feat(experimental-utils): allow rule options to be a readonly tuple (typescript-eslint#1924)

ulrichb · web-flow · commit 4ef67884962b · 2020-04-21T09:30:43.000-07:00
diff --git a/packages/experimental-utils/src/eslint-utils/RuleCreator.ts b/packages/experimental-utils/src/eslint-utils/RuleCreator.ts
@@ -17,7 +17,7 @@ function RuleCreator(urlCreator: (ruleName: string) => string) {
   // This function will get much easier to call when this is merged https://github.com/Microsoft/TypeScript/pull/26349
   // TODO - when the above PR lands; add type checking for the context.report `data` property
   return function createRule<
-    TOptions extends unknown[],
+    TOptions extends readonly unknown[],
     TMessageIds extends string,
     TRuleListener extends RuleListener = RuleListener
   >({
diff --git a/packages/experimental-utils/src/eslint-utils/applyDefault.ts b/packages/experimental-utils/src/eslint-utils/applyDefault.ts
@@ -7,12 +7,14 @@ import { deepMerge, isObjectNotArray } from './deepMerge';
  * @param userOptions the user opts
  * @returns the options with defaults
  */
-function applyDefault<TUser extends unknown[], TDefault extends TUser>(
+function applyDefault<TUser extends readonly unknown[], TDefault extends TUser>(
   defaultOptions: TDefault,
   userOptions: TUser | null,
 ): TDefault {
   // clone defaults
-  const options: TDefault = JSON.parse(JSON.stringify(defaultOptions));
+  const options: AsMutable<TDefault> = JSON.parse(
+    JSON.stringify(defaultOptions),
+  );
 
   if (userOptions === null || userOptions === undefined) {
     return options;
@@ -33,4 +35,6 @@ function applyDefault<TUser extends unknown[], TDefault extends TUser>(
   return options;
 }
 
+type AsMutable<T extends {}> = { -readonly [TKey in keyof T]: T[TKey] };
+
 export { applyDefault };
