derdrodt / lit-element-1

An ultra-light custom element base class with a simple but expressive API

🛠 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:

  • Runs in all supported browsers: StackBlitz, Glitch

  • Runs in browsers with JavaScript Modules: JSFiddle, JSBin, CodePen.

• 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.

  1. Add LitElement to your project:

     npm i @polymer/lit-element

  2. Create an element by extending LitElement and calling customElements.define with your class (see the examples below).

  3. Install the Polymer CLI:

     npm i -g polymer-cli@next

  4. 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

1. Create a class that extends LitElement.
2. Implement a static properties getter that returns the element's properties (which automatically become observed attributes).
3. 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 {

  // Public property API that triggers re-render (synced with attributes)
  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 method should return a `TemplateResult` using the provided lit-html `html` tag function
  _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