fix: use internal request id header vs "X-Request-Id"

src/Interceptor.ts

@@ -1,9 +1,19 @@
 import { Logger } from '@open-draft/logger'
-import { Emitter, EventMap, Listener } from 'strict-event-emitter'
+import { Emitter, Listener } from 'strict-event-emitter'
 
 export type InterceptorEventMap = Record<string, any>
 export type InterceptorSubscription = () => void
 
+/**
+ * Request header name to detect when a single request
+ * is being handled by nested interceptors (XHR -> ClientRequest).
+ * Obscure by design to prevent collisions with user-defined headers.
+ * Ideally, come up with the Interceptor-level mechanism for this.
+ * @see https://github.com/mswjs/interceptors/issues/378
+ */
+export const INTERNAL_REQUEST_ID_HEADER_NAME =
+  'x-interceptors-internal-request-id'
+
 export function getGlobalSymbol<V>(symbol: Symbol): V | undefined {
   return (
     // @ts-ignore https://github.com/Microsoft/TypeScript/issues/24587

src/interceptors/ClientRequest/NodeClientRequest.ts

@@ -21,6 +21,7 @@ import { uuidv4 } from '../../utils/uuid'
 import { emitAsync } from '../../utils/emitAsync'
 import { getRawFetchHeaders } from '../../utils/getRawFetchHeaders'
 import { isPropertyAccessible } from '../../utils/isPropertyAccessible'
+import { INTERNAL_REQUEST_ID_HEADER_NAME } from '../../Interceptor'
 
 export type Protocol = 'http' | 'https'
 
@@ -189,8 +190,8 @@ export class NodeClientRequest extends ClientRequest {
     // in another (parent) interceptor (like XMLHttpRequest -> ClientRequest).
     // That means some interceptor up the chain has concluded that
     // this request must be performed as-is.
-    if (this.getHeader('X-Request-Id') != null) {
-      this.removeHeader('X-Request-Id')
+    if (this.hasHeader(INTERNAL_REQUEST_ID_HEADER_NAME)) {
+      this.removeHeader(INTERNAL_REQUEST_ID_HEADER_NAME)
       return this.passthrough(chunk, encoding, callback)
     }
 

src/interceptors/XMLHttpRequest/XMLHttpRequestController.ts

@@ -13,6 +13,7 @@ import { isDomParserSupportedType } from './utils/isDomParserSupportedType'
 import { parseJson } from '../../utils/parseJson'
 import { uuidv4 } from '../../utils/uuid'
 import { createResponse } from './utils/createResponse'
+import { INTERNAL_REQUEST_ID_HEADER_NAME } from '../../Interceptor'
 
 const IS_MOCKED_RESPONSE = Symbol('isMockedResponse')
 const IS_NODE = isNodeProcess()
@@ -48,7 +49,10 @@ export class XMLHttpRequestController {
   private responseBuffer: Uint8Array
   private events: Map<keyof XMLHttpRequestEventTargetEventMap, Array<Function>>
 
-  constructor(readonly initialRequest: XMLHttpRequest, public logger: Logger) {
+  constructor(
+    readonly initialRequest: XMLHttpRequest,
+    public logger: Logger
+  ) {
     this.events = new Map()
     this.requestId = uuidv4()
     this.requestHeaders = new Headers()
@@ -99,7 +103,7 @@ export class XMLHttpRequestController {
           case 'addEventListener': {
             const [eventName, listener] = args as [
               keyof XMLHttpRequestEventTargetEventMap,
-              Function
+              Function,
             ]
 
             this.registerEvent(eventName, listener)
@@ -119,7 +123,7 @@ export class XMLHttpRequestController {
 
           case 'send': {
             const [body] = args as [
-              body?: XMLHttpRequestBodyInit | Document | null
+              body?: XMLHttpRequestBodyInit | Document | null,
             ]
 
             if (body != null) {
@@ -180,7 +184,10 @@ export class XMLHttpRequestController {
                  * handle the same request at the same time (e.g. emit the "response" event twice).
                  */
                 if (IS_NODE) {
-                  this.request.setRequestHeader('X-Request-Id', this.requestId!)
+                  this.request.setRequestHeader(
+                    INTERNAL_REQUEST_ID_HEADER_NAME,
+                    this.requestId!
+                  )
                 }
 
                 return invoke()
@@ -517,7 +524,7 @@ export class XMLHttpRequestController {
   private trigger<
     EventName extends keyof (XMLHttpRequestEventTargetEventMap & {
       readystatechange: ProgressEvent<XMLHttpRequestEventTarget>
-    })
+    }),
   >(eventName: EventName, options?: ProgressEventInit): void {
     const callback = this.request[`on${eventName}`]
     const event = createEvent(this.request, eventName, options)

test/modules/XMLHttpRequest/intercept/XMLHttpRequest.browser.test.ts

@@ -1,13 +1,15 @@
+import { invariant } from 'outvariant'
 import { HttpServer } from '@open-draft/test-server/http'
 import { RequestHandler } from 'express-serve-static-core'
 import { test, expect } from '../../../playwright.extend'
-import { invariant } from 'outvariant'
+import { INTERNAL_REQUEST_ID_HEADER_NAME } from '../../../../src/Interceptor'
 
 const httpServer = new HttpServer((app) => {
   const strictCorsMiddleware: RequestHandler = (req, res, next) => {
     invariant(
-      !req.header('x-request-id'),
-      'Found unexpected "X-Request-Id" request header in the browser for "%s %s"',
+      !req.header(INTERNAL_REQUEST_ID_HEADER_NAME),
+      'Found unexpected internal "%s" request header in the browser for "%s %s"',
+      INTERNAL_REQUEST_ID_HEADER_NAME,
       req.method,
       req.url
     )