[JS] FlexBuffers Fix for wrong type of offset and length values (#6107)

* Adding FlexBuffers support for Dart language

* Introduce snapshot method.

* Fix docu

* Replacing extension methods with static methods in order to support older Dart version

* Improving code based on PR feedback. Mainly rename refactoring.

* Addressing all PR feedback which does not need clarification

* exchange dynamic type with Object

* Adds better API documentation.
[] operator throws a very descriptive exception in case of a bad key.

* Implementation of JavaScript FlexBuffers decoder

* implements JS FlexBuffers builder

* replacing _toF32 with Math.fround

* Introducing test for BigInt number

* Moving functions from BitWitdth & ValueType object into BitWidthUtil and ValueTypeUtil accordingly.
Removing defensive checks.
Introducing fix for large int numbers by converting them to BigInt type.
Introducing handling for BigInt type in `add` method.
Using TextEncoder and Decoder to handle string / utf8 conversion.

* rename variable

* Lets user turn deduplication strategies for strings, keys and vector of keys off while building FlexBuffer.
Implements quick sort and choses quick sort if the number of keys is bigger then 20.
Removes unnecessary dict lookups in BitWidthUtil helper functions

* make iwidth and uwidth computation simpler and faster

* Making redInt and readUint a bit faster and shim the BigInt / BigUint usage

* Fixing a bug in FlexBuffers JS, where offsets and lengths are stored and read as int and not as uint values.

* Fixing a bug in FlexBuffers Dart, where offset and length values are stored and read as int values instead of uint values

6 files changed, 130 insertions, 50 deletions

diff --git a/dart/lib/src/builder.dart b/dart/lib/src/builder.dart
index afdf7232..5ce46dcb 100644
--- a/dart/lib/src/builder.dart
+++ b/dart/lib/src/builder.dart
@@ -111,9 +111,9 @@ class Builder {
     }
     final utf8String = utf8.encode(value);
     final length = utf8String.length;
-    final bitWidth = BitWidthUtil.width(length);
+    final bitWidth = BitWidthUtil.uwidth(length);
     final byteWidth = _align(bitWidth);
-    _writeInt(length, byteWidth);
+    _writeUInt(length, byteWidth);
     final stringOffset = _offset;
     final newOffset = _newOffset(length + 1);
     _pushBuffer(utf8String);
@@ -149,9 +149,9 @@ class Builder {
   void addBlob(ByteBuffer value) {
     _integrityCheckOnValueAddition();
     final length = value.lengthInBytes;
-    final bitWidth = BitWidthUtil.width(length);
+    final bitWidth = BitWidthUtil.uwidth(length);
     final byteWidth = _align(bitWidth);
-    _writeInt(length, byteWidth);
+    _writeUInt(length, byteWidth);
     final blobOffset = _offset;
     final newOffset = _newOffset(length);
     _pushBuffer(value.asUint8List());
@@ -295,13 +295,13 @@ class Builder {
     final value = _stack[0];
     final byteWidth = _align(value.elementWidth(_offset, 0));
     _writeStackValue(value, byteWidth);
-    _writeInt(value.storedPackedType(), 1);
-    _writeInt(byteWidth, 1);
+    _writeUInt(value.storedPackedType(), 1);
+    _writeUInt(byteWidth, 1);
     _finished = true;
   }
   
   _StackValue _createVector(int start, int vecLength, int step, [_StackValue keys]) {
-    var bitWidth = BitWidthUtil.width(vecLength);
+    var bitWidth = BitWidthUtil.uwidth(vecLength);
     var prefixElements = 1;
     if (keys != null) {
       var elemWidth = keys.elementWidth(_offset, 0);
@@ -330,10 +330,10 @@ class Builder {
     final fix = typed & ValueTypeUtils.isNumber(vectorType) && vecLength >= 2 && vecLength <= 4;
     if (keys != null) {
       _writeStackValue(keys, byteWidth);
-      _writeInt(1 << keys.width.index, byteWidth);
+      _writeUInt(1 << keys.width.index, byteWidth);
     }
     if (fix == false) {
-      _writeInt(vecLength, byteWidth);
+      _writeUInt(vecLength, byteWidth);
     }
     final vecOffset = _offset;
     for (var i = start; i < _stack.length; i += step) {
@@ -341,7 +341,7 @@ class Builder {
     }
     if (typed == false) {
       for (var i = start; i < _stack.length; i += step) {
-        _writeInt(_stack[i].storedPackedType(), 1);
+        _writeUInt(_stack[i].storedPackedType(), 1);
       }
     }
     if (keys != null) {
@@ -442,7 +442,7 @@ class Builder {
     if (value.isOffset) {
       final relativeOffset = _offset - value.offset;
       if (byteWidth == 8 || relativeOffset < (1 << (byteWidth * 8))) {
-        _writeInt(relativeOffset, byteWidth);
+        _writeUInt(relativeOffset, byteWidth);
       } else {
         throw StateError('Unexpected size $byteWidth. This might be a bug. Please create an issue https://github.com/google/flatbuffers/issues/new');
       }
@@ -452,9 +452,9 @@ class Builder {
     _offset = newOffset;
   }
 
-  void _writeInt(int value, int byteWidth) {
+  void _writeUInt(int value, int byteWidth) {
     final newOffset = _newOffset(byteWidth);
-    _pushInt(value, BitWidthUtil.fromByteWidth(byteWidth));
+    _pushUInt(value, BitWidthUtil.fromByteWidth(byteWidth));
     _offset = newOffset;
   }
 
@@ -492,6 +492,24 @@ class Builder {
     }
   }
 
+  void _pushUInt(int value, BitWidth width) {
+    switch (width) {
+
+      case BitWidth.width8:
+        _buffer.setUint8(_offset, value);
+        break;
+      case BitWidth.width16:
+        _buffer.setUint16(_offset, value, Endian.little);
+        break;
+      case BitWidth.width32:
+        _buffer.setUint32(_offset, value, Endian.little);
+        break;
+      case BitWidth.width64:
+        _buffer.setUint64(_offset, value, Endian.little);
+        break;
+    }
+  }
+
   void _pushBuffer(List<int> value) {
     _buffer.buffer.asUint8List().setAll(_offset, value);
   }
@@ -541,7 +559,7 @@ class _StackValue {
       final width = 1 << i;
       final offsetLoc = size + BitWidthUtil.paddingSize(size, width) + index * width;
       final offset = offsetLoc - _offset;
-      final bitWidth = BitWidthUtil.width(offset);
+      final bitWidth = BitWidthUtil.uwidth(offset);
       if (1 << bitWidth.index == width) {
         return bitWidth;
       }
diff --git a/dart/lib/src/reference.dart b/dart/lib/src/reference.dart
index ad3a2a0a..3954f068 100644
--- a/dart/lib/src/reference.dart
+++ b/dart/lib/src/reference.dart
@@ -188,16 +188,16 @@ class Reference {
     if(ValueTypeUtils.isFixedTypedVector(_valueType)) {
       _length = ValueTypeUtils.fixedTypedVectorElementSize(_valueType);
     } else if(_valueType == ValueType.Blob || ValueTypeUtils.isAVector(_valueType) || _valueType == ValueType.Map){
-      _length = _readInt(_indirect - _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
+      _length = _readUInt(_indirect - _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
     } else if (_valueType == ValueType.Null) {
       _length = 0;
     } else if (_valueType == ValueType.String) {
       final indirect = _indirect;
       var size_byte_width = _byteWidth;
-      var size = _readInt(indirect - size_byte_width, BitWidthUtil.fromByteWidth(size_byte_width));
+      var size = _readUInt(indirect - size_byte_width, BitWidthUtil.fromByteWidth(size_byte_width));
       while (_buffer.getInt8(indirect + size) != 0) {
         size_byte_width <<= 1;
-        size = _readInt(indirect - size_byte_width, BitWidthUtil.fromByteWidth(size_byte_width));
+        size = _readUInt(indirect - size_byte_width, BitWidthUtil.fromByteWidth(size_byte_width));
       }
       _length = size;
     } else if (_valueType == ValueType.Key) {
@@ -270,7 +270,7 @@ class Reference {
   /// value is not cached. Callers that need to use it more than once should
   /// cache the return value in a local variable.
   int get _indirect {
-    final step = _readInt(_offset, _parentWidth);
+    final step = _readUInt(_offset, _parentWidth);
     return _offset - step;
   }
 
@@ -324,8 +324,8 @@ class Reference {
   int _keyIndex(String key) {
     final input = utf8.encode(key);
     final keysVectorOffset = _indirect - _byteWidth * 3;
-    final indirectOffset = keysVectorOffset - _readInt(keysVectorOffset, BitWidthUtil.fromByteWidth(_byteWidth));
-    final byteWidth = _readInt(keysVectorOffset + _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
+    final indirectOffset = keysVectorOffset - _readUInt(keysVectorOffset, BitWidthUtil.fromByteWidth(_byteWidth));
+    final byteWidth = _readUInt(keysVectorOffset + _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
     var low = 0;
     var high = length - 1;
     while (low <= high) {
@@ -343,7 +343,7 @@ class Reference {
 
   int _diffKeys(List<int> input, int index, int indirect_offset, int byteWidth) {
     final keyOffset = indirect_offset + index * byteWidth;
-    final keyIndirectOffset = keyOffset - _readInt(keyOffset, BitWidthUtil.fromByteWidth(byteWidth));
+    final keyIndirectOffset = keyOffset - _readUInt(keyOffset, BitWidthUtil.fromByteWidth(byteWidth));
     for (var i = 0; i < input.length; i++) {
       final dif = input[i] - _buffer.getUint8(keyIndirectOffset + i);
       if (dif != 0) {
@@ -369,10 +369,10 @@ class Reference {
 
   String _keyForIndex(int index) {
     final keysVectorOffset = _indirect - _byteWidth * 3;
-    final indirectOffset = keysVectorOffset - _readInt(keysVectorOffset, BitWidthUtil.fromByteWidth(_byteWidth));
-    final byteWidth = _readInt(keysVectorOffset + _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
+    final indirectOffset = keysVectorOffset - _readUInt(keysVectorOffset, BitWidthUtil.fromByteWidth(_byteWidth));
+    final byteWidth = _readUInt(keysVectorOffset + _byteWidth, BitWidthUtil.fromByteWidth(_byteWidth));
     final keyOffset = indirectOffset + index * byteWidth;
-    final keyIndirectOffset = keyOffset - _readInt(keyOffset, BitWidthUtil.fromByteWidth(byteWidth));
+    final keyIndirectOffset = keyOffset - _readUInt(keyOffset, BitWidthUtil.fromByteWidth(byteWidth));
     var length = 0;
     while (_buffer.getUint8(keyIndirectOffset + length) != 0) {
       length += 1;
diff --git a/dart/lib/src/types.dart b/dart/lib/src/types.dart
index 09e55206..8aed2725 100644
--- a/dart/lib/src/types.dart
+++ b/dart/lib/src/types.dart
@@ -22,6 +22,16 @@ class BitWidthUtil {
     }
     return value == _toF32(value) ? BitWidth.width32 : BitWidth.width64;
   }
+  static BitWidth uwidth(num value) {
+    if (value.toInt() == value) {
+      var v = value.toInt().abs();
+      if (v >> 8 == 0) return BitWidth.width8;
+      if (v >> 16 == 0) return BitWidth.width16;
+      if (v >> 32 == 0) return BitWidth.width32;
+      return BitWidth.width64;
+    }
+    return value == _toF32(value) ? BitWidth.width32 : BitWidth.width64;
+  }
   static BitWidth fromByteWidth(int value) {
     if (value == 1) {
       return BitWidth.width8;
diff --git a/dart/test/flex_reader_test.dart b/dart/test/flex_reader_test.dart
index 47e7a3f9..ec30367b 100644
--- a/dart/test/flex_reader_test.dart
+++ b/dart/test/flex_reader_test.dart
@@ -1,6 +1,6 @@
 import 'dart:typed_data';
 
-import 'package:flat_buffers/flex_buffers.dart' show Reference;
+import 'package:flat_buffers/flex_buffers.dart' show Reference, Builder;
 import 'package:test/test.dart';
 
 void main() {
@@ -261,6 +261,24 @@ void main() {
     expect(flx.mapValueIterable.map((e) => e.json).toList(), [flx['address'].json, flx['age'].json, flx['flags'].json, flx['name'].json, flx['weight'].json]);
     expect(flx['flags'].vectorIterable.map((e) => e.boolValue).toList(), [true, false, true, true]);
   });
+
+  test('bug where offest were stored as int instead of uint', (){
+    const data = [99, 104, 97, 110, 110, 101, 108, 115, 95, 105, 110, 0,
+      100, 105, 108, 97, 116, 105, 111, 110, 95, 104, 101, 105, 103, 104, 116, 95, 102, 97, 99, 116, 111, 114, 0,
+      100, 105, 108, 97, 116, 105, 111, 110, 95, 119, 105, 100, 116, 104, 95, 102, 97, 99, 116, 111, 114, 0,
+      102, 117, 115, 101, 100, 95, 97, 99, 116, 105, 118, 97, 116, 105, 111, 110, 95, 102, 117, 110, 99, 116, 105, 111, 110, 0,
+      112, 97, 100, 95, 118, 97, 108, 117, 101, 115, 0, 112, 97, 100, 100, 105, 110, 103, 0,
+      115, 116, 114, 105, 100, 101, 95, 104, 101, 105, 103, 104, 116, 0,
+      115, 116, 114, 105, 100, 101, 95, 119, 105, 100, 116, 104, 0,
+      8, 130, 119, 97, 76, 51, 41, 34, 21, 8, 1, 8, 64, 1, 1, 1, 1, 0, 1, 1, 4, 4, 4, 4, 4, 4, 4, 4, 16, 36, 1];
+    var flx = Reference.fromBuffer(b(data));
+    expect(flx.json, '{"channels_in":64,"dilation_height_factor":1,"dilation_width_factor":1,"fused_activation_function":1,"pad_values":1,"padding":0,"stride_height":1,"stride_width":1}');
+    const object = {"channels_in":64,"dilation_height_factor":1,"dilation_width_factor":1,"fused_activation_function":1,"pad_values":1,"padding":0,"stride_height":1,"stride_width":1};
+    var data1 = Builder.buildFromObject(object).asUint8List();
+    expect(data1.length, data.length);
+    var flx1 = Reference.fromBuffer(b(data1));
+    expect(flx1.json, '{"channels_in":64,"dilation_height_factor":1,"dilation_width_factor":1,"fused_activation_function":1,"pad_values":1,"padding":0,"stride_height":1,"stride_width":1}');
+  });
 }
 
 ByteBuffer b(List<int> values) {
diff --git a/js/flexbuffers.js b/js/flexbuffers.js
index eff06f2f..59737237 100644
--- a/js/flexbuffers.js
+++ b/js/flexbuffers.js
@@ -202,7 +202,7 @@ flexbuffers.toReference = (buffer) => {
   }
 
   function indirect(dataView, offset, width) {
-    const step = readInt(dataView, offset, width);
+    const step = readUInt(dataView, offset, width);
     return offset - step;
   }
 
@@ -210,8 +210,8 @@ flexbuffers.toReference = (buffer) => {
     const input = toUTF8Array(key);
     const keysVectorOffset = indirect(dataView, offset, parentWidth) - byteWidth * 3;
     const bitWidth = flexbuffers.BitWidthUtil.fromByteWidth(byteWidth);
-    const indirectOffset = keysVectorOffset - readInt(dataView, keysVectorOffset, bitWidth);
-    const _byteWidth = readInt(dataView, keysVectorOffset + byteWidth, bitWidth);
+    const indirectOffset = keysVectorOffset - readUInt(dataView, keysVectorOffset, bitWidth);
+    const _byteWidth = readUInt(dataView, keysVectorOffset + byteWidth, bitWidth);
     let low = 0;
     let high = length - 1;
     while (low <= high) {
@@ -229,7 +229,7 @@ flexbuffers.toReference = (buffer) => {
 
   function diffKeys(input, index, dataView, offset, width) {
     const keyOffset = offset + index * width;
-    const keyIndirectOffset = keyOffset - readInt(dataView, keyOffset, flexbuffers.BitWidthUtil.fromByteWidth(width));
+    const keyIndirectOffset = keyOffset - readUInt(dataView, keyOffset, flexbuffers.BitWidthUtil.fromByteWidth(width));
     for (let i = 0; i < input.length; i++) {
       const dif = input[i] - dataView.getUint8(keyIndirectOffset + i);
       if (dif !== 0) {
@@ -248,12 +248,12 @@ flexbuffers.toReference = (buffer) => {
 
   function keyForIndex(index, dataView, offset, parentWidth, byteWidth) {
     const keysVectorOffset = indirect(dataView, offset, parentWidth) - byteWidth * 3;
-    const bitWidth = flexbuffers.BitWidthUtil.fromByteWidth(byteWidth)
-    const indirectOffset = keysVectorOffset - readInt(dataView, keysVectorOffset, bitWidth);
-    const _byteWidth = readInt(dataView, keysVectorOffset + byteWidth, bitWidth);
+    const bitWidth = flexbuffers.BitWidthUtil.fromByteWidth(byteWidth);
+    const indirectOffset = keysVectorOffset - readUInt(dataView, keysVectorOffset, bitWidth);
+    const _byteWidth = readUInt(dataView, keysVectorOffset + byteWidth, bitWidth);
     const keyOffset = indirectOffset + index * _byteWidth;
-    const keyIndirectOffset = keyOffset - readInt(dataView, keyOffset, flexbuffers.BitWidthUtil.fromByteWidth(_byteWidth));
-    var length = 0;
+    const keyIndirectOffset = keyOffset - readUInt(dataView, keyOffset, flexbuffers.BitWidthUtil.fromByteWidth(_byteWidth));
+    let length = 0;
     while (dataView.getUint8(keyIndirectOffset + length) !== 0) {
       length++;
     }
@@ -363,16 +363,16 @@ flexbuffers.toReference = (buffer) => {
         } else if (valueType === flexbuffers.ValueType.BLOB
           || valueType === flexbuffers.ValueType.MAP
           || flexbuffers.ValueTypeUtil.isAVector(valueType)) {
-          length = readInt(dataView, indirect(dataView, offset, parentWidth) - byteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth))
+          length = readUInt(dataView, indirect(dataView, offset, parentWidth) - byteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth))
         } else if (valueType === flexbuffers.ValueType.NULL) {
           length = 0;
         } else if (valueType === flexbuffers.ValueType.STRING) {
           const _indirect = indirect(dataView, offset, parentWidth);
           let sizeByteWidth = byteWidth;
-          size = readInt(dataView, _indirect - sizeByteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth));
+          size = readUInt(dataView, _indirect - sizeByteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth));
           while (dataView.getInt8(_indirect + size) !== 0) {
             sizeByteWidth <<= 1;
-            size = readInt(dataView, _indirect - sizeByteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth));
+            size = readUInt(dataView, _indirect - sizeByteWidth, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth));
           }
           length = size;
         } else if (valueType === flexbuffers.ValueType.KEY) {
@@ -411,7 +411,10 @@ flexbuffers.toReference = (buffer) => {
         if (this.isBool()) {
           return this.boolValue();
         }
-        return this.numericValue() || this.blobValue() || this.stringValue();
+        if (this.isNumber()) {
+          return this.numericValue();
+        }
+        return this.blobValue() || this.stringValue();
       }
     }
   }
@@ -509,11 +512,17 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     offset = newOffset;
   }
 
+  function writeUInt(value, byteWidth) {
+    const newOffset = computeOffset(byteWidth);
+    pushUInt(value, flexbuffers.BitWidthUtil.fromByteWidth(byteWidth));
+    offset = newOffset;
+  }
+
   function writeBlob(arrayBuffer) {
     const length = arrayBuffer.byteLength;
-    const bitWidth = flexbuffers.BitWidthUtil.iwidth(length);
+    const bitWidth = flexbuffers.BitWidthUtil.uwidth(length);
     const byteWidth = align(bitWidth);
-    writeInt(length, byteWidth);
+    writeUInt(length, byteWidth);
     const blobOffset = offset;
     const newOffset = computeOffset(length);
     new Uint8Array(buffer).set(new Uint8Array(arrayBuffer), blobOffset);
@@ -528,9 +537,9 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     }
     const utf8 = toUTF8Array(str);
     const length = utf8.length;
-    const bitWidth = flexbuffers.BitWidthUtil.iwidth(length);
+    const bitWidth = flexbuffers.BitWidthUtil.uwidth(length);
     const byteWidth = align(bitWidth);
-    writeInt(length, byteWidth);
+    writeUInt(length, byteWidth);
     const stringOffset = offset;
     const newOffset = computeOffset(length + 1);
     new Uint8Array(buffer).set(utf8, stringOffset);
@@ -564,7 +573,7 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     if (value.isOffset) {
       const relativeOffset = offset - value.offset;
       if (byteWidth === 8 || BigInt(relativeOffset) < (1n << BigInt(byteWidth * 8))) {
-        writeInt(relativeOffset, byteWidth);
+        writeUInt(relativeOffset, byteWidth);
       } else {
         throw `Unexpected size ${byteWidth}. This might be a bug. Please create an issue https://github.com/google/flatbuffers/issues/new`
       }
@@ -744,7 +753,7 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
   }
 
   function createVector(start, vecLength, step, keys = null) {
-    let bitWidth = flexbuffers.BitWidthUtil.iwidth(vecLength);
+    let bitWidth = flexbuffers.BitWidthUtil.uwidth(vecLength);
     let prefixElements = 1;
     if (keys !== null) {
       const elementWidth = keys.elementWidth(offset, 0);
@@ -773,10 +782,10 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     const fix = typed && flexbuffers.ValueTypeUtil.isNumber(vectorType) && vecLength >= 2 && vecLength <= 4;
     if (keys !== null) {
       writeStackValue(keys, byteWidth);
-      writeInt(1 << keys.width, byteWidth);
+      writeUInt(1 << keys.width, byteWidth);
     }
     if (!fix) {
-      writeInt(vecLength, byteWidth);
+      writeUInt(vecLength, byteWidth);
     }
     const vecOffset = offset;
     for (let i = start; i < stack.length; i += step) {
@@ -784,7 +793,7 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     }
     if (!typed) {
       for (let i = start; i < stack.length; i += step) {
-        writeInt(stack[i].storedPackedType(), 1);
+        writeUInt(stack[i].storedPackedType(), 1);
       }
     }
     if (keys !== null) {
@@ -809,7 +818,7 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
           const width = 1 << i;
           const offsetLoc = size + flexbuffers.BitWidthUtil.paddingSize(size, width) + index * width;
           const offset = offsetLoc - this.offset;
-          const bitWidth = flexbuffers.BitWidthUtil.iwidth(offset);
+          const bitWidth = flexbuffers.BitWidthUtil.uwidth(offset);
           if (1 << bitWidth === width) {
             return bitWidth;
           }
@@ -880,8 +889,8 @@ flexbuffers.builder = (size = 2048, deduplicateString = true, deduplicateKeys =
     const value = stack[0];
     const byteWidth = align(value.elementWidth(offset, 0));
     writeStackValue(value, byteWidth);
-    writeInt(value.storedPackedType(), 1);
-    writeInt(byteWidth, 1);
+    writeUInt(value.storedPackedType(), 1);
+    writeUInt(byteWidth, 1);
     finished = true;
   }
 
diff --git a/tests/JavaScriptFlexBuffersTest.js b/tests/JavaScriptFlexBuffersTest.js
index 41709566..11c46c97 100644
--- a/tests/JavaScriptFlexBuffersTest.js
+++ b/tests/JavaScriptFlexBuffersTest.js
@@ -15,6 +15,7 @@ function main() {
   testRoundTrip();
   testRoundTripWithBuilder();
   testDeduplicationOff();
+  testBugWhereOffestWereStoredAsIntInsteadOfUInt();
 }
 
 function testSingleValueBuffers() {
@@ -360,5 +361,29 @@ function testGoldBuffer() {
   });
 }
 
+function testBugWhereOffestWereStoredAsIntInsteadOfUInt() {
+  // Reported in https://github.com/google/flatbuffers/issues/5949#issuecomment-688421193
+  const object = {'channels_in': 64, 'dilation_height_factor': 1, 'dilation_width_factor': 1, 'fused_activation_function': 1, 'pad_values': 1, 'padding': 0, 'stride_height': 1, 'stride_width': 1};
+  let data1 = flexbuffers.encode(object);
+  const data = [99, 104, 97, 110, 110, 101, 108, 115, 95, 105, 110, 0,
+    100, 105, 108, 97, 116, 105, 111, 110, 95, 104, 101, 105, 103, 104, 116, 95, 102, 97, 99, 116, 111, 114, 0,
+    100, 105, 108, 97, 116, 105, 111, 110, 95, 119, 105, 100, 116, 104, 95, 102, 97, 99, 116, 111, 114, 0,
+    102, 117, 115, 101, 100, 95, 97, 99, 116, 105, 118, 97, 116, 105, 111, 110, 95, 102, 117, 110, 99, 116, 105, 111, 110, 0,
+    112, 97, 100, 95, 118, 97, 108, 117, 101, 115, 0, 112, 97, 100, 100, 105, 110, 103, 0,
+    115, 116, 114, 105, 100, 101, 95, 104, 101, 105, 103, 104, 116, 0,
+    115, 116, 114, 105, 100, 101, 95, 119, 105, 100, 116, 104, 0,
+    8, 130, 119, 97, 76, 51, 41, 34, 21, 8, 1, 8, 64, 1, 1, 1, 1, 0, 1, 1, 4, 4, 4, 4, 4, 4, 4, 4, 16, 36, 1];
+  let object2 = flexbuffers.toObject(new Uint8Array(data).buffer);
+  let object1 = flexbuffers.toObject(new Uint8Array(data1).buffer);
+  assert.deepStrictEqual(object, object2);
+  assert.deepStrictEqual(object, object1);
+  assert.strictEqual(data.length, data1.length);
+  let ref = flexbuffers.toReference(new Uint8Array(data).buffer);
+  assert.strictEqual(ref.isMap(), true);
+  assert.strictEqual(ref.length(), 8);
+  assert.strictEqual(ref.get("channels_in").numericValue(), 64);
+  assert.strictEqual(ref.get("padding").isNumber(), true);
+}
+
 main();