fix: use addEventListener for reliable response capture

The previous implementation overrode onreadystatechange directly,
which failed when handlers were set after send() was called.

This fix uses addEventListener('readystatechange', ...) instead,
which reliably captures responses regardless of when other handlers
are attached.

Author: claude

src/XHRInterceptor.ts

@@ -28,6 +28,7 @@ declare class XMLHttpRequest {
   setRequestHeader(header: string, value: string): void;
   getResponseHeader(name: string): string | null;
   getAllResponseHeaders(): string;
+  addEventListener(type: string, listener: (this: XMLHttpRequest, ev: any) => any): void;
 }
 
 // Callback types use 'any' to match React Native's XHRInterceptor API
@@ -85,6 +86,12 @@ function enableInterception(): void {
     username?: string | null,
     password?: string | null
   ): void {
+    const xhr = this as any;
+    xhr._interception = {
+      method,
+      url: url.toString(),
+    };
+
     openCallback(method, url.toString(), this);
 
     return originalXHROpen!.call(
@@ -113,12 +120,14 @@ function enableInterception(): void {
     const dataString = body === null || body === undefined ? '' : String(body);
     sendCallback(dataString, xhr);
 
-    // Listen for state changes to capture headers and response
-    const originalOnReadyStateChange = this.onreadystatechange;
-    this.onreadystatechange = function (event: any) {
+    // Use addEventListener which is more reliable than overriding onreadystatechange
+    // This works even when handlers are set after send() is called
+    this.addEventListener('readystatechange', function () {
       if (this.readyState === XMLHttpRequest.HEADERS_RECEIVED) {
-        if (!xhr._hasCalledHeaderReceived) {
-          xhr._hasCalledHeaderReceived = true;
+        if (!xhr._interception?.hasCalledHeaderReceived) {
+          if (xhr._interception) {
+            xhr._interception.hasCalledHeaderReceived = true;
+          }
           const contentType = this.getResponseHeader('content-type') || '';
           const contentLength = this.getResponseHeader('content-length');
           const responseSize = contentLength ? parseInt(contentLength, 10) : 0;
@@ -134,33 +143,34 @@ function enableInterception(): void {
       }
 
       if (this.readyState === XMLHttpRequest.DONE) {
-        let responseData = '';
-        if (this.responseType === '' || this.responseType === 'text') {
-          responseData = this.responseText || '';
-        } else if (this.responseType === 'json' && this.response) {
-          try {
-            responseData = JSON.stringify(this.response);
-          } catch {
-            responseData = '[Unable to stringify response]';
+        if (!xhr._interception?.hasCalledResponse) {
+          if (xhr._interception) {
+            xhr._interception.hasCalledResponse = true;
+          }
+          let responseData = '';
+          if (this.responseType === '' || this.responseType === 'text') {
+            responseData = this.responseText || '';
+          } else if (this.responseType === 'json' && this.response) {
+            try {
+              responseData = JSON.stringify(this.response);
+            } catch {
+              responseData = '[Unable to stringify response]';
+            }
+          } else if (this.response) {
+            responseData = '[Non-text response]';
           }
-        } else if (this.response) {
-          responseData = '[Non-text response]';
-        }
-
-        responseCallback(
-          this.status,
-          this.timeout,
-          responseData,
-          this.responseURL,
-          this.responseType,
-          xhr
-        );
-      }
 
-      if (originalOnReadyStateChange) {
-        originalOnReadyStateChange.call(this, event);
+          responseCallback(
+            this.status,
+            this.timeout,
+            responseData,
+            this.responseURL,
+            this.responseType,
+            xhr
+          );
+        }
       }
-    };
+    });
 
     return originalXHRSend!.call(this, body);
   };