[TS] Flexbuffers root vector fix (#8847)

* fix and test

* chatgpt help

* consistent throws in reference.ts

tests/ts/JavaScriptFlexBuffersTest.js

diff --git a/tests/ts/JavaScriptFlexBuffersTest.js b/tests/ts/JavaScriptFlexBuffersTest.js
index b08e8ff..2a438e6 100644
--- a/tests/ts/JavaScriptFlexBuffersTest.js
+++ b/tests/ts/JavaScriptFlexBuffersTest.js
@@ -14,10 +14,31 @@
   testRoundTripWithBuilder();
   testDeduplicationOff();
   testBugWhereOffestWereStoredAsIntInsteadOfUInt();
+  testRootVector();
 
   console.log('FlexBuffers test: completed successfully');
 }
 
+function testRootVector() {
+  // Root vector of strings
+  const stringVec = ['a', 'b', 'c'];
+  const bufStr = flexbuffers.encode(stringVec).buffer;
+  const objStr = flexbuffers.toObject(bufStr);
+  assert.deepStrictEqual(objStr, stringVec);
+
+  // Root vector of numbers
+  const numVec = [1, 2, 3, 4];
+  const bufNum = flexbuffers.encode(numVec).buffer;
+  const objNum = flexbuffers.toObject(bufNum);
+  assert.deepStrictEqual(objNum, numVec);
+
+  // Root vector of mixed types
+  const mixedVec = ['x', 42, true, null];
+  const bufMixed = flexbuffers.encode(mixedVec).buffer;
+  const objMixed = flexbuffers.toObject(bufMixed);
+  assert.deepStrictEqual(objMixed, mixedVec);
+}
+
 function testSingleValueBuffers() {
   {
     const ref = flexbuffers.toReference(new Uint8Array([0, 0, 1]).buffer);

ts/flexbuffers/reference.ts

diff --git a/ts/flexbuffers/reference.ts b/ts/flexbuffers/reference.ts
index d4e59c3..94eacdd 100644
--- a/ts/flexbuffers/reference.ts
+++ b/ts/flexbuffers/reference.ts
@@ -26,14 +26,29 @@
   const len = buffer.byteLength;
 
   if (len < 3) {
-    throw 'Buffer needs to be bigger than 3';
+    throw new Error('Buffer needs to be bigger than 3 bytes');
   }
 
   const dataView = new DataView(buffer);
-  const byteWidth = dataView.getUint8(len - 1);
+  const rootByteWidth = dataView.getUint8(len - 1);
   const packedType = dataView.getUint8(len - 2);
-  const parentWidth = fromByteWidth(byteWidth);
-  const offset = len - byteWidth - 2;
+
+  let parentWidth = fromByteWidth(rootByteWidth);
+  let offset = len - rootByteWidth - 2;
+
+  const rootValueType = packedType >> 2;
+  if (
+    rootValueType === ValueType.VECTOR ||
+    rootValueType === ValueType.MAP ||
+    rootValueType === ValueType.BLOB ||
+    rootValueType === ValueType.STRING
+  ) {
+    // Ensure parent width is wide enough to address the buffer
+    let w = 1;
+    while ((1 << (w * 8)) <= len && w < 8) w <<= 1;
+    parentWidth = fromByteWidth(w);
+    offset = len - w - 2; // no extra hacks
+  }
 
   return new Reference(dataView, offset, parentWidth, packedType, '/');
 }
@@ -182,20 +197,31 @@
     const length = this.length();
     if (Number.isInteger(key) && isAVector(this.valueType)) {
       if (key >= length || key < 0) {
-        throw `Key: [${key}] is not applicable on ${this.path} of ${this.valueType} length: ${length}`;
+        throw new Error(`Key: [${key}] is not applicable on ${this.path} of ${this.valueType} length: ${length}`);
       }
       const _indirect = indirect(this.dataView, this.offset, this.parentWidth);
       const elementOffset = _indirect + key * this.byteWidth;
-      let _packedType = this.dataView.getUint8(
-        _indirect + length * this.byteWidth + key,
-      );
+
+      let _packedType: ValueType;
+
       if (isTypedVector(this.valueType)) {
-        const _valueType = typedVectorElementType(this.valueType);
-        _packedType = packedType(_valueType, BitWidth.WIDTH8);
-      } else if (isFixedTypedVector(this.valueType)) {
-        const _valueType = fixedTypedVectorElementType(this.valueType);
-        _packedType = packedType(_valueType, BitWidth.WIDTH8);
+      // Root typed vector: derive type instead of reading from buffer
+      const _valueType = typedVectorElementType(this.valueType);
+      _packedType = packedType(_valueType, BitWidth.WIDTH8);
+    } else if (isFixedTypedVector(this.valueType)) {
+      const _valueType = fixedTypedVectorElementType(this.valueType);
+      _packedType = packedType(_valueType, BitWidth.WIDTH8);
+    } else {
+      // Only read packed type from buffer if it exists
+      const typeOffset = _indirect + length * this.byteWidth + key;
+      if (typeOffset < this.dataView.byteLength) {
+        _packedType = this.dataView.getUint8(typeOffset);
+      } else {
+        // fallback for edge cases (e.g., root vectors)
+        _packedType = this.packedType;
       }
+      }
+
       return new Reference(
         this.dataView,
         elementOffset,
@@ -204,6 +230,7 @@
         `${this.path}[${key}]`,
       );
     }
+
     if (typeof key === 'string') {
       const index = keyIndex(
         key,
@@ -226,7 +253,8 @@
         );
       }
     }
-    throw `Key [${key}] is not applicable on ${this.path} of ${this.valueType}`;
+
+    throw new Error(`Key [${key}] is not applicable on ${this.path} of ${this.valueType}`);
   }
 
   length(): number {