typescript-eslint · JoshuaKGoldberg · Sep 29, 2025 · Sep 18, 2025 · Sep 18, 2025 · Sep 18, 2025
diff --git a/packages/eslint-plugin/src/rules/no-base-to-string.ts b/packages/eslint-plugin/src/rules/no-base-to-string.ts
@@ -212,6 +212,36 @@ export default createRule<Options, MessageIds>({
       return Usefulness.Always;
     }
 
+    function hasBaseTypes(type: ts.Type): type is ts.InterfaceType {
+      return (
+        tsutils.isObjectType(type) &&
+        tsutils.isObjectFlagSet(
+          type,
+          ts.ObjectFlags.Interface | ts.ObjectFlags.Class,
+        )
+      );
+    }
+
+    function isIgnoredTypeOrBase(
+      type: ts.Type,
+      seen = new Set<ts.Type>(),
+    ): boolean {
+      if (seen.has(type)) {
+        return false;
+      }
+
+      seen.add(type);
+
+      const typeName = getTypeName(checker, type);
+      return (
+        ignoredTypeNames.includes(typeName) ||
+        (hasBaseTypes(type) &&
+          checker
+            .getBaseTypes(type)
+            .some(base => isIgnoredTypeOrBase(base, seen)))
+      );
+    }
+
     function collectToStringCertainty(
       type: ts.Type,
       visited: Set<ts.Type>,
@@ -249,7 +279,7 @@ export default createRule<Options, MessageIds>({
         return Usefulness.Always;
       }
 
-      if (ignoredTypeNames.includes(getTypeName(checker, type))) {
+      if (isIgnoredTypeOrBase(type)) {
         return Usefulness.Always;
       }
 

diff --git a/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts b/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts
@@ -324,6 +324,47 @@ error.toString();
       `,
       options: [{ ignoredTypeNames: ['MyError'] }],
     },
+    {
+      code: `
+interface Animal {}
+interface Serializable {}
+interface Cat extends Animal, Serializable {}
+
+declare const whiskers: Cat;
+whiskers.toString();
+      `,
+      options: [{ ignoredTypeNames: ['Animal'] }],
+    },
+    {
+      code: `
+interface MyError extends Error {}
+
+declare const error: MyError;
+error.toString();
+      `,
+    },
+    {
+      code: `
+class UnknownBase {}
+class CustomError extends UnknownBase {}
+
+declare const err: CustomError;
+err.toString();
+      `,
+      options: [{ ignoredTypeNames: ['UnknownBase'] }],
+    },
+    {
+      code: `
+interface Animal {}
+interface Dog extends Animal {}
+interface Cat extends Animal {}
+
+declare const dog: Dog;
+declare const cat: Cat;
+cat.toString();
+      `,
+      options: [{ ignoredTypeNames: ['Animal'] }],
+    },
     `
 function String(value) {
   return value;
@@ -2265,5 +2306,60 @@ v.join();
         },
       ],
     },
+    {
+      code: `
+interface Dog extends Animal {}
+
+declare const labrador: Dog;
+labrador.toString();
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: 'labrador',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
+    {
+      code: `
+interface A extends B {}
+interface B extends A {}
+
+declare const a: A;
+a.toString();
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: 'a',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
+    {
+      code: `
+        interface Base {}
+        interface Left extends Base {}
+        interface Right extends Base {}
+        interface Diamond extends Left, Right {}
+
+        declare const d: Diamond;
+        d.toString();
+      `,
+      errors: [
+        {
+          data: {
+            certainty: 'will',
+            name: 'd',
+          },
+          messageId: 'baseToString',
+        },
+      ],
+    },
   ],
 });