fix: add paragraph breaks to plaintext output

src/plaintext.ts

@@ -1,5 +1,8 @@
 import { stripHtml } from 'string-strip-html'
 import defu from 'defu'
+import { Text } from 'domhandler'
+import type { ChildNode, Element } from 'domhandler'
+import { parse, serialize } from './utils/ast/index.ts'
 
 const defaults = {
   dumpLinkHrefsNearby: {
@@ -8,6 +11,57 @@
   },
 }
 
+/**
+ * Elements that start a new paragraph in the plaintext output.
+ */
+const BLOCK_TAGS = new Set([
+  'address', 'article', 'aside', 'blockquote', 'center', 'dd', 'div', 'dl', 'dt',
+  'figcaption', 'figure', 'footer', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'header',
+  'hr', 'main', 'nav', 'ol', 'p', 'pre', 'section', 'table', 'td', 'th', 'tr', 'ul',
+])
+
+const PRESERVE_WHITESPACE_TAGS = new Set(['pre', 'textarea', 'script', 'style'])
+
+/**
+ * Text that only holds zero-width filler characters, like the `‍`
+ * in Spacer/Hr. Joiners inside real text are left alone, since
+ * emoji sequences and some scripts depend on them.
+ */
+const FILLER_ONLY = /^[\s\u00AD\u034F\u200B-\u200D\u2060\uFEFF]+$/
+
+/**
+ * Collapse source whitespace like a browser would, then surround
+ * block-level elements with line breaks, so that the plaintext
+ * structure doesn't depend on how the HTML was formatted.
+ */
+function addLineBreaks(nodes: ChildNode[], preserveWhitespace = false): ChildNode[] {
+  return nodes.flatMap<ChildNode>((node) => {
+    if (node.type === 'text') {
+      if (!preserveWhitespace) {
+        const text = node as Text
+        text.data = FILLER_ONLY.test(text.data) ? ' ' : text.data.replace(/[ \t\n\r\f]+/g, ' ')
+      }
+      return [node]
+    }
+
+    if (!('children' in node)) return [node]
+
+    const el = node as Element
+    el.children = addLineBreaks(el.children, preserveWhitespace || PRESERVE_WHITESPACE_TAGS.has(el.name))
+
+    if (el.name === 'br') return [el, new Text('\n')]
+    if (el.name === 'li') return [new Text('\n'), el]
+    if (BLOCK_TAGS.has(el.name)) return [new Text('\n\n'), el, new Text('\n\n')]
+
+    return [el]
+  })
+}
+
 export function createPlaintext(html: string, options?: Record<string, unknown>): string {
-  return stripHtml(html, defu(options, defaults)).result
+  const { result } = stripHtml(serialize(addLineBreaks(parse(html))), defu(options, defaults))
+
+  return result
+    .replace(/[ \t]+\n/g, '\n')
+    .replace(/\n{3,}/g, '\n\n')
+    .trim()
 }

src/tests/plaintext.test.ts

@@ -28,7 +28,44 @@ describe('createPlaintext', () => {
       '<div>Hello</div><br/>World',
       { ignoreTags: ['br'] },
     )
-    expect(result).toContain('<br/>')
+    expect(result).toContain('<br>')
+  })
+
+  it('separates block-level elements with blank lines', () => {
+    const result = createPlaintext('<h1>Hello Jane</h1><p>You were invited to join <strong>Acme</strong>.</p><div><a href="https://example.com">Accept</a></div><p>Acme Inc.</p>')
+    expect(result).toBe('Hello Jane\n\nYou were invited to join Acme.\n\nAccept\n\nhttps://example.com\n\nAcme Inc.')
+  })
+
+  it('breaks lines at <br> and list items', () => {
+    expect(createPlaintext('<p>One<br>Two</p>')).toBe('One\nTwo')
+    expect(createPlaintext('<ul><li>One</li><li>Two</li></ul>')).toBe('One\nTwo')
+  })
+
+  it('does not depend on source formatting', () => {
+    const minified = '<div><p>Hello <strong>Jane</strong>,</p><p>Welcome!</p></div>'
+    const formatted = `<div>
+  <p>
+    Hello
+    <strong>Jane</strong>,
+  </p>
+  <p>Welcome!</p>
+</div>`
+    expect(createPlaintext(formatted)).toBe(createPlaintext(minified))
+    expect(createPlaintext(minified)).toBe('Hello Jane,\n\nWelcome!')
+  })
+
+  it('keeps line breaks in <pre>', () => {
+    expect(createPlaintext('<pre>a\nb</pre>')).toBe('a\nb')
+  })
+
+  it('removes zero-width filler text', () => {
+    const result = createPlaintext('<p>Hello</p><div role="separator">&zwj;</div><p>World</p><div>&zwj;</div>')
+    expect(result).toBe('Hello\n\nWorld')
+  })
+
+  it('keeps joiners inside text', () => {
+    expect(createPlaintext('<p>&#x1F469;&zwj;&#x1F4BB;</p>')).toBe(String.fromCodePoint(0x1F469, 0x200D, 0x1F4BB))
+    expect(createPlaintext('<p>&#x645;&#x6CC;&zwnj;&#x62E;</p>')).toBe(String.fromCodePoint(0x645, 0x6CC, 0x200C, 0x62E))
   })
 
   it('strips a full email template', () => {