FEATURE: add strikethrough rich editor extension

app/assets/javascripts/discourse/app/static/prosemirror/extensions/register-default.js

@@ -0,0 +1,12 @@
+import { registerRichEditorExtension } from "discourse/lib/composer/rich-editor-extensions";
+import strikethrough from "./strikethrough";
+
+/**
+ * List of default extensions
+ * ProsemirrorEditor autoloads them when includeDefault=true (the default)
+ *
+ * @type {RichEditorExtension[]}
+ */
+const defaultExtensions = [strikethrough];
+
+defaultExtensions.forEach(registerRichEditorExtension);

app/assets/javascripts/discourse/app/static/prosemirror/extensions/strikethrough.js

@@ -0,0 +1,36 @@
+/** @type {RichEditorExtension} */
+const extension = {
+  markSpec: {
+    strikethrough: {
+      before: "link",
+      parseDOM: [
+        { tag: "s" },
+        { tag: "del" },
+        {
+          getAttrs: (value) =>
+            /(^|[\s])line-through([\s]|$)/u.test(value) && null,
+          style: "text-decoration",
+        },
+      ],
+      toDOM() {
+        return ["s"];
+      },
+    },
+  },
+  inputRules: ({ schema, markInputRule }) =>
+    markInputRule(/~~([^~]+)~~$/, schema.marks.strikethrough),
+  parse: {
+    s: { mark: "strikethrough" },
+    bbcode_s: { mark: "strikethrough" },
+  },
+  serializeMark: {
+    strikethrough: {
+      open: "~~",
+      close: "~~",
+      mixable: true,
+      expelEnclosingWhitespace: true,
+    },
+  },
+};
+
+export default extension;

app/assets/javascripts/discourse/tests/integration/components/prosemirror-editor/strikethrough-test.gjs

@@ -0,0 +1,39 @@
+import { module, test } from "qunit";
+import { setupRenderingTest } from "discourse/tests/helpers/component-test";
+import { testMarkdown } from "discourse/tests/helpers/rich-editor-helper";
+
+module(
+  "Integration | Component | prosemirror-editor - strikethrough extension",
+  function (hooks) {
+    setupRenderingTest(hooks);
+
+    const testCases = {
+      strikethrough: [
+        ["[s]Hello[/s]", "<p><s>Hello</s></p>", "~~Hello~~"],
+        ["~~Hello~~", "<p><s>Hello</s></p>", "~~Hello~~"],
+        [
+          "Hey [s]wrod[/s] ~~uord~~ World",
+          "<p>Hey <s>wrod</s> <s>uord</s> World</p>",
+          "Hey ~~wrod~~ ~~uord~~ World",
+        ],
+      ],
+      "with other marks": [
+        [
+          "___[s][`Hello`](https://example.com)[/s]___",
+          '<p><em><strong><s><a href="https://example.com"><code>Hello</code></a></s></strong></em></p>',
+          "***~~[`Hello`](https://example.com)~~***",
+        ],
+      ],
+    };
+
+    Object.entries(testCases).forEach(([name, tests]) => {
+      tests.forEach(([markdown, expectedHtml, expectedMarkdown]) => {
+        test(name, async function (assert) {
+          this.siteSettings.rich_editor = true;
+
+          await testMarkdown(assert, markdown, expectedHtml, expectedMarkdown);
+        });
+      });
+    });
+  }
+);