feat(XHR): support xhr.responseType to be "json"

Author: Xmader

python/pythonmonkey/builtin_modules/XMLHttpRequest.js

@@ -515,8 +515,8 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
   {
     if (this.#state === XMLHttpRequest.LOADING || this.#state === XMLHttpRequest.DONE)
       throw new DOMException('responseType can only be set before send()', 'InvalidStateError');
-    if (!['', 'text', 'arraybuffer'].includes(t))
-      throw new DOMException('only responseType "text" or "arraybuffer" is supported', 'NotSupportedError');
+    if (!['', 'text', 'arraybuffer', 'json'].includes(t))
+      throw new DOMException('only responseType "text" or "arraybuffer" or "json" is supported', 'NotSupportedError');
     this.#responseType = t;
   }
 
@@ -550,7 +550,29 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
       this.#responseObject = this.#mergeReceivedBytes().buffer;
       return this.#responseObject;
     }
+    
+    if (this.#responseType === 'json') // step 8
+    {
+      // step 8.2
+      if (this.#receivedLength === 0) // response’s body is null
+        return null;
+      // step 8.3
+      let jsonObject = null;
+      try
+      {
+        // TODO: use proper TextDecoder API
+        const str = decodeStr(this.#mergeReceivedBytes(), 'utf-8'); // only supports utf-8, see https://infra.spec.whatwg.org/#parse-json-bytes-to-a-javascript-value
+        jsonObject = JSON.parse(str);
+      }
+      catch (exception)
+      {
+        return null;
+      }
+      // step 8.4
+      this.#responseObject = jsonObject;
+    }
 
+    // step 6 and step 7 ("blob" or "document") are not supported
     throw new DOMException(`unsupported responseType "${this.#responseType}"`, 'InvalidStateError');
   }
 
@@ -600,7 +622,7 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
   #responseType = '';
   /** 
    * cache for converting receivedBytes to the desired response type
-   * @type {ArrayBuffer | string}
+   * @type {ArrayBuffer | string | Record<any, any>}
    */
   #responseObject = null;