Updated code as per suggestions on-behalf-of: @MermaidChart <hello@mermaidchart.com>

packages/mermaid/src/diagrams/treemap/parser.ts

@@ -4,6 +4,7 @@ import { log } from '../../logger.js';
 import { populateCommonDb } from '../common/populateCommonDb.js';
 import type { TreemapNode, TreemapAst, TreemapDB } from './types.js';
 import { buildHierarchy } from './utils.js';
+import { TreeMapDB } from './db.js';
 
 /**
  * Populates the database with data from the Treemap AST
@@ -83,7 +84,7 @@ const getItemName = (item: { name?: string | number }): string => {
 };
 
 export const parser: ParserDefinition = {
-  parser: { yy: {} as TreemapDB },
+  parser: { yy: new TreeMapDB() },
   parse: async (text: string): Promise<void> => {
     try {
       // Use a generic parse that accepts any diagram type
@@ -91,7 +92,13 @@ export const parser: ParserDefinition = {
       const parseFunc = parse as (diagramType: string, text: string) => Promise<TreemapAst>;
       const ast = await parseFunc('treemap', text);
       log.debug('Treemap AST:', ast);
-      populate(ast, parser.parser?.yy as TreemapDB);
+      const db = parser.parser?.yy;
+      if (!(db instanceof TreeMapDB)) {
+        throw new Error(
+          'parser.parser?.yy was not a TreemapDB. This is due to a bug within Mermaid, please report this issue at https://github.com/mermaid-js/mermaid/issues.'
+        );
+      }
+      populate(ast, db);
     } catch (error) {
       log.error('Error parsing treemap:', error);
       throw error;