.log,
I’ve avoided further tests to keep the sample brief.

Calling YAHOO.log has another advantage: Such calls can be easily
stripped out from the source once finished and tested by a simple regular
expression search and replace. This partly explains the
three flavors of the YUI files. Each YUI component comes in a -debug,
a -min and a raw version, the first two being the suffixes in the file
names. The -debug version is full of calls to YAHOO.log and
it is good to use when you are debugging. The Logger
window lets you
filter messages by type so you can concentrate on what you want. The raw
version, with no suffix, is made from the -debug version with the calls to YAHOO.log
stripped out. The -min version is derived from this last one by using
the YUI Compressor. Then there are the aggregate files, the result of
concatenating several often-used groups of components together. These are
always minified even though they lack the -min suffix. We have used
one of those in the example: yuiloader-dom-event.js contains the YUI
, the YAHOO global object and the
Dom, Event and
Get utilities, all of
which are so basic that it is easier and faster to load them all in one
go. It is a good idea to adopt the same naming convention for your library
files. The YUI Loader has a filter property that allows you to select the
version of files you want, and it uses that naming convention to figure out
which file is which version.

Element will save the
object it wraps in property element. We fetch it and we append two
HTML elements to it, one a <label>
element and the other a <div> which will contain input
fields. None of these are attached to the document yet; it’s always a good idea
to delay appending complex HTML to the document until everything is ready,
because this prevents the browser’s rendering engine from trying to repaint the
page (which is both resource-intensive and can cause unwanted flickers). As
with all the rest of the HTML elements, we are generous about adding classNames
to every element. We get the references to each of the elements created in
initAttributes and add classNames to each. We also set the for
attribute of the <label> element to point to the input field.

To actually build the input element we call _renderInputEl. Each type of
element, text inputs/areas, radio buttons, and checkboxes will have its own
renderer, so we make this method separate so it can be overridden. We simply
call it, pass it its container element and trust it to do the right thing.

When all the HTML markup is done, we append it to the parentEl. We
delay this until the very end so that the browser will calculate the layout of
the page just once.

One of the configuration attributes for the TextField Control is a validator
function. That function is expected to return an error message to be shown to
the user or false if everything is ok. We subscribe to the updateEvent
and call the function set in the validator attribute, adjusting the scope
to that of our object, and pass it the value entered. If any error message
is returned, we add a className that presumably will highlight the field
and set the error message as the title (tooltip) for the field.

Method _renderInputEl starts with an underscore. This is a
convention to indicate a private field. JavaScript does not support
private variables; the underscore only means that the library user should
refrain from using it because it is not part of the component’s public API and may change in subsequent releases. Why not make it really invisible? By placing our shortcuts to
YUI inside an anonymous function we made them invisible to anything outside of
that function. Can’t we do the same with this? Unfortunately, the
trick we used with the shortcuts only works with static variables, not with
instance variables such as _renderInputEl. We cannot make it
invisible; thus, we can only signal the user our intent by using the underscore convention.

_renderInputEl: function (containerEl) {
    var input = containerEl.appendChild(document.createElement('input')); = this.get('name'); = this.get('id') ||;
    input.value = this.get('value');
    Event.on(input, 'change', function(oArgs) {
        this.fireEvent('updateEvent', {
            event: oArgs,
            target: input
    }, this, true);
    Event.on(input,'keyup', function (oArgs) {
        this.set('value', input.value);
    }, this, true);

Method _renderInputEl fills the container given with an actual input
element. For our base object it will simply be a textbox. We draw it
using the name, id and value saved elsewhere. We then
listen to two events:

  1. on a change, we fire event updateEvent,
    which we declared in the constructor so as to communicate that the we are leaving the field and the value has changed;
  2. we also
    listen to event keyup so as to keep the value stored for this
    field constantly updated.

Adding a Destructor

JavaScript does not have the concept of a destructor. When objects are
no longer used, there is no destructor for the garbage collector to call and
free resources. Unfortunately, most components create and/or make use of
external resources that, if not freed, will take increasing amounts of
memory (because garbage collection in JavaScript is not well implemented in some popular browsers). Though JavaScript does not support destructors, there is nothing
preventing us from declaring one and documenting it for the programmer using our

destroy: function () {
    var el = this.get('element');
    Event.purgeElement(el, true);

Our destroy method takes the HTML element that represents our component, uses

YUI Event
purgeElement method to delete all event listeners from it and to
recurse down through all children and finally remove the element itself.
This will take the HTML element away from the document tree. We still have to
delete the reference to our component, which cannot be done from within, that’s
up to the user.


To define any new component derived from this one, we use a similar process. Here’s how we might define a CheckboxField object based on the TextField object:

var CheckboxField = function(oConfigs) {, oConfigs);

YAHOO.SATYAM.CheckboxField = CheckboxField;

Lang.extend(CheckboxField, TextField, {

    initAttributes: function (oConfigs) {
        oConfigs = oConfigs || {};, oConfigs);
        this.setAttributeConfig('selected', {
            validator: Lang.isBoolean,
            method: function (value) {
                var c = this.get('checkboxEl');
                if (c) { c.checked = value; }
        this.setAttributeConfig('checkboxEl', {
            writeOnce: true
    _renderInputEl: function (containerEl) {
        containerEl.innerHTML += '<input type="checkbox" name="' + this.get('name') + 
            '" value="' + this.get('value') + '"' + (this.get('selected')?' checked ':'') + 
            ' id="' + this.get('id') + '" />';
        this.set('checkboxEl', containerEl.firstChild);
        Dom.addClass(containerEl, CSS_PREFIX + 'checkbox');
        Event.on(containerEl, 'click', function(oArgs) {
            var target = Event.getTarget(oArgs); 
            this.set('value', target.checked);
            this.fireEvent('updateEvent', {
                event: oArgs,
                target: target
        }, this, true);


In this case we are defining a field of type Checkbox which we base on our
previous TextField. Since this declaration is still within the outer
anonymous function, all previous shortcuts such as TextField are still

The constructor for our CheckboxField component simply calls the base
constructor. In other languages we might have skipped this declaration,
but in JavaScript we declare an object by declaring its constructor, even if it
is trivial. We
make our component public by copying a reference to YAHOO.SATYAM.CheckboxField
and then we make it an extension of TextField.

Finally, to change its behavior, we override a couple of methods of TextField.
We override initAttributes, which TextField itself overrode, to
add a couple more attributes. We then override _renderInputEl
to draw our checkbox element. For the checkbox field we fire the updateEvent
when we receive a click on the box.

The source code contains a definition for a RadioField
which is quite similar to CheckboxField.

Bundling Functionality: The Field Object and the Fields Object

Since input fields seldom go alone, it would be easier to create several of
them at once. First, we will reduce the number of different objects
we have to deal with. We will create a simple Field object that
will be able to create any type of field. To do that we will add a type
property to our configuration attributes, then a single Field object can
create any type of field.

var Field = function(oConfig) {
    var Constructor = oConfig && oConfig.type && Field.types[oConfig.type];
    if (Constructor) {
        return new Constructor(oConfig);
    return undefined;

YAHOO.SATYAM.Field = Field;

Field.types = {
    'text': TextField,
    'radio': RadioField,

This constructor is an unusual one in that it returns a value. Constructors
by default return a reference to the object being constructed. However, a
constructor can explicitly return something other than itself. Here, we
first locate the constructor of the field type requested; we verify that there
is an oConfig object, that that oConfig object has a type
property and that we have the constructor for that type in our types
table. If so, we return a new object of the type that that constructor creates;
otherwise, we return undefined. We make public that Field
object as YAHOO.SATYAM.Field and we also create the types table,
which is static, since there can only be one set of fields for the

A curiosity of this object, of no practical value whatsoever, is that since
it has no reference to this at all and the only property is static
instead of an instance property, it can either be called as a constructor, using
new, or as a plain function. These two statements produce the same
results, though we will prefer to use the new keyword for consistency:

var checkLeft = new YAHOO.SATYAM.Field({
    type: 'checkbox',
    name: 'check',
    label: 'Checkbox',
    selected: true,
    id: 'c4546'
var checkLeft = YAHOO.SATYAM.Field({
    type: 'checkbox',
    name: 'check',
    label: 'Checkbox',
    selected: true,
    id: 'c4546'

How come function Field can have a property types? In
JavaScript, all functions are instances of Function, so all functions are
objects — and as objects they can be augmented with additional properties such as types.

If the library user decides to create a new type of input field, it is easy
to incorporate it into the Field object:

YAHOO.SATYAM.Field.types['newTypeName'] = YAHOO.SATYAM.NewInputField;

This standardization in the way to create fields makes it easier for us to
create a set of fields all at once. We do it via the Fields object:

var Fields = function (oConfig) {
    this.fields = {};
    if (!Lang.isArray(oConfig)) oConfig = [oConfig];
    for (var f = 0; f < oConfig.length; f++) {
        var cfg = oConfig[f];
        var name =;
        if (!name || this.fields[name]) {
            YAHOO.log('Missing or duplicate field name','error','Field');
            return undefined;
        this.fields[name] = new Field(cfg);

YAHOO.SATYAM.Fields = Fields;

Fields.prototype = {
    render: function (container) {
        for (var name in this.fields) {
            if (this.fields.hasOwnProperty(name)) {
    destroy: function () {
        for (var name in this.fields) {
            if (this.fields.hasOwnProperty(name)) {

The Fields object will take an array of configuration attributes (as does Field), or it can even take a single object. It first
checks that the argument is an array and, if not, makes it one. It
then loops through its elements reading the name property. It will store
the fields created in the fields object, using the name as the key. If there
is no name property or the name is already in use, it will issue an error in the

YUI Logger
and return nothing. If the name exists and it is unique, it will
create the field using the Field object seen above, each with its own cfg
attributes and file it under the field name. As usually, once the
constructor is defined, we make it public. We then define two methods, render
and destroy, which simply loop through the individual fields and call the
corresponding method in each.

In the example code, we build textLeft and radioLeft
via their corresponding field objects, TextField and RadioField. Then we create checkLeft using the generic Field object and the three
fields in the second, mirrored fieldset all at once via Fields.

Actually, the Field object and its types table provide such a simple
and flexible mechanism that we can add to the Field.types object any other object
with a similar interface. So far we have been handling individual fields
but since Fields allows us to define sets of input elements, we could add
a family of components to contain such groups of fields. The base object could be a Group object
that draws a <div> element allowing us to manipulate hiding,
disabling or styling for the whole group at once. A fields
configuration attribute in this Group
object would contain the definitions for the fields contained within it.

From Group we could derive a FieldSet object that would draw a <FieldSet> instead of a <div>
and would also have a legend configuration attribute. We could also derive a FoldingGroup
or FoldingFieldSet which would allow these groups to be collapsed or
expanded, optionally using the Animation
utility, if loaded. The ultimate object derived from Group would be the Form
which would also contain methods for form submittal and hooks for form-level
validation. The code for this family of objects is not included since it
would not show any relevant new techniques for the purpose of this article.

Progressive Enhancement

If we want to reach the widest user population, we have to code for the
widest variety of browsers, something the YUI Library helps us with, but we also
have to consider those users who don’t have JavaScript enabled. The YUI
Library supports those users via ‘progressive enhancement’. It assumes
that, since there won’t be any JavaScript active to draw the page, the server
will provide a functional page in the traditional web style of a full-page
reload per transaction. As a library developer we have to consider that
our library might be used in those two circumstances, when we are fully in
charge of drawing the elements on the page — as we have seen so far — or when we
have a page with elements already in it and our library is used to enhance it,
improving its appearance and providing extra functionality such as client-side
validation and submission via XHR.

The most important enhancement in this process is adding listeners for all
the events our library captures, which makes the page active and
responsive. We would need to listen to form submission and handle it via
XHR after passing through the configured validation. We would listen to
changes in each of the fields and validate the input in each. We may add
buttons to show calendars on the date input boxes, conditionally hide/show or
enable/disable groups of controls based on inputs elsewhere in the form, offer
autocomplete and so on.

If we mean to support progressive enhancement our input fields library has to be capable
of finding out if the fields are already in the page and only if they are not
there, draw them. We cannot be expected to support any arbitrary format
for the fields we are meant to enhance. To start with, fields in a passive
web page make no sense without an enclosing form so we will only enhance full
forms. This makes our task easier since fields within forms don’t make
sense without a name (they could not be submitted otherwise) and that name
is the key to locate them in the DOM as form['name'] or
So, our Fields.render method would have to first find out if the
container is an HTML form and, if so, for each input element, we would try to
locate a field with that name and enhance it with the rest of the information in
the field configuration attribute. We might
also try to locate the <label> elements pointing to our fields and
also enhance them with the corresponding classNames and do likewise for the <div>
that should enclose them both.

We’re not going to delve further into this subject for the purposes of this already lengthy article. However, progressive enhancement has to be considered in the general
design of the library if we want it to reach such a broad audience as the YUI
Library does. An excellent use of this concept can be seen in the YUI Menu
component. The markup that the component can parse makes a perfectly
functional menu should JavaScript be disabled. On the other hand, the Calendar
component does not read existing markup since the functionality it provides
could not be handled without JavaScript. As you build your own components, you’ll need to make important decisions about which ones require progressive enhancement and which ones make sense only in a JavaScript-enabled environment.

CSS Sprites

There is only one further technique which I have not mentioned. When
signaling errors on the input fields I add a className to the container.
The visual designer may use this class to set any visual clue to the user.
In this case, an alert icon is shown ().
This icon is not added as an <img> tag to the field but as a
non-repeating background image in the container which has a padding set on the
corresponding side (depending on the skin used) to make space for it. The
visual designer might have changed the color of the field itself or use any
other visual clue; the code is, once again, completely agnostic in relation to
visual presentation.

If several of these icons need to be packaged with the component, a further
trick might be used: sprites. You can see a sprite
that comes with the YUI standard skin. It is a collection of graphical
elements of small size which can be used in backgrounds, some repeating, such as
shades for bars, some non-repeating like in the example above, each one
individually accessible by indicating the offset from the edge of the set (see: background-position).
This saves on connections to the server. Instead of loading each of the graphic
elements when required you get all the graphic elements within the sprite in
just one download. The size of the whole sprite is so small that it doesn’t hurt
to load something you might not use…it is better in most cases to have a slightly larger image download for the single sprite than to have all the extra HTTP requests required for multiple images.

Final Thoughts

JavaScript is a very flexible language which is very good for the component
developer, but it can be tough for those using the components if the component
developer does not follow some common patterns. The YUI Library provides a useful
component-development paradigm with EventProvider for managed custom events and AttributeProvider for managed attributes; this article describes that paradigm, which characterizes YUI’s most recent UI controls.

Author’s Note: This article was done with the help of two Erics. One is Eric Abouaf, whose inputEx
library of controls, featured in the YUI
, was the reason to write this article. I realized there was no
documentation to guide library developers on how to write a library. I
have also used, with permission, his idea of a widget to draw sets of input
fields. The code for the article, though (made for clarity rather than
performance or completeness) was done anew. The other is Eric
Miraglia who made significant edits and suggestions over my initial draft.


  1. Outstanding!
    This is the type of information I really want.
    When is your book coming out?

  2. Dear friend.

    This post is great. So, do you have any library developed by you?

    My blog about YUI (in portuguese):

  3. Superb! Another outstanding post.
    I am starting a new web project and this information is really vital to start.
    Thanks for the post!

  4. Louis and Felipe

    Nope, there is no book coming and no library of general use I could publish. Actually, the article documents how most of the newest YUI components themselves are coded so, as for examples, there are plenty, YUI itself!

  5. Great article ! (Although I still miss some informations about AttributeProvider, such as how the attribute are inherited, set at startup, etc…)

    I expect many YUI library will popup…

  6. This is great – but why do all that work? Just to have it fit into the YUI framework? I’ve created many YUI-based components that are re-usable as javascript objects but don’t require all of the overhead of extending other objects, needing the class loader, etc. Most of them are under 400 bytes long!

  7. Jason,

    The recommendations above are not meant for your library to ‘fit into the YUI framework’. If you will never share your library with anyone else, you are certainly free to do it whichever way you want. However, some people do care to make their libraries public.

    The above recommendations are just a summary of what you can see in the source code of the most recent YUI visual components (widgets). They are in no way arbitrary, they are the result of perfecting the library over and over again with each release, and there has been many of those.

    If you don’t feel any of the above is of any use whatsoever for you, feel free to ignore.

  8. Sudesh Chandra said:
    June 30, 2008 at 3:42 pm

    Dear Satyam,
    Thanks for the superb post. The YUI Widgets are definitely a perfect example of the fine tuning and perfections that have been iterated over the releases. Outstanding quality by all means.

  9. Thanks for a very informative article. I was able to accomplish my goal based on this great information.

  10. The widgets in the YUI release appear to be at varying stages of following this pattern. Is there any particular widget that is a best example of what you consider to be the current best practises for building a widget?

    I also note many YUI widgets extend YAHOO.util.Element with an init method. This post makes no reference to that method. Is this fair game to make use of when building our own widgets?

  11. SiggyTheViking said:
    August 12, 2008 at 7:50 am

    Hello, Satyam, and thanks for the wisdom.

    There is as particular bit of code that I don’t understand:

    var Constructor = oConfig && oConfig.type && Field.types[oConfig.type];

    What does this do? Does it assign a ref to oConfig to Constructor if oConfig has .type, and a .type in .types?

    Does it only happen when yo call new?

    I am very fond of the so called LISP in c’s clothing (javascript), but I don’t get this one yet.


  12. Siggy,

    The wisdom is that of those who designed it, I just wrote about it and was amazed myself of all the goodies in it.

    So, regarding your question, the && operator returns false if the left-hand-side expression is false or the value of the right-hand-side expression. This is important, it will not return true, it will return the value of the right-hand-side expression.

    If the right-hand-side expression involves a further &&, it will keep going in the same manner. In other words, a chain of && operators will return false if any one of the expressions evaluates to false, otherwise, it returns the value of the right-most expression.

    It is also important to know that it stops any further evaluation as soon as it gets a false. Anything after the first false will not even be bothered with.

    So, the above reads, more or less, if there is an oConfig (which is not null nor undefined) and that oConfig has a type property (not null nor undefined) give me the constructor stored in the Fields.types array under that oConfig.type key.

    In the line next to that one it checks whether there was such a constructor and calls it if there was one. Actually, that conditional could have been merged into this chain of &&’s. The body of the whole function could have been reduced to a single line starting with “return oConfig && …”. It would have been inscrutable for the purpose of the article but it is a tempting challenge.

    You want to try?

  13. Worth pointing out that as of YUI 2.6.0, the container family (Module, Overlay, Tooltip, Panel, Dialog, SimpleDialog) do not extend Element, meaning that config properties are not managed in the fashion described in this article.

    For example, if you extend Panel and then try to wire up your custom attributes by extending initAttributes() your custom attributes won’t get added because Panel does not call initAttributes. Instead of calling this.setAttributeConfig(), you need to use calls to this.cfg.addProperty() somewhere else, such as inside an overriden init method.



  14. sagar dhinooja said:
    December 13, 2008 at 9:26 am


    i am using a yahoo my project
    i put one text box after the text box i put
    one image when i am clicking image it doesn’t
    show me a calendar anybody can help how can i display calender by clicking on the image?

  15. In the third example where you have your own branch of the YAHOO namespace on you’re doing the constructor, you have this line:,

    Is it correct that the line could be replaced like this:,

    It took me a while to find a way to use “this” instead of YAHOO…….. to start that line off with, but I’m not sure if that’s “theoretically” correct. It did work for me though lol.

  16. Floydian,

    You are correct when B inherits from A but if C inherits from B, all breaks apart. If C calls its superclass constructor it will get B’s constructor. But then, B constructor will call ‘this’ superclass constructor and it so happens that in this case, ‘this’ points no longer to B but to C so B’s constructor will end up calling C’s superclass constructor which is itself.

    This means that in each subclass you have to be absolutely specific about which superclass method you are calling and not only for constructors. Relative references via ‘this’ work, but not too far.

    How I came to learn this? Thanks to DataSource. Try to inherit from any of its type-specific DataSources: LocalDataSource, XDRDataSource, &c., they all inherit from DataSourceBase and they all call their superclass constructor via ‘this’, they work, but if you make a subclass of any of them, it will hang in a loop as soon as instanced.

  17. Doesn’t work anywhere except on your server.
    Where are the examples?

  18. Hi,
    I finally got inheritance working. I was also able to create my own “widget”. However I am unable to mix Scriptaculous syntax with YUI. Examples:
    1) Unable to create a new scriptaculous “Element”
    var elem = new Element(“div”,{class:”yui-g”});
    2) Unable to use Scriptaculous inheritance syntax
    with yui
    MyView = Class.create(YAHOO.widget.Panel,{


    then elsewhere var myView = new MyView(args);

  19. I am using the latest version of YUI. So far it is awesome! I am using the template found in the “menu” folder. The problem I am having is getting it to accept and 3rd level of menu? I see the nested template and am thinking it is possible, but I cannot seem to figure this one out.


  20. @Charles Scott

    A better place to ask this type of support question would be the YUI forum at It’s a lively community of YUI developers who will likely be able to give you some advice.


  21. Hi,

    According to Element documentation one should defer dom manipulation with Element. So am I right that the call to render method should be deferred? What is the best concept for widget rendering? I tried to look through widgets, but they are implemented in different ways.

    Thank you!

  22. Element has many of the methods that help in dealing with a widget but it is not a widget itself, it is not in the YAHOO.widget branch but in YAHOO.util, and it doesn’t have a render method or a destroy one. It really doesn’t deal with the lifetime of the widget as a whole but it helps with plenty of common tasks such as configuration and event handling.

    All the newest widgets are derived from Element. The container family uses a Config object, a forerunner of Attribute and AttributeProvider, earlier widgets as TreeView barely use anything at all. In YUI3, Element turns into Node though far more powerful.

    So, Element is the way to go.

  23. Dennis McCarthy said:
    August 25, 2009 at 6:27 am

    I notice that in some cases you use configuration attributes, while in other you use ordinary JavaScript properties (e.g., the fields property of the Fields control). How do you decide when to use a configuration attribute and when to use a property?


  24. Dennis,

    Good question, I should have addressed that. In the future that won’t be an issue any more, ECMAScript 5 will have setters and getters on all properties and many browsers already support that via proprietary solutions, and there are very good reasons for that.

    Check out the the properties of Attribute, they provide getter, setter, validator and conversion functions, all features that might be desirable for a regular property but that the language, so far, does not provide.

    Currently, properties are totally passive. If you have, say, a width property, the change won’t be reflected until the widget is redrawn, if you have a width attribute, the change can be effected immediately.

    In the case you mention, the array is like a table of constants, it is not meant to be changed except, possibly, for developers who might add other field types. For the developer using the library, it is meant to be read-only, as if it was a constant, something JavaScript currently does not provide. It could have been declared (not really, but just for the sake of an example) as an attribute with its readOnly flag set, but that would have complicated access to it.

    In JavaScript we lack constants and private variables, we rely on naming conventions or documentation to declare the intent so, in the case of field types, we trust they’ll be properly used.

  25. Is it possible to use this technique to create a component that internally uses a layout to organize its HTML elements? If it is, do I call the layout’s render method in my component’s render method?

  26. Stephen Nelson said:
    April 9, 2010 at 6:40 pm

    I’ve been attempting to follow this example to create my own custom YUI component. At least with YUI 2.8, the initAttributes() method on Element appears to be an empty stub which does not call its superclass. I can work around it by providing the oConfigs values to the attributeConfig ‘value’ param directly. Was this changed?