fix(sanitize-html): emit transformTags text on empty tags when textFilter is set (#5494)

When a transformTags handler adds text to an allowed tag that originally had
no text content, the injected text was silently dropped if any textFilter was
configured. The onopentag branch that emits frame.innerText was guarded by
!options.textFilter, deferring emission to ontext so the filter could run
there. For an empty element htmlparser2 never fires ontext, so the text was
emitted by neither branch.

Emit frame.innerText through options.textFilter here when present (mirroring
the discard path), so the transformTags text contract holds for empty tags
regardless of whether a textFilter is set.

Author: spokodev

packages/sanitize-html/index.js

@@ -540,8 +540,13 @@ function sanitizeHtml(html, options, _recursing) {
         result += ' />';
       } else {
         result += '>';
-        if (frame.innerText && !hasText && !options.textFilter) {
-          result += escapeHtml(frame.innerText);
+        if (frame.innerText && !hasText) {
+          const escaped = escapeHtml(frame.innerText);
+          if (options.textFilter) {
+            result += options.textFilter(escaped, name);
+          } else {
+            result += escaped;
+          }
           addedText = true;
         }
       }

packages/sanitize-html/test/test.js

@@ -323,6 +323,24 @@ describe('sanitizeHtml', function() {
     }), '<a href="http://somelink">some new text</a>');
   });
 
+  it('should add new text to an empty tag when transforming function sets it and an identity textFilter is present', function () {
+    assert.equal(sanitizeHtml('<a></a>', {
+      allowedTags: [ 'a' ],
+      textFilter: function (text) {
+        return text;
+      },
+      transformTags: {
+        a: function (tagName, attribs) {
+          return {
+            tagName,
+            attribs,
+            text: 'some new text'
+          };
+        }
+      }
+    }), '<a>some new text</a>');
+  });
+
   it('should preserve text when initially set and replace attributes when they are changed by transforming function', function () {
     assert.equal(sanitizeHtml('<a href="http://somelink">some initial text</a>', {
       transformTags: {