notebook document, is untitled.

src/vs/vscode.proposed.d.ts

@@ -1432,6 +1432,7 @@ declare module 'vscode' {
 		readonly fileName: string;
 		readonly viewType: string;
 		readonly isDirty: boolean;
+		readonly isUntitled: boolean;
 		readonly cells: NotebookCell[];
 		languages: string[];
 		displayOrder?: GlobPattern[];

src/vs/workbench/api/common/extHostNotebook.ts

@@ -215,6 +215,10 @@ export class ExtHostNotebookDocument extends Disposable implements vscode.Notebo
 		this._proxy.$updateNotebookLanguages(this.viewType, this.uri, this._languages);
 	}
 
+	get isUntitled() {
+		return this.uri.scheme === Schemas.untitled;
+	}
+
 	private _metadata: Required<vscode.NotebookDocumentMetadata> = notebookDocumentMetadataDefaults;
 	private _metadataChangeListener: IDisposable;