🛠 Status: In Development
LitElement is currently in development. It's on the fast track to a 1.0 release, so we encourage you to use it and give us your feedback, but there are things that haven't been finalized yet and you can expect some changes.
LitElement
A simple base class for creating custom elements rendered with lit-html.
LitElement uses lit-html to render into the
element's Shadow DOM
and Polymer's
PropertiesMixin
to help manage element properties and attributes. LitElement reacts to changes in properties
and renders declaratively using lit-html
.
-
React to changes: LitElement reacts to changes in properties and attributes by
asynchronously rendering, ensuring changes are batched. This reduces overhead
and maintains consistent state.
-
Declarative rendering LitElement uses lit-html
to declaratively describe
how an element should render. Then lit-html
ensures that updates
are fast by creating the static DOM once and smartly updating only the parts of
the DOM that change. Pass a JavaScript string to the html
tag function,
describing dynamic parts with standard JavaScript template expressions:
- static elements:
html`<div>Hi</div>`
- expression:
html`<div>${disabled ? 'Off' : 'On'}</div>`
- attribute:
html`<div class$="${color} special"></div>`
- event handler:
html`<button on-click="${(e) => this._clickHandler(e)}"></button>`
Getting started
-
The easiest way to try out LitElement is to use one of these online tools:
-
You can also copy this HTML file into a local file and run it in any browser that supports JavaScript Modules.
-
When you're ready to use LitElement in a project, install it via npm. To run the project in the browser, a module-compatible toolctain is required. We recommend installing the Polymer CLI to and using its development server as follows.
-
Add LitElement to your project:
npm i @polymer/lit-element
-
Create an element by extending LitElement and calling customElements.define
with your class (see the examples below).
-
Install the Polymer CLI:
npm i -g polymer-cli@next
-
Run the development server and open a browser pointing to its URL:
polymer serve
LitElement is published on npm using JavaScript Modules.
This means it can take advantage of the standard native JavaScript module loader available in all current major browsers.
However, since LitElement uses npm convention to reference dependencies by name, a light transform to rewrite specifiers to URLs is required to get it to run in the browser. The polymer-cli's development server polymer serve
automatically handles this transform.
Tools like WebPack and Rollup can also be used to serve and/or bundle LitElement.
Minimal Example
- Create a class that extends
LitElement
. - Implement a static
properties
getter that returns the element's properties
(which automatically become observed attributes). - Then implement a
_render(props)
method and use the element's
current properties (props) to return a lit-html
template result to render
into the element. This is the only method that must be implemented by subclasses.
<script src="node_modules/@webcomponents/webcomponents-bundle.js"></script>
<script type="module">
import {LitElement, html} from '@polymer/lit-element';
class MyElement extends LitElement {
static get properties() { return { mood: String }}
_render({mood}) {
return html`<style> .mood { color: green; } </style>
Web Components are <span class="mood">${mood}</span>!`;
}
}
customElements.define('my-element', MyElement);
</script>
<my-element mood="happy"></my-element>
API Documentation
See the source
for detailed API info, here are some highlights. Note, the leading underscore
is used to indicate that these methods are
protected;
they are not private and can and should be implemented by subclasses.
These methods generally are called as part of the rendering lifecycle and should
not be called in user code unless otherwise indicated.
-
_createRoot()
: Implement to customize where the
element's template is rendered by returning an element into which to
render. By default this creates a shadowRoot for the element.
To render into the element's childNodes, return this
.
-
_firstRendered()
: Called after the element DOM is rendered for the first time.
-
_shouldRender(props, changedProps, prevProps)
: Implement to control if rendering
should occur when property values change or invalidate
is called.
By default, this method always returns true, but this can be customized as
an optimization to avoid rendering work when changes occur which should not be rendered.
-
_render(props)
: Implement to describe the element's DOM using lit-html
. Ideally,
the _render
implementation is a pure function using only props
to describe
the element template. This is the only method that must be implemented by subclasses.
-
_didRender(props, changedProps, prevProps)
: Called after element DOM has been rendered.
Implement to directly control rendered DOM. Typically this is not needed as lit-html
can be used in the _render
method to set properties, attributes, and
event listeners. However, it is sometimes useful for calling methods on
rendered elements, for example focusing an input:
this.shadowRoot.querySelector('input').focus()
.
-
renderComplete
: Returns a promise which resolves after the element next renders.
-
_requestRender
: Call to request the element to asynchronously re-render regardless
of whether or not any property changes are pending.
Bigger Example
import {LitElement, html} from '@polymer/lit-element';
class MyElement extends LitElement {
static get properties() {
return {
foo: String,
whales: Number
}
}
constructor() {
super();
this.foo = 'foo';
this.addEventListener('click', async (e) => {
this.whales++;
await this.renderComplete;
this.dispatchEvent(new CustomEvent('whales', {detail: {whales: this.whales}}))
});
}
_render({foo, whales}) {
return html`
<style>
:host {
display: block;
}
:host([hidden]) {
display: none;
}
</style>
<h4>Foo: ${foo}</h4>
<div>whales: ${'🐳'.repeat(whales)}</div>
<slot></slot>
`;
}
}
customElements.define('my-element', MyElement);
<my-element whales="5">hi</my-element>
Supported Browsers
The last 2 versions of all modern browsers are supported, including
Chrome, Safari, Opera, Firefox, Edge. In addition, Internet Explorer 11 is also supported.
Known Issues
- When the Shady DOM polyfill and ShadyCSS shim are used, styles may be out of order.
- Rendering is not supported in IE11 due to a lit-html issue.