A file format for describing custom elements.

The schema is published as a JSON Schema file, in schema.json. The schema is written in TypeScript (see schema.d.ts) and then compiled to JSON Schema.

Install:

npm i -D custom-elements-manifest

Require the JSON Schema:

const customElementManifestSchema = require('custom-elements-manifest');

Import the TypeScript types:

import * as schema from 'custom-elements-manifest/schema';

In order to allow tools to find npm packages with custom element manifests without having to download package tarballs, packages should have a "customElements" field in their package.json that points to the manifest:

{
  "name": "example-package",
  "customElements": "custom-elements.json",
}

The schema has a schemaVersion field in the top-level object to facilitate evolution of the schema. The schema follows semver versioning, the current schema version is 2.1.0.

This version may not always match the npm package version, as some changes to the npm package might not have changes to the schema.

Given the following source code in directory my-project:

my-project/my-element.js:

/**
 * This is the description of the class
 */
export class MyElement extends HTMLElement {
  static get observedAttributes() {
    return ['disabled'];
  }

  set disabled(val) {
    this.__disabled = val;
  }
  get disabled() {
    return this.__disabled;
  }

  fire() {
    this.dispatchEvent(new Event('disabled-changed'));
  }
}

customElements.define('my-element', MyElement);

The manifest would look like:

my-project/custom-elements.json:

{
  "schemaVersion": "2.1.0",
  "readme": "README.md",
  "modules": [
    {
      "kind": "javascript-module",
      "path": "my-project/my-element.js",
      "declarations": [
        {
          "kind": "class",
          "customElement": true,
          "name": "MyElement",
          "tagName": "my-element",
          "description": "This is the description of the class",
          "members": [
            {
              "kind": "field",
              "name": "disabled"
            },
            {
              "kind": "method",
              "name": "fire"
            }
          ],
          "events": [
            {
              "name": "disabled-changed",
              "type": {
                "text": "Event"
              }
            }
          ],
          "attributes": [
            {
              "name": "disabled"
            }
          ],
          "superclass": {
            "name": "HTMLElement"
          }
        }
      ],
      "exports": [
        {
          "kind": "js",
          "name": "MyElement",
          "declaration": {
            "name": "MyElement"
          }
        },
        {
          "kind": "custom-element-definition",
          "name": "my-element",
          "declaration": {
            "name": "MyElement"
          }
        }
      ]
    }
  ]
}

Many tools need some machine-readable descriptions of custom elements: IDEs, documentation viewers, linters, graphical design tools, etc.

There have been several efforts in this area, including:

• Polymer Analyzer's analysis.json file
• VS Code Custom Data format
• https://github.com/JetBrains/web-types

This repository is an effort to bring together tool owners to standardize on a common specification for a description format.

Developers using custom elements should be able to get full-featured IDE support including auto-completion, hover-documentation, unknown symbol warnings, etc. These features should be available in HTML files, and in various template syntaxes via template-specific tools.

Documentation viewers should be able to display all the relevant information about a custom element, such as its tag name, attributes, properties, definition module, CSS variables and parts, etc.

Using a custom-elements manifest, it would be easy to generate or display demos for your component using tools such as api-viewer-element, or automatically generate Storybook knobs for your components.

Linters should be able to produce warnings based on custom element defintions, such as warning if unknown elements are used in HTML templates.

React currently is the only major framework where custom elements require some special handling. React will pass all data to a custom element in the form of HTML attributes, and cannot listen for DOM events coming from Custom Elements without the use of a workaround.

The solution for this is to create a wrapper React component that handles these things. Using a custom elements manifest, creation of these wrapper components could be automated.

Some component libraries like Fast or Shoelace provide specific instructions on how to integrate with certain frameworks. Automating this integration layer could make development easier for both authors of component libraries, but also for consumers of libraries.

A major use-case of custom elements manifests is that they allow us to reliably detect NPM packages that for certain contain custom elements. These packages could be stored, and displayed on a custom elements catalog, effectively a potential reboot of webcomponents.org. This catalog would be able to show rich demos and documentation of the custom elements contained in a package, by importing its components from a CDN like unpkg, and its custom elements manifest.

Tooling would be able to detect whether or not the public API of a custom element has changed, based on a snapshot of the current custom elements manifest file to decide the impact of an update, and potentially prevent breaking API change in patch versions.