@polymer/lit-element

🛠 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 fast, lightweight web components

LitElement uses lit-html to render into the element's Shadow DOM and adds API to help manage element properties and attributes. LitElement reacts to changes in properties and renders declaratively using lit-html. See the lit-html guide for additional information on how to create templates for lit-element.

• Setup properties: LitElement supports observable properties that cause the element to update. These properties can be declared in a few ways:

  • As class fields with the @property() decorator, if you're using a compiler that supports them, like TypeScript or Babel.
  • With a static properties getter.
  • By manually writing getters and setters. This can be useful if tasks should be performed when a property is set, for example validation. Call requestUpdate(name, oldValue) in the setter to trigger an update and use any configured property options.

  Properties can be given an options argument which is an object that describes how to process the property. This can be done either in the @property({...}) decorator or in the object returned from the properties getter, e.g. static get properties { return { foo: {...} }.

  Property options include:

  • attribute: Indicates how and whether the property becomes an observed attribute. If the value is false, the property is not added to the static observedAttributes getter. If true or absent, the lowercased property name is observed (e.g. fooBar becomes foobar). If a string, the string value is observed (e.g attribute: 'foo-bar').
  • type: Indicates how to serialize and deserialize the attribute to/from a property. The value can be a function used for both serialization and deserialization, or it can be an object with individual functions via the optional keys, fromAttribute and toAttribute. type defaults to the String constructor, and so does the toAttribute and fromAttribute keys.
  • reflect: Indicates whether the property should reflect to its associated attribute (as determined by the attribute option). If true, when the property is set, the attribute which name is determined according to the rules for the attribute property option, will be set to the value of the property serialized using the rules from the type property option. Note, type: Boolean has special handling by default which means that truthy values result in the presence of the attribute, whereas falsy values result in the absence of the attribute.
  • hasChanged: A function that indicates whether a property should be considered changed when it is set and thus result in an update. The function should take the newValue and oldValue and return true if an update should be requested.

• 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>${this.disabled ? 'Off' : 'On'}</div>`
  • property: html`<x-foo .bar="${this.bar}"></x-foo>`
  • attribute: html`<div class="${this.color} special"></div>`
  • boolean attribute: html`<input type="checkbox" ?checked=${checked}>`
  • event handler: html`<button @click="${this._clickHandler}"></button>`

Getting started

• The easiest way to try out LitElement is to use one of these online tools:

  • Runs in all supported browsers: 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 toolchain is required. We recommend installing the Polymer CLI and using its development server as follows.

  1. Add LitElement to your project:

     npm i @polymer/lit-element

  2. Install the webcomponents polyfill. If you're developing a reusable package, this should be a dev dependency which you load in your tests, demos, etc.

     npm i -D @webcomponents/webcomponentsjs

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

  4. Install the Polymer CLI:

     npm i -g polymer-cli

  5. 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. Use a @property decorator to create a property (or implement a static properties getter that returns the element's properties). (which automatically become observed attributes).
3. Then implement a render() method and use the element's current properties to return a lit-html template result to render into the element.

  <script src="node_modules/@webcomponents/webcomponentsjs/webcomponents-bundle.js"></script>
  <script type="module">
    import {LitElement, html} from '@polymer/lit-element';

    class MyElement extends LitElement {

      static get properties() {
        return {
          mood: {type: String}
        };
      }

      constructor() {
        super();
        this.mood = 'happy';
      }

      render() {
        return html`<style> .mood { color: green; } </style>
          Web Components are <span class="mood">${this.mood}</span>!`;
      }

    }

    customElements.define('my-element', MyElement);
  </script>

  <my-element mood="happy"></my-element>

API Documentation

• render() (protected): Implement to describe the element's DOM using lit-html. Ideally, the render implementation is a pure function using only the element's current properties to describe the element template. Note, since render() is called by update(), setting properties does not trigger an update, allowing property values to be computed and validated.

• shouldUpdate(changedProperties) (protected): Implement to control if updating and rendering should occur when property values change or requestUpdate() is called. The changedProperties argument is a Map with keys for the changed properties pointing to their previous values. By default, this method always returns true, but this can be customized as an optimization to avoid updating work when changes occur, which should not be rendered.

• update(changedProperties) (protected): This method calls render() and then uses lit-html in order to render the template DOM. It also updates any reflected attributes based on property values. Setting properties inside this method will not trigger another update.

• firstUpdated(changedProperties): (protected) Called after the element's DOM has been updated the first time, immediately before updated() is called. This method can be useful for capturing references to rendered static nodes that must be directly acted upon, for example in updated(). Setting properties inside this method will trigger the element to update.

• updated(changedProperties): (protected) Called whenever the element's DOM has been updated and rendered. Implement to perform post updating tasks via DOM APIs, for example, focusing an element. Setting properties inside this method will trigger the element to update.

• updateComplete: Returns a Promise that resolves when the element has completed updating. The Promise value is a boolean that is true if the element completed the update without triggering another update. The Promise result is false if a property was set inside updated(). This getter can be implemented to await additional state. For example, it is sometimes useful to await a rendered element before fulfilling this Promise. To do this, first await super.updateComplete then any subsequent state.

• requestUpdate(name?, oldValue?): Call to request the element to asynchronously update regardless of whether or not any property changes are pending. This should be called when an element should update based on some state not triggered by setting a property. In this case, pass no arguments. It should also be called when manually implementing a property setter. In this case, pass the property name and oldValue to ensure that any configured property options are honored. Returns the updateComplete Promise which is resolved when the update completes.

• createRenderRoot() (protected): 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.

Advanced: Update Lifecycle

• A property is set (e.g. element.foo = 5).
• If the property's hasChanged(value, oldValue) returns false, the element does not update. If it returns true, requestUpdate() is called to schedule an update.
• requestUpdate(): Updates the element after awaiting a microtask (at the end of the event loop, before the next paint).
• shouldUpdate(changedProperties): The update proceeds if this returns true, which it does by default.
• update(changedProperties): Updates the element. Setting properties inside this method will not trigger another update.
  • render(): Returns a lit-html TemplateResult (e.g. html`Hello ${world}`) to render element DOM. Setting properties inside this method will not trigger the element to update.
• firstUpdated(changedProperties): Called after the element is updated the first time, immediately before updated is called. Setting properties inside this method will trigger the element to update.
• updated(changedProperties): Called whenever the element is updated. Setting properties inside this method will trigger the element to update.
• updateComplete Promise is resolved with a boolean that is true if the element is not pending another update, and any code awaiting the element's updateComplete Promise runs and observes the element in the updated state.

Bigger Example

Note, this example uses decorators to create properties. Decorators are a proposed standard currently available in TypeScript or Babel.

import {LitElement, html, property} from '@polymer/lit-element';

class MyElement extends LitElement {

  // Public property API that triggers re-render (synced with attributes)
  @property()
  foo = 'foo';

  @property({type: Number})
  whales = 5;

  constructor() {
    super();
    this.addEventListener('click', async (e) => {
      this.whales++;
      await this.updateComplete;
      this.dispatchEvent(new CustomEvent('whales', {detail: {whales: this.whales}}))
    });
  }

  // Render method should return a `TemplateResult` using the provided lit-html `html` tag function
  render() {
    return html`
      <style>
        :host {
          display: block;
        }
        :host([hidden]) {
          display: none;
        }
      </style>
      <h4>Foo: ${this.foo}</h4>
      <div>whales: ${'🐳'.repeat(this.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

• On very old versions of Safari (<=9) or Chrome (<=41), properties created for native platform properties like (id or name) may not have default values set in the element constructor. On these browsers native properties appear on instances and therefore their default value will overwrite any element default (e.g. if the element sets this.id = 'id' in the constructor, the 'id' will become '' since this is the native platform default).