Move toEntry to be a method so that it can be override, if necessary

testsuite/tests/util/string.test.ts

@@ -59,4 +59,8 @@ describe('string functions', () => {
     expect(string.replaceUnicode(String.raw`x\\\\\U{61}`)).toBe(String.raw`x\\\\a`);
   });
 
+  test('toEntity()', () => {
+    expect(string.toEntity('\u200B')).toBe('​');
+  });
+
 });

ts/core/MmlTree/SerializedMmlVisitor.ts

@@ -25,9 +25,7 @@
 import { MmlVisitor } from './MmlVisitor.js';
 import { MmlNode, TextNode, XMLNode } from './MmlNode.js';
 import { HtmlNode } from './MmlNodes/HtmlNode.js';
-
-export const toEntity = (c: string) =>
-  '&#x' + c.codePointAt(0).toString(16).toUpperCase() + ';';
+import { toEntity } from '../../util/string.js';
 
 /*****************************************************************/
 /**
@@ -161,7 +159,17 @@ export class SerializedMmlVisitor extends MmlVisitor {
       .replace(/</g, '&lt;')
       .replace(/>/g, '&gt;')
       .replace(/"/g, '&quot;')
-      .replace(/[\uD800-\uDBFF]./g, toEntity)
-      .replace(/[\u0080-\uD7FF\uE000-\uFFFF]/g, toEntity);
+      .replace(/[\uD800-\uDBFF]./g, this.toEntity)
+      .replace(/[\u0080-\uD7FF\uE000-\uFFFF]/g, this.toEntity);
+  }
+
+  /**
+   * Access to the toEntity() function that can be overridden in subclasses.
+   *
+   * @param {string} c   The character to encode.
+   * @returns {string}   The numeric entity for the character.
+   */
+  protected toEntity(c: string): string {
+    return toEntity(c);
   }
 }

ts/util/string.ts

@@ -100,3 +100,13 @@ export function replaceUnicode(text: string): string {
     (_m, pre, h1, h2) => pre + String.fromCodePoint(parseInt(h1 || h2, 16))
   );
 }
+
+/**
+ * Turn a character into its numeric entity for HTML
+ *
+ * @param {string} c   The character to convert
+ * @returns {string}   The entity string for the character
+ */
+export function toEntity(c: string): string {
+  return `&#x${c.codePointAt(0).toString(16).toUpperCase()};`;
+}