fix(eslint-plugin): [prefer-nullish-coalescing] handle case when type of left side is null or undefined (typescript-eslint#7225)

auvred · web-flow · commit b62affe8ddac · 2023-07-17T15:13:07.000-04:00
diff --git 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
@@ -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',
+        },
+      ],
+    },
   ],
 });

Original file line number	Diff line number	Diff line change
`@@ -310,6 +310,8 @@ export default util.createRule<Options, MessageIds>({`
`310`	`310`	`.filter((flag): flag is number => flag !== undefined)`
`311`	`311`	`.reduce((previous, flag) => previous \| flag, 0);`
`312`	`312`	`if (`
	`313`	`+ type.flags !== ts.TypeFlags.Null &&`
	`314`	`+ type.flags !== ts.TypeFlags.Undefined &&`
`313`	`315`	`(type as ts.UnionOrIntersectionType).types.some(t =>`
`314`	`316`	`tsutils.isTypeFlagSet(t, ignorableFlags),`
`315`	`317`	`)`