From c1fe8e8927a78d1c9d0fefcbe5fe99ee05d08145 Mon Sep 17 00:00:00 2001
From: Galen Kistler <109082771+gtk-grafana@users.noreply.github.com>
Date: Fri, 16 Jun 2023 07:01:40 -0500
Subject: [PATCH] Prometheus: Expr undefined at runtime (#70010)

* null/undefined expression default to empty string
---
 .../app/plugins/datasource/prometheus/querybuilder/state.ts  | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/public/app/plugins/datasource/prometheus/querybuilder/state.ts b/public/app/plugins/datasource/prometheus/querybuilder/state.ts
index 8676fdfc999..24be5dd5dc0 100644
--- a/public/app/plugins/datasource/prometheus/querybuilder/state.ts
+++ b/public/app/plugins/datasource/prometheus/querybuilder/state.ts
@@ -36,7 +36,7 @@ function getDefaultEditorMode(expr: string, defaultEditor: QueryEditorMode = Que
  * Returns query with defaults, and boolean true/false depending on change was required
  */
 export function getQueryWithDefaults(
-  query: PromQuery,
+  query: PromQuery & { expr?: string },
   app: CoreApp | undefined,
   defaultEditor?: QueryEditorMode
 ): PromQuery {
@@ -47,7 +47,8 @@ export function getQueryWithDefaults(
   }
 
   // default query expr is now empty string, set in getDefaultQuery
-  if (query.expr === '') {
+  // While expr is required in the types, it is not always defined at runtime, so we need to check for undefined and default to an empty string to prevent runtime errors
+  if (!query.expr) {
     result = { ...result, expr: '', legendFormat: LegendFormatMode.Auto };
   }