MarkdownPlugin: serialize \n within a text child of a paragraph as line break

.changeset/neat-ravens-lick.md

@@ -0,0 +1,5 @@
+---
+"@platejs/markdown": patch
+---
+
+Serialize new lines (\n) within a paragraph's text node

packages/markdown/src/lib/rules/defaultRules.ts

@@ -4,6 +4,7 @@ import {
   type TListElement,
   type TMentionElement,
   type TText,
+  type Descendant,
   getPluginKey,
   getPluginType,
   KEYS,
@@ -744,20 +745,47 @@ export const defaultRules: MdRules = {
       return elements.length === 1 ? elements[0] : elements;
     },
     serialize: (node, options) => {
-      let enrichedChildren = node.children;
-
-      enrichedChildren = enrichedChildren.map((child) => {
+      // a child may be split in multiple parts, therefore use flatMap
+      const enrichedChildren = node.children.flatMap((child) => {
         if (child.text === '\n') {
-          return {
-            type: 'break',
-          } as any;
+          return [
+            {
+              type: 'break',
+            } as Descendant,
+          ];
         }
 
         if (child.text === '' && options.preserveEmptyParagraphs !== false) {
-          return { ...child, text: '\u200B' };
+          return [{ ...child, text: '\u200B' }];
+        }
+
+        //support linebreaks within a single text node
+        if (
+          child.text &&
+          typeof child.text === 'string' &&
+          child.text.includes('\n')
+        ) {
+          const enrichedParts: Descendant[] = [];
+          const childParts = child.text.split('\n');
+          childParts.forEach((part, index) => {
+            if (part.length === 0) {
+              enrichedParts.push({
+                type: 'break',
+              } as Descendant);
+            } else {
+              enrichedParts.push({ ...child, text: part });
+              // do not add line break after the last text that ends this child node (read: paragraph)
+              if (childParts.length !== index + 1) {
+                enrichedParts.push({
+                  type: 'break',
+                } as Descendant);
+              }
+            }
+          });
+          return enrichedParts;
         }
 
-        return child;
+        return [child];
       });
 
       const convertedNodes = convertNodesSerialize(

packages/markdown/src/lib/serializer/serializeMd.spec.tsx

@@ -254,6 +254,24 @@ describe('serializeMd', () => {
     expect(serializeMd(editor as any, { value: slateNodes })).toMatchSnapshot();
   });
 
+  it('should serialize new lines WITHIN a single text node to line breaks in Markdown', () => {
+    const slateNodes = [
+      {
+        children: [
+          {
+            text: 'Text followed by two empty lines\n\n\nFollowed by more text.',
+          },
+        ],
+        type: 'p',
+      },
+    ];
+
+    const result = serializeMd(editor as any, { value: slateNodes });
+    expect(result).toEqual(
+      `Text followed by two empty lines\\\n\\\n\\\nFollowed by more text.\n`
+    );
+  });
+
   it('should serialize lists with spread option correctly', () => {
     const listFragment = [
       {