@open-wc/scoped-elements

---

permalink: 'scoped-elements/index.html' title: Scoped elements section: guides tags:

• guides

---

Scoped elements

Part of Open Web Components: guides, tools and libraries for modern web development and web components

Scope element tag names avoiding naming collision and allowing to use different versions of the same web component in your code.

Installation

npm i --save @open-wc/scoped-elements

Usage

1. Import ScopedElementsMixin from @open-wc/scoped-elements.

   import { ScopedElementsMixin } from '@open-wc/scoped-elements';
   

2. Import the classes of the components you want to use.

   import { MyButton } from './MyButton.js';
   import { MyPanel } from './MyPanel.js';
   

3. Apply ScopedElementsMixin and define the tags you want to use for your components.

   class MyElement extends ScopedElementsMixin(LitElement) {
     static get scopedElements() {
       return {
         'my-button': MyButton,
         'my-panel': MyPanel,
       };
     }
   }
   

   WARNING: If you are going to use elements that are globally defined you have to declare them in scopedElements as well. This is required because we are trying to work as close as possible to the future Scoped Custom Element Registries feature and, by the moment, there is not going ot be inheritance between registries.

   You can declare them like in the following example:

    static get scopedElements() {
      return {
        'old-button': customElements.get('old-button'),
        'my-panel': MyPanel,
      };
    }
   

   If you try to register the same element globally AND locally with the exact same name AND class instance it will reuse the global tag name and NOT scope it.

4. Use your components in your html.

   render() {
     return html`
       <my-panel class="panel">
         <my-button>${this.text}</my-button>
       </my-panel>
     `;
   }
   

Complete example

import { css, LitElement } from 'lit-element';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyButton } from './MyButton.js';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-button': MyButton,
      'my-panel': MyPanel,
    };
  }

  static get styles() {
    return css`
      .panel {
        padding: 10px;
        background-color: grey;
      }
    `;
  }

  static get properties() {
    return {
      text: String,
    };
  }

  render() {
    return html`
      <my-panel class="panel">
        <my-button>${this.text}</my-button>
      </my-panel}>
    `;
  }
}

Lazy scoped components

In some situations may happen that you want to use a component in your templates that is not already loaded at the moment of defining the scoped elements map. The ScopedElementsMixin provides the defineScopedElement method to define scoped elements at any time.

import { LitElement } from 'lit-element';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-panel': MyPanel,
    };
  }

  constructor() {
    super();

    import('./MyButton.js').then(({ MyButton }) => this.defineScopedElement('my-button', MyButton));
  }

  render() {
    return html`
      <my-panel class="panel">
        <my-button>${this.text}</my-button>
      </my-panel>
    `;
  }
}

Obtaining a scoped tag name

Maybe you want to create a scoped element programmatically and don't know which one is the scoped tag name? No problem, there is a static method called getScopedTagName that would help you for that.

import { LitElement } from 'lit-element';
import { ScopedElementsMixin } from '@open-wc/scoped-elements';
import { MyButton } from './MyButton.js';
import { MyPanel } from './MyPanel.js';

export class MyElement extends ScopedElementsMixin(LitElement) {
  static get scopedElements() {
    return {
      'my-panel': MyPanel,
      'my-button': MyButton,
    };
  }

  constructor() {
    super();

    const scopedTagName = this.constructor.getScopedTagName('my-button');

    // do whatever you need with the scopedTagName
  }

  // ...
}

Motivation

Complex Web Component applications are often developed by several teams across organizations. In that scenario it is common that shared component libraries are used by teams to create a homogeneous look and feel or just to avoid creating the same components multiple times, but as those libraries evolve problems between different versions of the same library may appear, as teams may not be able to evolve and update their code at the same velocity. This causes bottlenecks in software delivery that should be managed by the teams and complex build systems, to try to alleviate the problem.

Scoped Custom Element Registries is a proposal that will solve this problem, but until it is ready, or a polyfill becomes available, we have to scope custom element tag names if we want to use different versions of those custom elements in our code. This package allows you to forget about how custom elements are defined, registered and scopes their tag names if it is necessary, and avoids the name collision problem.

Use case and demos

Consider the following setup

• Team Blue owns Page A
• Team Green owns Page B
• Team Black owns Feature A & B

1. Everything is good and your app is live [code] with both pages.
2. Team Black releases a new version (2.x) of Feature B which unfortunately needs to be breaking in order to support new use-cases.
3. Team Blue (on Page A) does not use any of those new use cases, and they have a tight deadline to meet, so they cannot update right now.
4. Team Green (on Page B) has to deliver an important functionality to your end users, but they need to upgrade to Feature B 2.x since it can only be solved with this new version.
5. Since Feature A 1.x & 2.x are both used in the same app, this will lead to nested dependencies, which then will lead to catastrophic failure, and errors [code].

Two possible solutions come to mind:

1. Temporarily (!) allow shipping similar source code (most breaking releases are not a total rewrite) and scope them via @open-wc/scoped-elements; see the "fixed" example with-scope [code] running with nested dependencies.
2. Synchronizing updates of shared dependencies - e.g. make sure Team Blue & Team Green always use the same version when releasing. This can be a viable solution however it comes with a high organizational overhead and is hard to scale up (for 10+ teams)

Technical explanation of scenario

The simplified app has the following dependencies

• app
  • page-a
    • feature-a 1.x
    • feature-b 1.x
  • page-b
    • feature-a 2.x
    • feature-b 1.x

which leads to the following node_modules tree

├── node_modules
│   ├── feature-a
│   ├── feature-b
│   ├── page-a
│   └── page-b
│       └── node_modules
│           └── feature-a
├── demo-app.js
└── index.html

To demonstrate, we made three demos:

1. before-nesting [code] In this demo, everything works fine as Page A and B both are using the same version of Feature A

2. no-scope [code] Feature A version 1.x and 2.x are imported via self registering entry points which leads to the following error message, because the feature-a component tries to register multiple times:

   Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': the name "feature-a" has already been used with this registry
     at [...]/node_modules/page-b/node_modules/feature-a/feature-a.js:3:16
   

3. with-scope [code] This example successfully fixes the problem by using ScopedElementsMixin on both Page A and Page B.

How it works

ScopedElementsMixin is mixed into your LitElement and via static get scopedElements() you define the tags and classes you wanna use in your elements template. Under the hood it changes your template so <my-button>${this.text}</my-button> becomes <my-button-2748>${this.text}</my-button-2748>.

Every auto-defined scoped elements gets a random* 4 digits number suffix. This suffix changes every time to make sure developers are not inclined to use it the generated tag name as a styling hook. Additionally the suffix allows scoped-elements and traditional self-defined elements to coexist, avoiding name collision.

* it is actually a global counter that gets initialized with a random starting number on load

Limitations

1. Components imported via npm SHOULD NOT be self registering components. If a shared component (installed from npm) does not offer an export to the class alone, without the registration side effect, then this component may not be used. E.g. every component that calls customElement.define.

   export class MyEl { ... }
   customElement.define('my-el', MyEl);
   

   Or uses the customElement typescript decorator

   @customElement('my-el')
   export class MyEl { ... }
   

   Only side effects free class exports may be used

   export class MyEl { ... }
   

2. Every component that uses sub components should use scoped-elements. Any import to a self registering component can potentially result in a browser exception - completely breaking the whole application.

3. Imported elements should be fully side effect free (not only element registration)

4. Currently, only lit-element is supported (though other elements/rendering engines could be incorporated in the future).

5. You cannot use tag selectors in css, but you could use an id, a class name or even a property instead.

   🚫 my-panel {
     width: 300px;
   }
   ✅ .panel {
     width: 300px;
   }
   

6. You cannot use tag names using javascript querySelectors, but you could use an id, a class name or even a property instead.

   🚫 this.shadowRoot.querySelector('my-panel');
   ✅ this.shadowRoot.querySelector('.panel');
   

7. Using scoped-elements may result in a performance degradation of up to 8%.

8. Loading of duplicate/similar source code (most breaking releases are not a total rewrite) should always be a temporary solution.

9. Often, temporary solutions tend to become more permanent. Be sure to focus on keeping the lifecycle of nested dependencies short.

Performance

We are using Tachometer to measure the performance penalty of using the scoped elements feature. The chosen test application is a slight variation of the Polymer Shop Application.

This is an example of the results obtained running the performance test.

⠋ Auto-sample 560 (timeout in 16m27s)
┌─────────────┬───────────────┐
│     Version │ <none>        │
├─────────────┼───────────────┤
│     Browser │ chrome        │
│             │ 80.0.3987.106 │
├─────────────┼───────────────┤
│ Sample size │ 610           │
└─────────────┴───────────────┘
┌─────────────────────────────┬────────────┬─────────────────────┬─────────────────┬──────────────────────────┐
│ Benchmark                   │ Bytes      │            Avg time │  vs lit-element │ vs scoped-elements-mixin │
├─────────────────────────────┼────────────┼─────────────────────┼─────────────────┼──────────────────────────┤
│ lit-element                 │ 281.24 KiB │ 285.72ms - 286.69ms │                 │                   faster │
│                             │            │                     │        -        │                  2% - 2% │
│                             │            │                     │                 │           5.68ms - 7.1ms │
├─────────────────────────────┼────────────┼─────────────────────┼─────────────────┼──────────────────────────┤
│ scoped-elements-mixin       │ 283.21 KiB │ 292.08ms - 293.11ms │          slower │                          │
│                             │            │                     │         2% - 2% │                 -        │
│                             │            │                     │ 5.68ms - 7.10ms │                          │
└─────────────────────────────┴────────────┴─────────────────────┴─────────────────┴──────────────────────────┘

Special thanks

This package was initially inspired by carehtml and we would like to thank @bashmish for his work on it.