fix(handler): onSubscribe can return readonly GraphQLErrors

Author: enisdenjo

docs/interfaces/HandlerOptions.md

@@ -123,11 +123,11 @@ ___
 
 ### onSubscribe
 
-• `Optional` **onSubscribe**: (`req`: [`Request`](Request.md)<`RawRequest`\>, `params`: [`RequestParams`](RequestParams.md)) => `void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
+• `Optional` **onSubscribe**: (`req`: [`Request`](Request.md)<`RawRequest`\>, `params`: [`RequestParams`](RequestParams.md)) => `void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
 
 #### Type declaration
 
-▸ (`req`, `params`): `void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
+▸ (`req`, `params`): `void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
 
 The subscribe callback executed right after processing the request
 before proceeding with the GraphQL operation execution.
@@ -159,7 +159,7 @@ further execution.
 
 ##### Returns
 
-`void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
+`void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs` \| `Promise`<`void` \| readonly `GraphQLError`[] \| [`Response`](../README.md#response) \| `ExecutionArgs`\>
 
 ___
 

src/handler.ts

@@ -118,9 +118,9 @@ export interface HandlerOptions<RawRequest = unknown> {
     req: Request<RawRequest>,
     params: RequestParams,
   ) =>
-    | Promise<ExecutionArgs | GraphQLError[] | Response | void>
+    | Promise<ExecutionArgs | readonly GraphQLError[] | Response | void>
     | ExecutionArgs
-    | GraphQLError[]
+    | readonly GraphQLError[]
     | Response
     | void;
   /**

src/utils.ts

@@ -12,7 +12,7 @@ export function isObject(val: unknown): val is Record<PropertyKey, unknown> {
 }
 
 /** @private */
-export function areGraphQLErrors(obj: unknown): obj is GraphQLError[] {
+export function areGraphQLErrors(obj: unknown): obj is readonly GraphQLError[] {
   return (
     Array.isArray(obj) &&
     // must be at least one error