backend onChange undefined about hocuspocus HOT 2 CLOSED

...
import { TiptapTransformer } from '@hocuspocus/transformer'

...
async onChange(data) {
  const prosemirrorJSON = TiptapTransformer.fromYdoc(data.document);
  console.log(prosemirrorJSON)
},

[onChange] item.toDelta is not a function
file:///xxxx/hocuspocus/node_modules/y-prosemirror/src/lib.js:331
      const delta = item.toDelta()
                         ^

TypeError: item.toDelta is not a function
    at serialize (file:///xxxx/hocuspocus/node_modules/y-prosemirror/src/lib.js:331:26)
    at Array.map (<anonymous>)
    at yXmlFragmentToProsemirrorJSON (file:///xxxx/hocuspocus/node_modules/y-prosemirror/src/lib.js:375:20)
    at yDocToProsemirrorJSON (file:///xxxx/hocuspocus/node_modules/y-prosemirror/src/lib.js:308:10)      
    at Prosemirror.fromYdoc (file:///xxxx/hocuspocus/node_modules/@hocuspocus/transformer/dist/hocuspocus-transformer.esm.js:2625:20)
    at Tiptap.fromYdoc (file:///xxxx/hocuspocus/node_modules/@hocuspocus/transformer/dist/hocuspocus-transformer.esm.js:2667:39)
    at Object.onChange (file:///xxxx/hocuspocus/index.js:14:47)
    at file:///xxxx/hocuspocus/node_modules/@hocuspocus/server/dist/hocuspocus-server.esm.js:2806:109    
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)