chore: setup new linting rule

Author: RebeccaStevens

.eslintrc.json

@@ -30,6 +30,8 @@
   "rules": {
     "@typescript-eslint/no-unnecessary-condition": "off",
     "import/no-relative-parent-imports": "error",
+    "functional/prefer-readonly-type": "off",
+    "functional/prefer-readonly-type-declaration": "warn",
     "node/no-unsupported-features/es-builtins": "off",
     "node/no-unsupported-features/es-syntax": "off",
     // enable once supported in all our supported node versions.

tests/helpers/util.ts

@@ -82,10 +82,8 @@ export function createDummyRule(
   } as unknown as Rule.RuleModule;
 }
 
-export type RuleTesterTests = {
-  // eslint-disable-next-line functional/prefer-readonly-type
+export type MutableRuleTesterTests = {
   valid?: Array<ESLintRuleTester.ValidTestCase | string>;
-  // eslint-disable-next-line functional/prefer-readonly-type
   invalid?: Array<ESLintRuleTester.InvalidTestCase>;
 };
 
@@ -94,8 +92,8 @@ export type RuleTesterTests = {
  */
 export function addFilename(
   filename: string,
-  tests: RuleTesterTests
-): RuleTesterTests {
+  tests: MutableRuleTesterTests
+): MutableRuleTesterTests {
   const { valid, invalid } = tests;
   return {
     invalid: invalid?.map((test) => ({ ...test, filename })),