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Declaring Templates

Basic templates

While you can create and update nodes in the Shadow DOM manually, FASTElement provides a streamlined templating system for the most common rendering scenarios. To create an HTML template for an element, import and use the html tagged template helper and pass the template to the @customElement decorator.

Here's how we would add a template for our name-tag component that renders some basic structure as well as our greeting:

Example: Adding a Template to a FASTElement

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

const template = html<NameTag>`
<div class="header">
<h3>${x => x.greeting.toUpperCase()}</h3>
<h4>my name is</h4>

<div class="body">TODO: Name Here</div>

<div class="footer"></div>

name: 'name-tag',
export class NameTag extends FASTElement {
@attr greeting: string = 'Hello';

There are several important details in the above example, so let's break them down one-by-one.

First, we create a template by using a tagged template literal. The tag, html, provides special processing for the HTML string that follows, returning an instance of ViewTemplate.

Within a template, we provide bindings that declare the dynamic parts of our template. These bindings are declared with arrow functions. Because the template is typed, the input to your arrow function will be an instance of the data model you declared in your html tag. When the html tag processes your template, it identifies these dynamic expressions and builds up an optimized model, capable of high-performance rendering, and efficient, incremental batched updates.

Finally, we associate the template with our custom element by using a new form of the @customElement decorator, which allows us to pass more options. In this configuration, we pass an options object specifying the name and the template.

With this in place, we now have a name-tag element that will render its template into the Shadow DOM and automatically update the h3 content whenever the name tag's greeting attribute changes. Give it a try!

Typed Templates

Your templates can be typed to the data model that they are rendering over. In TypeScript, we provide the type as part of the tag: html<NameTag>. For TypeScript 3.8 or higher, you can import ViewTemplate as a type:

import type { ViewTemplate } from '@microsoft/fast-element';

const template: ViewTemplate<NameTag> = html`
<div>${x => x.greeting}</div>

Understanding bindings

We've seen how arrow functions can be used to declare dynamic parts of templates. Let's look at a few more examples to see the breadth of what is available to you.


To bind the content of an element, simply provide the expression within the start and end tags of the element. It can be the sole content of the element or interwoven with other elements and text.

Example: Basic Text Content

<h3>${x => x.greeting.toUpperCase()}</h3>

Example: Interpolated Text Content

<h3>${x => x.greeting}, my name is ${x =>}.</h3>

Example: Heterogeneous Content

${x => x.greeting}, my name is
<span class="name">${x =>}</span>.

Dynamic content is set via the textContent HTML property for security reasons. You cannot set HTML content this way. See below for the explicit, opt-in mechanism for setting HTML.


You can also use an expression to set an attribute value on an HTML Element. Simply place the expression where the value of the HTML attribute would go. The template engine will then use your expression to set the value using setAttribute(...), whenever it needs to be updated. Additionally, some attributes are known as boolean attributes (e.g. required, readonly, disabled). These attributes behave differently from normal attributes and need special value handling. The templating engine will handle this for you if you prepend the attribute name with a ?.

Example: Basic Attribute Values

<a href="${x => x.aboutLink}">About</a>

Example: Interpolated Attribute Values

<a href="products/${x =>}">
${x =>}
<li class="list-item ${x => x.type}">

When binding to class, the underlying engine will not over-write classes added to the element via other mechanisms. It only adds and removes classes that result directly from the binding. This "safe by default" behavior does come at a slight performance cost. To opt-out of this feature and squeeze out every ounce of performance by always overwriting all classes, use a property binding (see below) on the className property. e.g. :className="list-item ${x => x.type}".

<span style="text-decoration: ${x => x.done ? 'line-through' : ''}">
${x => x.description}

Example: ARIA Attributes

<div role="progressbar"
aria-valuenow="${x => x.value}"
aria-valuemin="${x => x.min}"
aria-valuemax="${x => x.max}">

Example: Boolean Attributes

<button type="submit" ?disabled="${x => !x.enabled}">Submit</button>

Example: Nullish value

Some cases may occur where an attribute needs to have a value when used however not present if unused. These are different than boolean attributes by where their presence alone triggers their effect. In this situation, a nullish value (null or undefined) may be provided so the attribute won't exist in that condition.

<div aria-hidden="${x => x.isViewable ? 'true' : null}"></div>


Properties can also be set directly on an HTML element. To do so, prepend the property name with : to indicate a property binding. The template engine will then use your expression to assign the element's property value.

Example: Basic Property Values

<my-element :myCustomProperty="${x => x.mySpecialData}">

Example: Inner HTML

<div :innerHTML="${x => x.someDangerousHTMLContent}"></div>

Avoid scenarios that require you to directly set HTML, especially when the content is coming from an external source. If you must do this, you should always sanitize the HTML.

The best way to accomplish HTML sanitization is to configure a trusted types policy with FASTElement's template engine. FASTElement ensures that all HTML strings pass through the configured policy. Also, by leveraging the platform's trusted types capabilities, you get native enforcement of the policy through CSP headers. Here's an example of how to configure a custom policy to sanitize HTML:

import { DOM } from '@microsoft/fast-element';

const myPolicy = trustedTypes.createPolicy('my-policy', {
createHTML(html) {
// TODO: invoke a sanitization library on the html before returning it
return html;


For security reasons, the HTML Policy can only be set once. For this reason, it should be set by application developers and not by component authors, and it should be done immediately during the startup sequence of the application.


Besides rendering content, attributes, and properties, you'll often want to add event listeners and execute code when events fire. To do that, prepend the event name with @ and provide the expression to be called when that event fires. Within an event binding, you also have access to a special context argument from which you can access the event object.

Example: Basic Events

<button @click="${x => x.remove()}">Remove</button>

Example: Accessing Event Details

<input type="text"
:value="${x => x.description}"
@input="${(x, c) => x.handleDescriptionChange(c.event)}">

In both examples above, after your event handler is executed, preventDefault() will be called on the event object by default. You can return true from your handler to opt-out of this behavior.

The second example demonstrates an important characteristic of the templating engine: it only supports unidirectional data flow (model => view). It does not support two-way data binding (model <=> view). As shown above, pushing data from the view back to the model should be handled with explicit events that call into your model's API.

Templating and the element lifecycle

It is during the connectedCallback phase of the Custom Element lifecycle that FASTElement creates templates and binds the resulting view. The creation of the template only occurs the first time the element is connected, while binding happens every time the element is connected (with unbinding happening during the disconnectedCallback for symmetry).


In the future, we're planning new optimizations that will enable us to safely determine when we do not need to unbind/rebind certain views.

In most cases, the template that FASTElement renders is determined by the template property of the Custom Element's configuration. However, you can also implement a method on your Custom Element class named resolveTemplate() that returns a template instance. If this method is present, it will be called during connectedCallback to obtain the template to use. This allows the element author to dynamically select completely different templates based on the state of the element at the time of connection.

In addition to dynamic template selection during the connectedCallback, the $fastController property of FASTElement enables dynamically changing the template at any time by setting the controller's template property to any valid template.


Check out our Cheat Sheet for a quick guide on bindings.