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Defining Elements

Basic elements

To define a custom element, begin by creating a class that extends FASTElement and decorate it with the @customElement decorator, providing the element name.

Example: A Basic FASTElement Definition

import { FASTElement, customElement } from '@microsoft/fast-element';

export class NameTag extends FASTElement {


With this in place, you can now use your name-tag element anywhere in HTML with the following markup:

Example: Using a Web Component


Web Component names must contain a - in order to prevent future conflicts with built-in elements and to namespace components from different libraries. For more information on the basics of Web Components see this set of articles.


HTML has a few special tags known as "self-closing tags". Common examples include <input> and <img>. However, most HTML elements and all web components must have an explicit closing tag.

We've got a basic Web Component in place, but it doesn't do much. So, let's add an attribute and make it render something.

Example: Adding Attributes to a FASTElement

import { FASTElement, customElement, attr } from '@microsoft/fast-element';

export class NameTag extends FASTElement {
@attr greeting: string = 'Hello';

// optional method
greetingChanged() {

To add attributes to your HTML element, create properties decorated by the @attr decorator. All attributes defined this way will be automatically registered with the platform so that they can be updated through the browser's native setAttribute API as well as the property. You can optionally add a method with the naming convention propertyNameChanged to your class (e.g. greeting and greetingChanged()), and this method will be called whenever your property changes, whether it changes through the property or the attribute API.


All properties decorated with @attr are also observable. See observables and state for information about how observables enable efficient rendering.

By default, anything extending from FASTElement will automatically have a ShadowRoot attached in order to enable encapsulated rendering.

To see it in action, you can use the same HTML as above, or change the default greeting with the following:

Example: Using a Web Component with Attributes

<name-tag greeting="Hola"></name-tag>

Customizing attributes

By default, any attribute created with @attr will perform no explicit type coercion other than when it reflects its value to the HTML DOM via the setAttribute API. However, you can convert DOM attribute string values to and from arbitrary types as well as control the mode that is used to reflect property values to the DOM. There are three modes available through the mode property of the attribute configuration:

  • reflect - The default mode that is used if none is specified. This reflects property changes to the DOM. If a converter is supplied, it will invoke the converter before calling the setAttribute DOM API.
  • boolean - This mode causes your attribute to function using the HTML boolean attribute behavior. When your attribute is present in the DOM or equal to its own name, the value will be true. When the attribute is absent from the DOM, the value of the property will be false. Setting the property will also update the DOM by adding/removing the attribute.
  • fromView - This mode skips reflecting the value of the property back to the HTML attribute, but does receive updates when changed through setAttribute.

In addition to setting the mode, you can also supply a custom ValueConverter by setting the converter property of the attribute configuration. The converter must implement the following interface:

interface ValueConverter {
toView(value: any): string;
fromView(value: string): any;

Here's how it works:

  • When the DOM attribute value changes, the converter's fromView method will be called, allowing custom code to coerce the value to the proper type expected by the property.
  • When the property value changes, the converter's fromView method will also be called, ensuring that the type is correct. After this, the mode will be determined. If the mode is set to reflect then the converter's toView method will be called to allow the type to be formatted before writing to the attribute using setAttribute.

When the mode is set to boolean, a built-in booleanConverter is automatically used to ensure type correctness so that the manual configuration of the converter is not needed in this common scenario.

Example: An Attribute in Reflect Mode with No Special Conversion

import { FASTElement, customElement, attr } from '@microsoft/fast-element';

export class NameTag extends FASTElement {
@attr greeting: string = 'Hello';

Example: An Attribute in Boolean Mode with Boolean Conversion

import { FASTElement, customElement, attr } from '@microsoft/fast-element';

export class MyCheckbox extends FASTElement {
@attr({ mode: 'boolean' }) disabled: boolean = false;

Example: An Attribute in Reflect Mode with Custom Conversion

import { FASTElement, customElement, attr, ValueConverter } from '@microsoft/fast-element';

const numberConverter: ValueConverter = {
toView(value: any): string {
// convert numbers to strings

fromView(value: string): any {
// convert strings to numbers

export class MyCounter extends FASTElement {
@attr({ converter: numberConverter }) count: number = 0;

The element lifecycle

All Web Components support a series of lifecycle events that you can tap into to execute custom code at specific points in time. FASTElement implements several of these callbacks automatically in order to enable features of its templating engine (described in declaring templates). However, you can override them to provide your own code. Here's an example of how you would execute custom code when your element is inserted into the DOM.

Example: Tapping into the Custom Element Lifecycle

import { FASTElement, customElement, attr } from '@microsoft/fast-element';

export class NameTag extends FASTElement {
@attr greeting: string = 'Hello';

greetingChanged() {
this.shadowRoot!.innerHTML = this.greeting;

connectedCallback() {
console.log('name-tag is now connected to the DOM');

The full list of available lifecycle callbacks is:

constructorRuns when the element is created or upgraded. FASTElement will attach the shadow DOM at this time.
connectedCallbackRuns when the element is inserted into the DOM. On first connect, FASTElement hydrates the HTML template, connects template bindings, and adds the styles.
disconnectedCallbackRuns when the element is removed from the DOM. FASTElement will remove template bindings and clean up resources at this time.
attributeChangedCallback(attrName, oldVal, newVal)Runs any time one of the element's custom attributes changes. FASTElement uses this to sync the attribute with its property. When the property updates, a render update is also queued, if there was a template dependency.
adoptedCallbackRuns if the element was moved from its current document into a new document via a call to the adoptNode(...) API.

Working without decorators

The examples above and those throughout our documentation leverage TypeScript, and in particular, the decorators feature of the language. Decorators are an upcoming feature planned for a future version of JavaScript, but their design is not yet finished. While the syntax for decorator usage is not likely to change in the final version of the feature, some of our community members may feel uncomfortable using this feature at this stage. Additionally, since decorators are transpiled into code that uses helper functions (both in TypeScript and Babel) the compiled output will be larger than the equivalent non-decorator code.

While there are size implications of using decorators prior to full language support, they do present the most declarative and readable form of the API, and we recommend their use for the average project. To strike a balance between declarative readability and size, we recommend that TypeScript be used in combination with the "importHelpers": true compiler option. When this option is set, instead of generating helper functions for decorators into every file, TypeScript will import a set of shared helpers published in the tslib package.

For those that require the smallest possible builds, FAST Elements can be completely defined in Vanilla JS, without using decorators, by leveraging a static definition field on your class. The definition field only needs to present the same configuration as the @customElement decorator. Here's an example that shows the use of the definition field along with a manual call to define the element:

import { FASTElement, html, css } from '@microsoft/fast-element';

const template = html`...`;
const styles = css`...`;
const converter = { ... };

export class MyElement extends FASTElement {
static definition = {
name: 'my-element',
attributes: [
'value', // same attr/prop
{ attribute: 'some-attr', property: 'someAttr' }, // different attr/prop
{ property: 'count', converter } // derive attr; add converter

value = '';
someAttr = '';
count = 0;


The definition can also be separated from the class and passed into the define call directly if desired. Here's what that would look like: FASTElement.define(MyElement, myDefinition);