perf: Avoid CORS preflight request by removing upload listener when not used

src/RESTController.js

@@ -169,24 +169,24 @@ const RESTController = {
         headers[key] = customHeaders[key];
       }
 
-      function handleProgress(type, event) {
-        if (options && typeof options.progress === 'function') {
+      if (options && typeof options.progress === 'function') {
+        const handleProgress = function (type, event) {
           if (event.lengthComputable) {
             options.progress(event.loaded / event.total, event.loaded, event.total, { type });
           } else {
             options.progress(null, null, null, { type });
           }
-        }
-      }
-
-      xhr.onprogress = event => {
-        handleProgress('download', event);
-      };
+        };
 
-      if (xhr.upload) {
-        xhr.upload.onprogress = event => {
-          handleProgress('upload', event);
+        xhr.onprogress = event => {
+          handleProgress('download', event);
         };
+
+        if (xhr.upload) {
+          xhr.upload.onprogress = event => {
+            handleProgress('upload', event);
+          };
+        }
       }
 
       xhr.open(method, url, true);

src/__tests__/RESTController-test.js

@@ -570,6 +570,20 @@ describe('RESTController', () => {
     );
   });
 
+  it('does not set upload progress listener when callback is not provided to avoid CORS pre-flight', () => {
+    const xhr = {
+      setRequestHeader: jest.fn(),
+      open: jest.fn(),
+      upload: jest.fn(),
+      send: jest.fn(),
+    };
+    RESTController._setXHR(function () {
+      return xhr;
+    });
+    RESTController.ajax('POST', 'users', {});
+    expect(xhr.upload.onprogress).toBeUndefined();
+  });
+
   it('does not upload progress when total is uncomputable', done => {
     const xhr = mockXHR([{ status: 200, response: { success: true } }], {
       progress: {

src/__tests__/test_helpers/mockXHR.js

@@ -35,8 +35,14 @@ function mockXHR(results, options = {}) {
       this.readyState = 4;
       attempts++;
       this.onreadystatechange();
-      this.onprogress(options.progress);
-      this.upload.onprogress(options.progress);
+
+      if (typeof this.onprogress === 'function') {
+        this.onprogress(options.progress);
+      }
+
+      if (typeof this.upload.onprogress === 'function') {
+        this.upload.onprogress(options.progress);
+      }
     },
   };
   return XHR;