# ReactiveElement

A simple low level base class for creating fast, lightweight web components.

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## Documentation

Full documentation is available at [lit.dev/docs/api/ReactiveElement/](https://lit.dev/docs/api/ReactiveElement/).

## Overview

`ReactiveElement` is a base class for writing web components that react to changes in properties and attributes. `ReactiveElement` adds reactive properties and a batching, asynchronous update lifecycle to the standard web component APIs. Subclasses can respond to changes and update the DOM to reflect the element state.

`ReactiveElement` doesn't include a DOM template system, but can easily be extended to add one by overriding the `update()` method to call the template library. `LitElement` is such an extension that adds `lit-html` templating.

## Example

```ts
import {
  ReactiveElement,
  html,
  css,
  customElement,
  property,
  PropertyValues,
} from '@lit/reactive-element';

// This decorator defines the element.
@customElement('my-element')
export class MyElement extends ReactiveElement {
  // This decorator creates a property accessor that triggers rendering and
  // an observed attribute.
  @property()
  mood = 'great';

  static styles = css`
    span {
      color: green;
    }
  `;

  contentEl?: HTMLSpanElement;

  // One time setup of shadowRoot content.
  createRenderRoot() {
    const shadowRoot = super.createRenderRoot();
    shadowRoot.innerHTML = `Web Components are <span></span>!`;
    this.contentEl = shadowRoot.firstElementChild;
    return shadowRoot;
  }

  // Use a DOM rendering library of your choice or manually update the DOM.
  update(changedProperties: PropertyValues) {
    super.update(changedProperties);
    this.contentEl.textContent = this.mood;
  }
}
```

```html
<my-element mood="awesome"></my-element>
```

Note, this example uses decorators to create properties. Decorators are a proposed
standard currently available in [TypeScript](https://www.typescriptlang.org/) or [Babel](https://babeljs.io/docs/en/babel-plugin-proposal-decorators). ReactiveElement also supports a [vanilla JavaScript method](https://lit.dev/docs/components/properties/#declaring-properties-in-a-static-properties-field) of declaring reactive properties.

## Installation

```bash
$ npm install @lit/reactive-element
```

Or use from `lit`:

```bash
$ npm install lit
```

## Contributing

Please see [CONTRIBUTING.md](../../CONTRIBUTING.md).
