typescript-eslint · bradzacher · Jan 17, 2022 · Jan 9, 2022 · Jan 9, 2022 · Jan 9, 2022
diff --git a/packages/type-utils/src/isTypeReadonly.ts b/packages/type-utils/src/isTypeReadonly.ts
@@ -3,10 +3,10 @@ import {
   isConditionalType,
   isObjectType,
   isUnionType,
-  isUnionOrIntersectionType,
   unionTypeParts,
   isPropertyReadonlyInType,
   isSymbolFlagSet,
+  isIntersectionType,
 } from 'tsutils';
 import * as ts from 'typescript';
 import { getTypeOfPropertyOfType } from './propertyTypes';
@@ -224,6 +224,32 @@ function isTypeReadonlyRecurser(
     return readonlyness;
   }
 
+  if (isIntersectionType(type)) {
+    // Special case for handling arrays/tuples (as readonly arrays/tuples always have mutable methods).
+    if (
+      type.types.some(t => checker.isArrayType(t) || checker.isTupleType(t))
+    ) {
+      const allReadonlyParts = type.types.every(
+        t =>
+          seenTypes.has(t) ||
+          isTypeReadonlyRecurser(checker, t, options, seenTypes) ===
+            Readonlyness.Readonly,
+      );
+      return allReadonlyParts ? Readonlyness.Readonly : Readonlyness.Mutable;
+    }
+
+    // Normal case.
+    const isReadonlyObject = isTypeReadonlyObject(
+      checker,
+      type,
+      options,
+      seenTypes,
+    );
+    if (isReadonlyObject !== Readonlyness.UnknownType) {
+      return isReadonlyObject;
+    }
+  }
+
   if (isConditionalType(type)) {
     const result = [type.root.node.trueType, type.root.node.falseType]
       .map(checker.getTypeFromTypeNode)
@@ -240,7 +266,7 @@ function isTypeReadonlyRecurser(
 
   // all non-object, non-intersection types are readonly.
   // this should only be primitive types
-  if (!isObjectType(type) && !isUnionOrIntersectionType(type)) {
+  if (!isObjectType(type)) {
     return Readonlyness.Readonly;
   }
 

diff --git a/packages/type-utils/tests/isTypeReadonly.test.ts b/packages/type-utils/tests/isTypeReadonly.test.ts
@@ -165,6 +165,50 @@ describe('isTypeReadonly', () => {
         });
       });
 
+      describe('Intersection', () => {
+        describe('is readonly', () => {
+          const runTests = runTestIsReadonly;
+
+          it.each([
+            [
+              'type Test = Readonly<{ foo: string; bar: number; }> & Readonly<{ bar: number; }>;',
+            ],
+          ])('handles an intersection of 2 fully readonly types', runTests);
+
+          it.each([
+            [
+              'type Test = Readonly<{ foo: string; bar: number; }> & { foo: string; };',
+            ],
+          ])(
+            'handles an intersection of a fully readonly type with a mutable subtype',
+            runTests,
+          );
+
+          // Array - special case.
+          // Note: Methods are mutable but arrays are treated special; hence no failure.
+          it.each([
+            ['type Test = ReadonlyArray<string> & Readonly<{ foo: string; }>;'],
+            [
+              'type Test = readonly [string, number] & Readonly<{ foo: string; }>;',
+            ],
+          ])('handles an intersections involving a readonly array', runTests);
+        });
+
+        describe('is not readonly', () => {
+          const runTests = runTestIsNotReadonly;
+
+          it.each([
+            ['type Test = { foo: string; bar: number; } & { bar: number; };'],
+            [
+              'type Test = { foo: string; bar: number; } & Readonly<{ bar: number; }>;',
+            ],
+            [
+              'type Test = Readonly<{ bar: number; }> & { foo: string; bar: number; };',
+            ],
+          ])('handles an intersection of non fully readonly types', runTests);
+        });
+      });
+
       describe('Conditional Types', () => {
         describe('is readonly', () => {
           const runTests = runTestIsReadonly;