From 841d45f8b45715083cebe612ce970220cb1a5be2 Mon Sep 17 00:00:00 2001
From: auvred <aauvred@gmail.com>
Date: Fri, 14 Jul 2023 19:52:57 +0000
Subject: [PATCH] fix(eslint-plugin): [prefer-nullish-coalescing] handle case
 when type of left side is null or undefined

---
 .../src/rules/prefer-nullish-coalescing.ts    |  2 +
 .../rules/prefer-nullish-coalescing.test.ts   | 42 +++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts b/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
index 99bc76429d21..f9a953f501cf 100644
--- a/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
+++ b/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
@@ -310,6 +310,8 @@ export default util.createRule<Options, MessageIds>({
           .filter((flag): flag is number => flag !== undefined)
           .reduce((previous, flag) => previous | flag, 0);
         if (
+          type.flags !== ts.TypeFlags.Null &&
+          type.flags !== ts.TypeFlags.Undefined &&
           (type as ts.UnionOrIntersectionType).types.some(t =>
             tsutils.isTypeFlagSet(t, ignorableFlags),
           )
diff --git a/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts b/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
index 022eb5cdb8bc..9ba8d2dd34f2 100644
--- a/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
@@ -1208,5 +1208,47 @@ x || y;
         },
       ],
     },
+    {
+      code: `
+declare const x: null;
+x || y;
+      `,
+      errors: [
+        {
+          messageId: 'preferNullishOverOr',
+        },
+      ],
+    },
+    {
+      code: `
+const x = undefined;
+x || y;
+      `,
+      errors: [
+        {
+          messageId: 'preferNullishOverOr',
+        },
+      ],
+    },
+    {
+      code: `
+null || y;
+      `,
+      errors: [
+        {
+          messageId: 'preferNullishOverOr',
+        },
+      ],
+    },
+    {
+      code: `
+undefined || y;
+      `,
+      errors: [
+        {
+          messageId: 'preferNullishOverOr',
+        },
+      ],
+    },
   ],
 });