backwards compatible

src/assets/js/datastore.ts

@@ -161,6 +161,16 @@ export default class DataStore {
     return await this._set(this._lineKey_(row), line);
   }
 
+  // for backwards compatibility - checks whether the line was struck through
+  // in the old-style format
+  public async _isStruckThroughOldFormat(row: Row): Promise<boolean> {
+    const line = await this._get<any>(this._lineKey_(row), []);
+    if (typeof line !== 'string' && line.length && line[0].properties.strikethrough) {
+      return true;
+    }
+    return false;
+  }
+
   public async getParents(row: Row): Promise<Array<Row>> {
     return await this._get(this._parentsKey_(row), [], decodeParents);
   }

src/plugins/todo/index.tsx

@@ -33,6 +33,10 @@ registerPlugin<void>(
     });
 
     async function isStruckThrough(session: Session, row: Row) {
+      // for backwards compatibility
+      const isStruckThroughOldStyle = await session.document.store._isStruckThroughOldFormat(row);
+      if (isStruckThroughOldStyle) { return true; }
+
       const text = await session.document.getText(row);
       return (text.slice(0, 2) === '~~') && (text.slice(-2) === '~~');
     }