fix: use private "__kind" property instead of instanceof

src/browser/setupWorker/start/createFallbackRequestListener.ts

@@ -8,7 +8,7 @@ import { XMLHttpRequestInterceptor } from '@mswjs/interceptors/XMLHttpRequest'
 import { SetupWorkerInternalContext, StartOptions } from '../glossary'
 import type { RequiredDeep } from '~/core/typeUtils'
 import { handleRequest } from '~/core/utils/handleRequest'
-import { toRequestHandlersOnly } from '~/core/utils/internal/filterRequestHandlers'
+import { toRequestHandlersOnly } from '~/core/utils/internal/toRequestHandlersOnly'
 
 export function createFallbackRequestListener(
   context: SetupWorkerInternalContext,

src/browser/setupWorker/start/createRequestListener.ts

@@ -13,7 +13,7 @@ import { handleRequest } from '~/core/utils/handleRequest'
 import { RequiredDeep } from '~/core/typeUtils'
 import { devUtils } from '~/core/utils/internal/devUtils'
 import { toResponseInit } from '~/core/utils/toResponseInit'
-import { toRequestHandlersOnly } from '~/core/utils/internal/filterRequestHandlers'
+import { toRequestHandlersOnly } from '~/core/utils/internal/toRequestHandlersOnly'
 
 export const createRequestListener = (
   context: SetupWorkerInternalContext,

src/core/handlers/RequestHandler.ts

@@ -117,6 +117,8 @@ export abstract class RequestHandler<
     StrictRequest<DefaultBodyType>
   >()
 
+  private readonly __kind: 'RequestHandler'
+
   public info: HandlerInfo & RequestHandlerInternalInfo
   /**
    * Indicates whether this request handler has been used
@@ -151,6 +153,7 @@ export abstract class RequestHandler<
     }
 
     this.isUsed = false
+    this.__kind = 'RequestHandler'
   }
 
   /**

src/core/handlers/WebSocketHandler.ts

@@ -28,6 +28,8 @@ const kStopPropagationPatched = Symbol('kStopPropagationPatched')
 const KOnStopPropagation = Symbol('KOnStopPropagation')
 
 export class WebSocketHandler {
+  private readonly __kind: 'WebSocketHandler'
+
   public id: string
   public callFrame?: string
 
@@ -38,6 +40,7 @@ export class WebSocketHandler {
 
     this[kEmitter] = new Emitter()
     this.callFrame = getCallFrame(new Error())
+    this.__kind = 'WebSocketHandler'
   }
 
   public parse(args: {

src/core/utils/internal/toRequestHandlersOnly.test.ts

@@ -0,0 +1,40 @@
+import { GraphQLHandler } from '../../handlers/GraphQLHandler'
+import { HttpHandler } from '../../handlers/HttpHandler'
+import { RequestHandler } from '../../handlers/RequestHandler'
+import { WebSocketHandler } from '../../handlers/WebSocketHandler'
+import { toRequestHandlersOnly } from './toRequestHandlersOnly'
+
+it('returns true for HttpHandler', () => {
+  expect(toRequestHandlersOnly(new HttpHandler('*', '*', () => {}))).toBe(true)
+})
+
+it('returns true for GraphQLHandler', () => {
+  expect(
+    toRequestHandlersOnly(new GraphQLHandler('all', '*', '*', () => {})),
+  ).toBe(true)
+})
+
+it('returns true for a custom RequestHandler', () => {
+  class MyHandler extends RequestHandler {
+    constructor() {
+      super({ info: { header: '*' }, resolver: () => {} })
+    }
+    predicate = () => false
+    log() {}
+  }
+
+  expect(toRequestHandlersOnly(new MyHandler())).toBe(true)
+})
+
+it('returns false for a WebSocketHandler', () => {
+  expect(toRequestHandlersOnly(new WebSocketHandler('*'))).toBe(false)
+})
+
+it('returns false for an arbitrary values', () => {
+  expect(toRequestHandlersOnly(undefined)).toBe(false)
+  expect(toRequestHandlersOnly(null)).toBe(false)
+  expect(toRequestHandlersOnly({})).toBe(false)
+  expect(toRequestHandlersOnly([])).toBe(false)
+  expect(toRequestHandlersOnly(123)).toBe(false)
+  expect(toRequestHandlersOnly('hello')).toBe(false)
+})

src/core/utils/internal/filterRequestHandlers.ts → src/core/utils/internal/toRequestHandlersOnly.ts

@@ -6,5 +6,10 @@ import { RequestHandler } from '../../handlers/RequestHandler'
  * out other handlers, like `WebSocketHandler`.
  */
 export function toRequestHandlersOnly(input: unknown): input is RequestHandler {
-  return input instanceof RequestHandler
+  return (
+    input != null &&
+    typeof input === 'object' &&
+    '__kind' in input &&
+    input.__kind === 'RequestHandler'
+  )
 }

src/node/SetupServerCommonApi.ts

@@ -20,7 +20,7 @@ import { InternalError, devUtils } from '~/core/utils/internal/devUtils'
 import type { SetupServerCommon } from './glossary'
 import { handleWebSocketEvent } from '~/core/ws/handleWebSocketEvent'
 import { webSocketInterceptor } from '~/core/ws/webSocketInterceptor'
-import { toRequestHandlersOnly } from '~/core/utils/internal/filterRequestHandlers'
+import { toRequestHandlersOnly } from '~/core/utils/internal/toRequestHandlersOnly'
 
 export const DEFAULT_LISTEN_OPTIONS: RequiredDeep<SharedOptions> = {
   onUnhandledRequest: 'warn',