Merge pull request #18 from JoshuaKGoldberg/readonly-modifiers-check

fix: account for mapped types without modifiers in isPropertyReadonlyInType

src/types/utilities.test.ts

@@ -0,0 +1,26 @@
+import * as ts from "typescript";
+import { describe, expect, it } from "vitest";
+
+import { createSourceFileAndTypeChecker } from "../test/utils";
+import { isPropertyReadonlyInType } from "./utilities";
+
+describe("isPropertyReadonlyInType", () => {
+ it("does not crash when the type is a mapped type parameter extending any", () => {
+ const { sourceFile, typeChecker } = createSourceFileAndTypeChecker(`
+ type MyType<T> = {
+ [K in keyof T]: 'cat' | 'dog' | T[K];
+ };
+ type Test<A extends any[]> = MyType<A>;
+ `);
+ const node = sourceFile.statements.at(-1) as ts.TypeAliasDeclaration;
+ const type = typeChecker.getTypeAtLocation(node);
+
+ expect(
+ isPropertyReadonlyInType(
+ type,
+ ts.escapeLeadingUnderscores("length"),
+ typeChecker
+ )
+ ).toBe(false);
+ });
+});

src/types/utilities.ts

@@ -92,10 +92,11 @@ function isReadonlyPropertyFromMappedType(
  !/^__@[^@]+$/.test(name as string)
  )
  return declaration.readonlyToken.kind !== ts.SyntaxKind.MinusToken;
- return isPropertyReadonlyInType(
- (type as unknown as { modifiersType: ts.Type }).modifiersType,
- name,
- typeChecker
+
+ const { modifiersType } = type as { modifiersType?: ts.Type };
+
+ return (
+ modifiersType && isPropertyReadonlyInType(modifiersType, name, typeChecker)
  );
 }