fix: add W3C FileReader spec compliance for state transitions

FileReader was missing LOADING state guard (should throw
InvalidStateError if already reading per W3C spec), state transition
to LOADING before async read, and loadstart event dispatch. Added
all three to comply with the W3C FileReader specification.

Signed-off-by: Srikanth Patchava <spatchava@meta.com>
Signed-off-by: Srikanth Patchava <srikanth.patchava@outlook.com>

Author: srpatcha

packages/react-native/Libraries/Blob/FileReader.js

@@ -72,6 +72,13 @@ class FileReader extends EventTarget {
   }
 
   readAsArrayBuffer(blob: ?Blob): void {
+    if (this._readyState === LOADING) {
+      throw new DOMException(
+        'The object is in an invalid state.',
+        'InvalidStateError',
+      );
+    }
+
     this._aborted = false;
 
     if (blob == null) {
@@ -80,6 +87,9 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._readyState = LOADING;
+    this.dispatchEvent(new Event('loadstart'));
+
     NativeFileReaderModule.readAsDataURL(blob.data).then(
       (text: string) => {
         if (this._aborted) {
@@ -103,6 +113,13 @@ class FileReader extends EventTarget {
   }
 
   readAsDataURL(blob: ?Blob): void {
+    if (this._readyState === LOADING) {
+      throw new DOMException(
+        'The object is in an invalid state.',
+        'InvalidStateError',
+      );
+    }
+
     this._aborted = false;
 
     if (blob == null) {
@@ -111,6 +128,9 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._readyState = LOADING;
+    this.dispatchEvent(new Event('loadstart'));
+
     NativeFileReaderModule.readAsDataURL(blob.data).then(
       (text: string) => {
         if (this._aborted) {
@@ -130,6 +150,13 @@ class FileReader extends EventTarget {
   }
 
   readAsText(blob: ?Blob, encoding: string = 'UTF-8'): void {
+    if (this._readyState === LOADING) {
+      throw new DOMException(
+        'The object is in an invalid state.',
+        'InvalidStateError',
+      );
+    }
+
     this._aborted = false;
 
     if (blob == null) {
@@ -138,6 +165,9 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._readyState = LOADING;
+    this.dispatchEvent(new Event('loadstart'));
+
     NativeFileReaderModule.readAsText(blob.data, encoding).then(
       (text: string) => {
         if (this._aborted) {