Fix <br> at the beginning or end of <b> and <i>

Fixes bug mixmark-io#436

src/commonmark-rules.js

@@ -207,12 +207,26 @@ rules.referenceLink = {
   }
 }
 
+const WHITESPACE_START = /^(\\?\n| )+/
+const WHITESPACE_END = /(\\?\n| )+$/
 rules.emphasis = {
   filter: ['em', 'i'],
 
   replacement: function (content, node, options) {
     if (!content.trim()) return ''
-    return options.emDelimiter + content + options.emDelimiter
+    var startWhitespace = ''
+    var endWhitespace = ''
+    var m = WHITESPACE_START.exec(content)
+    if (m) {
+      startWhitespace = m[0]
+      content = content.slice(startWhitespace.length)
+    }
+    m = WHITESPACE_END.exec(content)
+    if (m) {
+      endWhitespace = m[0]
+      content = content.slice(0, -endWhitespace.length)
+    }
+    return startWhitespace + options.emDelimiter + content + options.emDelimiter + endWhitespace
   }
 }
 
@@ -221,7 +235,19 @@ rules.strong = {
 
   replacement: function (content, node, options) {
     if (!content.trim()) return ''
-    return options.strongDelimiter + content + options.strongDelimiter
+    var startWhitespace = ''
+    var endWhitespace = ''
+    var m = WHITESPACE_START.exec(content)
+    if (m) {
+      startWhitespace = m[0]
+      content = content.slice(startWhitespace.length)
+    }
+    m = WHITESPACE_END.exec(content)
+    if (m) {
+      endWhitespace = m[0]
+      content = content.slice(0, -endWhitespace.length)
+    }
+    return startWhitespace + options.strongDelimiter + content + options.strongDelimiter + endWhitespace
   }
 }
 

test/index.html

@@ -693,6 +693,13 @@ <h2>This is a header.</h2>
   <pre class="expected">Text with no space after the period. _Text in em with leading/trailing spaces_ **text in strong with trailing space**</pre>
 </div>
 
+<!-- Two trailing newlines are for the line break -->
+<div class="case" data-name="newline at the end of a strong element">
+    <div class="input"><strong>Text with a trailing break<br></strong>before continuation</div>
+    <pre class="expected">**Text with a trailing break**  
+before continuation</pre>
+</div>
+
 <div class="case" data-name="whitespace in nested inline elements">
   <div class="input">Text at root <strong><a href="http://www.example.com">link text with trailing space in strong </a></strong>more text at root</div>
   <pre class="expected">Text at root **[link text with trailing space in strong](http://www.example.com)** more text at root</pre>