From 0230a7173f3f281cc2a9f86aecec7532eadef5ec Mon Sep 17 00:00:00 2001
From: Michael Paulson <michael.b.paulson@gmail.com>
Date: Fri, 22 Jul 2016 11:11:56 -0700
Subject: [PATCH 1/3] feat(mutable-js): The mutable Scalar generation.

This is just the initial commit to start the conversation on adding mutation to javascript.
---
 src/idl_gen_js.cpp | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/idl_gen_js.cpp b/src/idl_gen_js.cpp
index d7c913e50..fec578029 100644
--- a/src/idl_gen_js.cpp
+++ b/src/idl_gen_js.cpp
@@ -79,7 +79,7 @@ class JsGenerator : public BaseGenerator {
     for (auto it = parser_.structs_.vec.begin();
          it != parser_.structs_.vec.end(); ++it) {
       auto &struct_def = **it;
-      GenStruct(struct_def, decl_code_ptr, exports_code_ptr);
+      GenStruct(parser_, struct_def, decl_code_ptr, exports_code_ptr);
     }
   }
   void GenNamespaces(std::string *code_ptr, std::string *exports_ptr) {
@@ -361,7 +361,7 @@ static void GenStructBody(const StructDef &struct_def,
 }
 
 // Generate an accessor struct with constructor for a flatbuffers struct.
-void GenStruct(StructDef &struct_def, std::string *code_ptr, std::string *exports_ptr) {
+void GenStruct(const Parser &parser, StructDef &struct_def, std::string *code_ptr, std::string *exports_ptr) {
   if (struct_def.generated) return;
   std::string &code = *code_ptr;
   std::string &exports = *exports_ptr;
@@ -550,6 +550,22 @@ void GenStruct(StructDef &struct_def, std::string *code_ptr, std::string *export
     }
     code += "};\n\n";
 
+    // Adds the mutable scalar value to the output
+    if (IsScalar(field.value.type.base_type) && parser.opts.mutable_buffer) {
+      std::string annotations = "@param {" + GenTypeName(field.value.type, true) + "} value\n";
+      GenDocComment(code_ptr, annotations +
+        "@returns {boolean}");
+
+      code += object_name + ".prototype.mutate_" + field.name + " = function(value) {\n";
+      code += "  var offset = this.bb.__offset(this.bb_pos, " + NumToString(field.value.offset) + ")\n\n";
+      code += "  if (offset === 0) {\n";
+      code += "    return false;\n";
+      code += "  }\n\n";
+      code += "  this.bb.write" + MakeCamel(GenType(field.value.type)) + "(this.bb_pos + offset, value);\n";
+      code += "  return true;\n";
+      code += "}\n\n";
+    }
+
     // Emit vector helpers
     if (field.value.type.base_type == BASE_TYPE_VECTOR) {
       // Emit a length helper

From a351124cfd3b12ca46572a664c7884e5844908e3 Mon Sep 17 00:00:00 2001
From: Michael Paulson <michael.b.paulson@gmail.com>
Date: Mon, 25 Jul 2016 14:33:46 -0700
Subject: [PATCH 2/3] chore(generate-code): Generate the JS code after mutation
 has been added.

---
 tests/monster_test_generated.js               | 390 ++++++++++++++++++
 .../namespace_test1_generated.js              |  45 ++
 .../namespace_test2_generated.js              |  15 +
 3 files changed, 450 insertions(+)

diff --git a/tests/monster_test_generated.js b/tests/monster_test_generated.js
index 7c4c28f82..774882d7c 100644
--- a/tests/monster_test_generated.js
+++ b/tests/monster_test_generated.js
@@ -127,6 +127,21 @@ MyGame.Example.Test.prototype.a = function() {
   return this.bb.readInt16(this.bb_pos);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Test.prototype.mutate_a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 0)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt16(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -134,6 +149,21 @@ MyGame.Example.Test.prototype.b = function() {
   return this.bb.readInt8(this.bb_pos + 2);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Test.prototype.mutate_b = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 2)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} a
@@ -191,6 +221,21 @@ MyGame.Example.TestSimpleTableWithEnum.prototype.color = function() {
   return offset ? /** @type {MyGame.Example.Color} */ (this.bb.readInt8(this.bb_pos + offset)) : MyGame.Example.Color.Green;
 };
 
+/**
+ * @param {MyGame.Example.Color} value
+ * @returns {boolean}
+ */
+MyGame.Example.TestSimpleTableWithEnum.prototype.mutate_color = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 4)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  */
@@ -248,6 +293,21 @@ MyGame.Example.Vec3.prototype.x = function() {
   return this.bb.readFloat32(this.bb_pos);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Vec3.prototype.mutate_x = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 0)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -255,6 +315,21 @@ MyGame.Example.Vec3.prototype.y = function() {
   return this.bb.readFloat32(this.bb_pos + 4);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Vec3.prototype.mutate_y = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 4)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -262,6 +337,21 @@ MyGame.Example.Vec3.prototype.z = function() {
   return this.bb.readFloat32(this.bb_pos + 8);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Vec3.prototype.mutate_z = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 8)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -269,6 +359,21 @@ MyGame.Example.Vec3.prototype.test1 = function() {
   return this.bb.readFloat64(this.bb_pos + 16);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Vec3.prototype.mutate_test1 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 16)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {MyGame.Example.Color}
  */
@@ -276,6 +381,21 @@ MyGame.Example.Vec3.prototype.test2 = function() {
   return /** @type {MyGame.Example.Color} */ (this.bb.readInt8(this.bb_pos + 24));
 };
 
+/**
+ * @param {MyGame.Example.Color} value
+ * @returns {boolean}
+ */
+MyGame.Example.Vec3.prototype.mutate_test2 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 24)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {MyGame.Example.Test=} obj
  * @returns {MyGame.Example.Test}
@@ -364,6 +484,21 @@ MyGame.Example.Stat.prototype.val = function() {
   return offset ? this.bb.readInt64(this.bb_pos + offset) : this.bb.createLong(0, 0);
 };
 
+/**
+ * @param {flatbuffers.Long} value
+ * @returns {boolean}
+ */
+MyGame.Example.Stat.prototype.mutate_val = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 6)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -372,6 +507,21 @@ MyGame.Example.Stat.prototype.count = function() {
   return offset ? this.bb.readUint16(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Stat.prototype.mutate_count = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 8)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint16(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  */
@@ -474,6 +624,21 @@ MyGame.Example.Monster.prototype.mana = function() {
   return offset ? this.bb.readInt16(this.bb_pos + offset) : 150;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_mana = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 6)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt16(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -482,6 +647,21 @@ MyGame.Example.Monster.prototype.hp = function() {
   return offset ? this.bb.readInt16(this.bb_pos + offset) : 100;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_hp = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 8)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt16(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Encoding=} optionalEncoding
  * @returns {string|Uint8Array}
@@ -524,6 +704,21 @@ MyGame.Example.Monster.prototype.color = function() {
   return offset ? /** @type {MyGame.Example.Color} */ (this.bb.readInt8(this.bb_pos + offset)) : MyGame.Example.Color.Blue;
 };
 
+/**
+ * @param {MyGame.Example.Color} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_color = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 16)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {MyGame.Example.Any}
  */
@@ -532,6 +727,21 @@ MyGame.Example.Monster.prototype.testType = function() {
   return offset ? /** @type {MyGame.Example.Any} */ (this.bb.readUint8(this.bb_pos + offset)) : MyGame.Example.Any.NONE;
 };
 
+/**
+ * @param {MyGame.Example.Any} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_test_type = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 18)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Table} obj
  * @returns {?flatbuffers.Table}
@@ -649,6 +859,21 @@ MyGame.Example.Monster.prototype.testbool = function() {
   return offset ? !!this.bb.readInt8(this.bb_pos + offset) : false;
 };
 
+/**
+ * @param {boolean} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testbool = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 34)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -657,6 +882,21 @@ MyGame.Example.Monster.prototype.testhashs32Fnv1 = function() {
   return offset ? this.bb.readInt32(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashs32_fnv1 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 36)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -665,6 +905,21 @@ MyGame.Example.Monster.prototype.testhashu32Fnv1 = function() {
   return offset ? this.bb.readUint32(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashu32_fnv1 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 38)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {flatbuffers.Long}
  */
@@ -673,6 +928,21 @@ MyGame.Example.Monster.prototype.testhashs64Fnv1 = function() {
   return offset ? this.bb.readInt64(this.bb_pos + offset) : this.bb.createLong(0, 0);
 };
 
+/**
+ * @param {flatbuffers.Long} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashs64_fnv1 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 40)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {flatbuffers.Long}
  */
@@ -681,6 +951,21 @@ MyGame.Example.Monster.prototype.testhashu64Fnv1 = function() {
   return offset ? this.bb.readUint64(this.bb_pos + offset) : this.bb.createLong(0, 0);
 };
 
+/**
+ * @param {flatbuffers.Long} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashu64_fnv1 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 42)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -689,6 +974,21 @@ MyGame.Example.Monster.prototype.testhashs32Fnv1a = function() {
   return offset ? this.bb.readInt32(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashs32_fnv1a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 44)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -697,6 +997,21 @@ MyGame.Example.Monster.prototype.testhashu32Fnv1a = function() {
   return offset ? this.bb.readUint32(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashu32_fnv1a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 46)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {flatbuffers.Long}
  */
@@ -705,6 +1020,21 @@ MyGame.Example.Monster.prototype.testhashs64Fnv1a = function() {
   return offset ? this.bb.readInt64(this.bb_pos + offset) : this.bb.createLong(0, 0);
 };
 
+/**
+ * @param {flatbuffers.Long} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashs64_fnv1a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 48)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {flatbuffers.Long}
  */
@@ -713,6 +1043,21 @@ MyGame.Example.Monster.prototype.testhashu64Fnv1a = function() {
   return offset ? this.bb.readUint64(this.bb_pos + offset) : this.bb.createLong(0, 0);
 };
 
+/**
+ * @param {flatbuffers.Long} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testhashu64_fnv1a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 50)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeUint64(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {number} index
  * @returns {boolean}
@@ -746,6 +1091,21 @@ MyGame.Example.Monster.prototype.testf = function() {
   return offset ? this.bb.readFloat32(this.bb_pos + offset) : 3.14159;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testf = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 54)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -754,6 +1114,21 @@ MyGame.Example.Monster.prototype.testf2 = function() {
   return offset ? this.bb.readFloat32(this.bb_pos + offset) : 3.0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testf2 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 56)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -762,6 +1137,21 @@ MyGame.Example.Monster.prototype.testf3 = function() {
   return offset ? this.bb.readFloat32(this.bb_pos + offset) : 0.0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+MyGame.Example.Monster.prototype.mutate_testf3 = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 58)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeFloat32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {number} index
  * @param {flatbuffers.Encoding=} optionalEncoding
diff --git a/tests/namespace_test/namespace_test1_generated.js b/tests/namespace_test/namespace_test1_generated.js
index e6390567f..a8fea9ea6 100644
--- a/tests/namespace_test/namespace_test1_generated.js
+++ b/tests/namespace_test/namespace_test1_generated.js
@@ -64,6 +64,21 @@ NamespaceA.NamespaceB.TableInNestedNS.prototype.foo = function() {
   return offset ? this.bb.readInt32(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+NamespaceA.NamespaceB.TableInNestedNS.prototype.mutate_foo = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 4)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  */
@@ -121,6 +136,21 @@ NamespaceA.NamespaceB.StructInNestedNS.prototype.a = function() {
   return this.bb.readInt32(this.bb_pos);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+NamespaceA.NamespaceB.StructInNestedNS.prototype.mutate_a = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 0)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @returns {number}
  */
@@ -128,6 +158,21 @@ NamespaceA.NamespaceB.StructInNestedNS.prototype.b = function() {
   return this.bb.readInt32(this.bb_pos + 4);
 };
 
+/**
+ * @param {number} value
+ * @returns {boolean}
+ */
+NamespaceA.NamespaceB.StructInNestedNS.prototype.mutate_b = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 4)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt32(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {flatbuffers.Builder} builder
  * @param {number} a
diff --git a/tests/namespace_test/namespace_test2_generated.js b/tests/namespace_test/namespace_test2_generated.js
index f76d5a95a..856bbff92 100644
--- a/tests/namespace_test/namespace_test2_generated.js
+++ b/tests/namespace_test/namespace_test2_generated.js
@@ -70,6 +70,21 @@ NamespaceA.TableInFirstNS.prototype.fooEnum = function() {
   return offset ? /** @type {NamespaceA.NamespaceB.EnumInNestedNS} */ (this.bb.readInt8(this.bb_pos + offset)) : NamespaceA.NamespaceB.EnumInNestedNS.A;
 };
 
+/**
+ * @param {NamespaceA.NamespaceB.EnumInNestedNS} value
+ * @returns {boolean}
+ */
+NamespaceA.TableInFirstNS.prototype.mutate_foo_enum = function(value) {
+  var offset = this.bb.__offset(this.bb_pos, 6)
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb.writeInt8(this.bb_pos + offset, value);
+  return true;
+}
+
 /**
  * @param {NamespaceA.NamespaceB.StructInNestedNS=} obj
  * @returns {NamespaceA.NamespaceB.StructInNestedNS}

From d268d11ca29b95ce1b8186e6526b58b51f2c913a Mon Sep 17 00:00:00 2001
From: Michael Paulson <michael.b.paulson@gmail.com>
Date: Wed, 27 Jul 2016 13:32:53 -0700
Subject: [PATCH 3/3] feat(test): Added mutation testing for scalar values.

This is a port of the tests found in test.cpp
---
 tests/JavaScriptTest.js | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/tests/JavaScriptTest.js b/tests/JavaScriptTest.js
index c89ae85d7..c97ed2d08 100644
--- a/tests/JavaScriptTest.js
+++ b/tests/JavaScriptTest.js
@@ -64,7 +64,10 @@ function main() {
 
   fs.writeFileSync('monsterdata_javascript_wire.mon', new Buffer(fbb.asUint8Array()));
 
-  // Test it:
+  // Tests mutation first.  This will verify that we did not trample any other
+  // part of the byte buffer.
+  testMutation(fbb.dataBuffer());
+  
   testBuffer(fbb.dataBuffer());
 
   test64bit();
@@ -74,6 +77,21 @@ function main() {
   console.log('FlatBuffers test: completed successfully');
 }
 
+function testMutation(bb) {
+  var monster = MyGame.Example.Monster.getRootAsMonster(bb);
+
+  monster.mutate_hp(120);
+  assert.strictEqual(monster.hp(), 120);
+
+  monster.mutate_hp(80);
+  assert.strictEqual(monster.hp(), 80);
+
+  var manaRes = monster.mutate_mana(10);
+  assert.strictEqual(manaRes, false);  // Field was NOT present, because default value.
+
+  // TODO: There is not the availability to mutate structs or vectors.
+}
+
 function testBuffer(bb) {
   assert.ok(MyGame.Example.Monster.bufferHasIdentifier(bb));