Suppress uncaught DOMException error while upgrading <marp-pre> Web Component elements in Firefox

CHANGELOG.md

@@ -48,6 +48,10 @@
 * Upgrade dependent packages to the latest version ([#380](https://github.com/marp-team/marp-core/pull/380))
 * Switch package manager from yarn to npm ([#379](https://github.com/marp-team/marp-core/pull/379))
 
+### Fixed
+
+- Suppress uncaught `DOMException` error while upgrading `<marp-pre>` Web Component elements in Firefox ([#370](https://github.com/marp-team/marp-core/issues/370), [#384](https://github.com/marp-team/marp-core/pull/384))
+
 ## v3.9.0 - 2023-10-15
 
 ### Added

jest.config.js → jest.config.mjs

@@ -1,6 +1,9 @@
-const { defaults: tsjPreset } = require('ts-jest/presets')
+import { createDefaultPreset } from 'ts-jest'
 
-module.exports = {
+const tsjPreset = createDefaultPreset()
+
+/** @type {import('jest').Config} */
+const config = {
   collectCoverageFrom: ['src/**/*.{j,t}s'],
   coveragePathIgnorePatterns: ['/node_modules/', '.*\\.d\\.ts'],
   coverageThreshold: { global: { lines: 95 } },
@@ -12,3 +15,5 @@ module.exports = {
   },
   prettierPath: null,
 }
+
+export default config

src/custom-elements/browser/marp-custom-element.ts

@@ -5,7 +5,7 @@ export const createMarpCustomElement = <T extends Constructor<HTMLElement>>(
   { attrs = {}, style }: { attrs?: Record<string, string>; style?: string },
 ) =>
   class MarpCustomElement extends Base {
-    declare shadowRoot: ShadowRoot
+    declare readonly shadowRoot: ShadowRoot | null
 
     constructor(...args: any[]) {
       super(...args)
@@ -14,7 +14,7 @@ export const createMarpCustomElement = <T extends Constructor<HTMLElement>>(
         if (!this.hasAttribute(key)) this.setAttribute(key, value)
       }
 
-      this.attachShadow({ mode: 'open' })
+      this._shadow()
     }
 
     static get observedAttributes() {
@@ -29,19 +29,34 @@ export const createMarpCustomElement = <T extends Constructor<HTMLElement>>(
       this._update()
     }
 
+    _shadow() {
+      if (!this.shadowRoot) {
+        try {
+          this.attachShadow({ mode: 'open' })
+        } catch (e) {
+          if (!(e instanceof Error && e.name === 'NotSupportedError')) throw e
+        }
+      }
+      return this.shadowRoot
+    }
+
     _update() {
-      const styleTag = style ? `<style>:host { ${style} }</style>` : ''
-      let slotTag = '<slot></slot>'
+      const shadowRoot = this._shadow()
 
-      const { autoScaling } = this.dataset
+      if (shadowRoot) {
+        const styleTag = style ? `<style>:host { ${style} }</style>` : ''
+        let slotTag = '<slot></slot>'
 
-      if (autoScaling !== undefined) {
-        const downscale =
-          autoScaling === 'downscale-only' ? 'data-downscale-only' : ''
+        const { autoScaling } = this.dataset
 
-        slotTag = `<marp-auto-scaling exportparts="svg:auto-scaling" ${downscale}>${slotTag}</marp-auto-scaling>`
-      }
+        if (autoScaling !== undefined) {
+          const downscale =
+            autoScaling === 'downscale-only' ? 'data-downscale-only' : ''
+
+          slotTag = `<marp-auto-scaling exportparts="svg:auto-scaling" ${downscale}>${slotTag}</marp-auto-scaling>`
+        }
 
-      this.shadowRoot.innerHTML = styleTag + slotTag
+        shadowRoot.innerHTML = styleTag + slotTag
+      }
     }
   }

test/custom-elements/browser.ts

@@ -84,6 +84,34 @@ describe('The hydration script for custom elements', () => {
       )
     })
 
+    it('does not throw known DOMException error while upgrading <pre is="marp-pre"> to <marp-pre> (for Firefox)', () => {
+      document.body.innerHTML = '<pre is="marp-pre">1</pre>'
+
+      jest
+        .spyOn(HTMLElement.prototype, 'attachShadow')
+        .mockImplementationOnce(() => {
+          throw new DOMException(
+            'Element.attachShadow: Unable to attach ShadowDOM',
+            'NotSupportedError',
+          )
+        })
+
+      expect(() => browser.applyCustomElements()).not.toThrow()
+    })
+
+    it.skip('throws error if unknown error occured while upgrading <pre is="marp-pre"> to <marp-pre>', () => {
+      document.body.innerHTML = '<pre is="marp-pre">1</pre>'
+
+      jest.spyOn(console, 'error').mockImplementation(() => {})
+      jest
+        .spyOn(HTMLElement.prototype, 'attachShadow')
+        .mockImplementationOnce(() => {
+          throw new Error('Unknown error while attaching shadow')
+        })
+
+      expect(() => browser.applyCustomElements()).toThrow()
+    })
+
     it('does not replace <h1 is="marp-h1"> to <marp-h1>', () => {
       const html = '<h1 is="marp-h1">test</h1>'
       document.body.innerHTML = html