Developing a Component

This guide explains the techniques involved in the ongoing development a Spectrum control as a spectrum-web-components.

The components in spectrum-web-components are based on the CSS definitions in spectrum-css. Typically, component implementations contain very little code. The CSS from the spectrum-css project typically specifies most, if not all, of the presentation details.

What is a web component? #

According to, web components are:

... a set of web platform APIs that allow you to create new custom, reusable, encapsulated HTML tags to use in web pages and web apps. Custom components and widgets build on the Web Component standards, will work across modern browsers, and can be used with any JavaScript library or framework that works with HTML.

In order to add a new component to this library, you will need to develop a working knowledge of the following technologies:

Setting up the styling #

The most complicated part of implementing a Spectrum web component is getting the styles set up correctly. The shadow DOM is the heart of a web component. It isolates the component from the styles and DOM of the containing page. While this offers many benefits, it also means that we must structure our CSS very differently.

The CSS from the spectrum-css project is intended to be installed globally on a web page. Using it in the context of a web component requires that we modify it. To facilitate that, this project comes with a config-driven processor that can transform the Spectrum CSS into a format that can be consumed in a web component.

The first step is to create a directory and a spectrum-config.js file for your new component. This config file contains information about the structure of the web component in relation to the Spectrum CSS classes.

Below is a fragment of the spectrum-config.js file for sp-button.

const config = {
conversions: [
inPackage: '@spectrum-css/button',
outPackage: 'button',
fileName: 'button',
excludeByComponents: [builder.element('a')],
components: [
converter.classToAttribute('is-disabled', 'disabled'),
converter.pseudoToAttribute('disabled', 'disabled'),
['spectrum-Button--sizeS', 's'],
['spectrum-Button--sizeM', 'm'],
['spectrum-Button--sizeL', 'l'],
['spectrum-Button--sizeXL', 'xl'],
converter.classToSlotted('spectrum-Icon', 'icon'),
find: [
replace: [
replace: builder.attribute('name', 'icon', 'equal'),
hoist: false,
hoist: false,
find: builder.pseudoClass('empty'),
replace: builder.attribute('hidden'),

If we wanted to create a button component using this config file, the steps would be as follows:

  1. Make the directory src/components/button
  2. In that new directory, create a spectrum-config.js file with the above contents
  3. Run the command yarn process-spectrum to create the CSS file

When you do the above, the config-driven processor will look in the spectrum-css project for the matching CSS file. It will parse that file and restructure the CSS as per the configuration instructions.

Structure of a Spectrum Web Component #

If you look at an sp-button in the Chrome developer tools, you will see a DOM structure that looks like this.

<sp-button tabindex="0" variant="accent">
#shadow-root (open)
<button id="button" tabindex="0">
<div id="label>
▼ <slot>
↳ #text
Click Me"

If anything here looks unfamiliar, it is probably a good time to do some reading about web components.

You can compare this markup with the reference markup in the spectrum-css documentation

Host Class Mapping #

We need to determine what the main CSS class is for our component in the original spectrum-css. In the case of sp-button, we can see that the top-level class is .Spectrum-Button. We then need to determine where we want that CSS to be applied. In many cases, you will want that CSS to be applied to the actual web component via the :host selector. That is the default behaviour of the conversion script. In this case, we wanted to preserve all of the default behaviour of the button element in HTML. So, we want the main CSS to be applied to our <button> instead. If you look at the host definition in spectrum-config.js you can see that we have supplied a shadowSelector option. That tells the script to move all of the CSS for .Spectrum-Button to the #button element in the shadow DOM.

host: {
selector: '.spectrum-Button',
shadowSelector: '#button',

Shadow DOM Structure #

The next step is to fill out the remaining structure of the shadow DOM portion of the component. Note that, in the shadow DOM, we are using ids instead of long class names. We can do that because the namespace of each instance of our web component has it's own DOM scope. So, there can never be an id name collision.

Typically, you will reference the sample code from the spectrum-css documentation and recreate that structure in the shadow DOM of your component.

In the case of sp-checkbox, we turn this sample DOM code:

<label class="spectrum-Checkbox">
<input type="checkbox" class="spectrum-Checkbox-input" id="checkbox-0">
<span class="spectrum-Checkbox-box">
<svg class="spectrum-Icon spectrum-UIIcon-CheckmarkSmall spectrum-Checkbox-checkmark" focusable="false" aria-hidden="true">
<use xlink:href="#spectrum-css-icon-CheckmarkSmall" />
<svg class="spectrum-Icon spectrum-UIIcon-DashSmall spectrum-Checkbox-partialCheckmark" focusable="false" aria-hidden="true">
<use xlink:href="#spectrum-css-icon-DashSmall" />
<span class="spectrum-Checkbox-label">Checkbox</span>

into this code in our component's render method (actually implementation is slightly different):

return html`
<label id="root">
<span id="box">
<span id="label"><slot></slot></span>

You will notice that many of the spectrum-css classes are mapped to ids in the web component. For example, .spectrum-Checkbox-input and .spectrum-Checkbox-box become #input and #box. Those transformations are described in the ids section of the spectrum-config.js file.

ids: [
selector: '.spectrum-Checkbox-input',
name: 'input',
selector: '.spectrum-Checkbox-box',
name: 'box',
selector: '.spectrum-Checkbox-checkmark',
name: 'checkmark',
selector: '.spectrum-Checkbox-partialCheckmark',
name: 'partialCheckmark',
selector: '.spectrum-Checkbox-label',
name: 'label',

Properties and Attributes #

Most of our controls have options that affect how they are rendered. For example, Spectrum supports a number of different kinds of buttons (e.g primary, secondary or call-to-action). spectrum-css supports these visual styles using CSS classes. In web components, we typically support these options using attributes/properties on the component. For example, here is a call-to-action style button.

<sp-button variant="accent">CTA</sp-button>

We could conditionally add CSS classes to the elements of the shadow DOM during rendering, but it is much easier to just let the attributes on the DOM node drive the styling directly. In order to facilitate that, the spectrum-config.js file lets you specify how to map the various spectrum-css classes to CSS that is based on the attributes on the :host of the web component.

attributes: [
type: 'boolean',
selector: '.spectrum-Button--quiet',
type: 'boolean',
selector: ':disabled',
type: 'enum',
name: 'variant',
values: [
name: 'negative',
selector: '.spectrum-Button--warning',

We support two different kinds of attributes, booleans and enums. Booleans are turned on or off by the presence or absence of the attribute. Enum attributes must be set to one of the allowed values. The CSS generated will reference the attributes on the host: element directly.

Class to Class Mapping #

In some cases, you will need to retain the spectrum-css classes as classes. An example of that is when you need to apply CSS rules to multiple items in the shadow DOM. In that case, we simply map class names to shorter classnames. There is an example of remapping classes in the slider component.

classes: [
selector: '.spectrum-Slider-track',
name: 'track',

Slots #

Slot tags are how we host our child content (light DOM) in our component's shadow DOM. The spectrum-css for a component sometimes contains rules for laying out the child content. There is a slots section in the spectrum-config.js file for mapping those rules to the slotted content.

slots: [
name: 'icon',
selector: '.spectrum-Icon',

The above section tells our CSS processor to map CSS for the .spectrum-Icon class to the content that is being hosted in the slot with the name icon.

Coding the Component #

All of the spectrum-web-components are written using the lit-element framework and Typescript. Your best bet is to look at similar components and match the style.

We have a working specification for the APIs for each of the Spectrum components. If you file an issue for the component that you want to implement, we can provide the necessary specifications for it.

Documenting the component #

The documentation for each component in the documentation site is adopted from the in said package. The pages are written in Markdown. See one of the existing pages for an example.

To run the local documentation server, use the command:

yarn docs:start

The documentation automatically extracts the properties and attributes from the source code. You should document your component using the appropriate jsdoc tags. See Button.ts for an example.

Working with Storybook #

We use Storybook for developing our components. This gives us a rapid way to test our components in various configurations. The best way to start is to copy one of the existing stories.

To run Storybook, use the command:

yarn storybook