ref(utils): Simplify collection of xhr request payload (getsentry#4402)

This simplifies the implementation of XHR request body harvesting for breadcrumbs, introduced in getsentry#2904. Instead of caching the data and grabbing it at a later point, this grabs it as soon as it's available, in the xhr `send` method.

Author: lobsterkatie

packages/utils/src/instrument.ts

@@ -219,9 +219,6 @@ function instrumentXHR(): void {
     return;
   }
 
-  // Poor man's implementation of ES6 `Map`, tracking and keeping in sync key and value separately.
-  const requestKeys: XMLHttpRequest[] = [];
-  const requestValues: Array<any>[] = [];
   const xhrproto = XMLHttpRequest.prototype;
 
   fill(xhrproto, 'open', function(originalOpen: () => void): () => void {
@@ -251,20 +248,6 @@ function instrumentXHR(): void {
             /* do nothing */
           }
 
-          try {
-            const requestPos = requestKeys.indexOf(xhr);
-            if (requestPos !== -1) {
-              // Make sure to pop both key and value to keep it in sync.
-              requestKeys.splice(requestPos);
-              const args = requestValues.splice(requestPos)[0];
-              if (args[0] !== undefined) {
-                xhrInfo.body = args[0] as XHRSendInput;
-              }
-            }
-          } catch (e) {
-            /* do nothing */
-          }
-
           triggerHandlers('xhr', {
             args,
             endTimestamp: Date.now(),
@@ -291,8 +274,9 @@ function instrumentXHR(): void {
 
   fill(xhrproto, 'send', function(originalSend: () => void): () => void {
     return function(this: SentryWrappedXMLHttpRequest, ...args: any[]): void {
-      requestKeys.push(this);
-      requestValues.push(args);
+      if (this.__sentry_xhr__ && args[0] !== undefined) {
+        this.__sentry_xhr__.body = args[0];
+      }
 
       triggerHandlers('xhr', {
         args,