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Node.js C++ codebase

Hi! 👋 You've found the C++ code backing Node.js. This README aims to help you get started working on it and document some idioms you may encounter while doing so.

Coding style

Node.js has a document detailing its C++ coding style that can be helpful as a reference for stylistic issues.

V8 API documentation

A lot of the Node.js codebase is around what the underlying JavaScript engine, V8, provides through its API for embedders. Knowledge of this API can also be useful when working with native addons for Node.js written in C++, although for new projects N-API is typically the better alternative.

V8 does not provide much public API documentation beyond what is available in its C++ header files, most importantly v8.h, which can be accessed online in the following locations:

V8 also provides an introduction for V8 embedders, which can be useful for understanding some of the concepts it uses in its embedder API.

Important concepts when using V8 are the ones of Isolates and JavaScript value handles.

V8 supports fast API calls, which can be useful for improving the performance in certain cases.

libuv API documentation

The other major dependency of Node.js is libuv, providing the event loop and other operating system abstractions to Node.js.

There is a reference documentation for the libuv API.

File structure

The Node.js C++ files follow this structure:

The .h header files contain declarations, and sometimes definitions that don't require including other headers (e.g. getters, setters, etc.). They should only include other .h header files and nothing else.

The -inl.h header files contain definitions of inline functions from the corresponding .h header file (e.g. functions marked inline in the declaration or template functions). They always include the corresponding .h header file, and can include other .h and -inl.h header files as needed. It is not mandatory to split out the definitions from the .h file into an -inl.h file, but it becomes necessary when there are multiple definitions and contents of other -inl.h files start being used. Therefore, it is recommended to split a -inl.h file when inline functions become longer than a few lines to keep the corresponding .h file readable and clean. All visible definitions from the -inl.h file should be declared in the corresponding .h header file.

The .cc files contain definitions of non-inline functions from the corresponding .h header file. They always include the corresponding .h header file, and can include other .h and -inl.h header files as needed.

Helpful concepts

A number of concepts are involved in putting together Node.js on top of V8 and libuv. This section aims to explain some of them and how they work together.


The v8::Isolate class represents a single JavaScript engine instance, in particular a set of JavaScript objects that can refer to each other (the “heap”).

The v8::Isolate is often passed to other V8 API functions, and provides some APIs for managing the behaviour of the JavaScript engine or querying about its current state or statistics such as memory usage.

V8 APIs are not thread-safe unless explicitly specified. In a typical Node.js application, the main thread and any Worker threads each have one Isolate, and JavaScript objects from one Isolate cannot refer to objects from another Isolate.

Garbage collection, as well as other operations that affect the entire heap, happen on a per-Isolate basis.

Typical ways of accessing the current Isolate in the Node.js code are:

  • Given a FunctionCallbackInfo for a binding function, using args.GetIsolate().
  • Given a Context, using context->GetIsolate().
  • Given a Environment, using env->isolate().
  • Given a Realm, using realm->isolate().

V8 JavaScript values

V8 provides classes that mostly correspond to JavaScript types; for example, v8::Value is a class representing any kind of JavaScript type, with subclasses such as v8::Number (which in turn has subclasses like v8::Int32), v8::Boolean or v8::Object. Most types are represented by subclasses of v8::Object, e.g. v8::Uint8Array or v8::Date.

Internal fields

V8 provides the ability to store data in so-called “internal fields” inside v8::Objects that were created as instances of C++-backed classes. The number of fields needs to be defined when creating that class.

Both JavaScript values and void* pointers may be stored in such fields. In most native Node.js objects, the first internal field is used to store a pointer to a BaseObject subclass, which then contains all relevant information associated with the JavaScript object.

Typical ways of working with internal fields are:

  • obj->InternalFieldCount() to look up the number of internal fields for an object (0 for regular JavaScript objects).
  • obj->GetInternalField(i) to get a JavaScript value from an internal field.
  • obj->SetInternalField(i, v) to store a JavaScript value in an internal field.
  • obj->GetAlignedPointerFromInternalField(i) to get a void* pointer from an internal field.
  • obj->SetAlignedPointerInInternalField(i, p) to store a void* pointer in an internal field.

Contexts provide the same feature under the name “embedder data”.

JavaScript value handles

All JavaScript values are accessed through the V8 API through so-called handles, of which there are two types: Locals and Globals.

Local handles

A v8::Local handle is a temporary pointer to a JavaScript object, where “temporary” usually means that is no longer needed after the current function is done executing. Local handles can only be allocated on the C++ stack.

Most of the V8 API uses Local handles to work with JavaScript values or return them from functions.

Whenever a Local handle is created, a v8::HandleScope or v8::EscapableHandleScope object must exist on the stack. The Local is then added to that scope and deleted along with it.

When inside a binding function, a HandleScope already exists outside of it, so there is no need to explicitly create one.

EscapableHandleScopes can be used to allow a single Local handle to be passed to the outer scope. This is useful when a function returns a Local.

The following JavaScript and C++ functions are mostly equivalent:

function getFoo(obj) {
v8::Local<v8::Value> GetFoo(v8::Local<v8::Context> context,
                            v8::Local<v8::Object> obj) {
  v8::Isolate* isolate = context->GetIsolate();
  v8::EscapableHandleScope handle_scope(isolate);

  // The 'foo_string' handle cannot be returned from this function because
  // it is not “escaped” with `.Escape()`.
  v8::Local<v8::String> foo_string =
      v8::String::NewFromUtf8(isolate, "foo").ToLocalChecked();

  v8::Local<v8::Value> return_value;
  if (obj->Get(context, foo_string).ToLocal(&return_value)) {
    return handle_scope.Escape(return_value);
  } else {
    // There was a JS exception! Handle it somehow.
    return v8::Local<v8::Value>();

See exception handling for more information about the usage of .To(), .ToLocalChecked(), v8::Maybe and v8::MaybeLocal usage.

Casting local handles

If it is known that a Local<Value> refers to a more specific type, it can be cast to that type using .As<...>():

v8::Local<v8::Value> some_value;
// CHECK() is a Node.js utilitity that works similar to assert().
v8::Local<v8::Uint8Array> as_uint8 = some_value.As<v8::Uint8Array>();

Generally, using val.As<v8::X>() is only valid if val->IsX() is true, and failing to follow that rule may lead to crashes.

Detecting handle leaks

If it is expected that no Local handles should be created within a given scope unless explicitly within a HandleScope, a SealHandleScope can be used.

For example, there is a SealHandleScope around the event loop, forcing any functions that are called from the event loop and want to run or access JavaScript code to create HandleScopes.

Global handles

A v8::Global handle (sometimes also referred to by the name of its parent class Persistent, although use of that is discouraged in Node.js) is a reference to a JavaScript object that can remain active as long as the engine instance is active.

Global handles can be either strong or weak. Strong global handles are so-called “GC roots”, meaning that they will keep the JavaScript object they refer to alive even if no other objects refer to them. Weak global handles do not do that, and instead optionally call a callback when the object they refer to is garbage-collected.

v8::Global<v8::Object> reference;

void StoreReference(v8::Isolate* isolate, v8::Local<v8::Object> obj) {
  // Create a strong reference to `obj`.
  reference.Reset(isolate, obj);

// Must be called with a HandleScope around it.
v8::Local<v8::Object> LoadReference(v8::Isolate* isolate) {
  return reference.Get(isolate);
Eternal handles

v8::Eternal handles are a special kind of handles similar to v8::Global handles, with the exception that the values they point to are never garbage-collected while the JavaScript Engine instance is alive, even if the v8::Eternal itself is destroyed at some point. This type of handle is rarely used.


JavaScript allows multiple global objects and sets of built-in JavaScript objects (like the Object or Array functions) to coexist inside the same heap. Node.js exposes this ability through the vm module.

V8 refers to each of these global objects and their associated builtins as a Context.

Currently, in Node.js there is one main Context associated with the principal Realm of an Environment instance, and a number of subsidiary Contexts that are created with vm.Context or associated with ShadowRealm.

Most Node.js features will only work inside a context associated with a Realm. The only exception at the time of writing are MessagePort objects. This restriction is not inherent to the design of Node.js, and a sufficiently committed person could restructure Node.js to provide built-in modules inside of vm.Contexts.

Often, the Context is passed around for exception handling. Typical ways of accessing the current Context in the Node.js code are:

  • Given an Isolate, using isolate->GetCurrentContext().
  • Given an Environment, using env->context() to get the Environment's principal Realm's context.
  • Given a Realm, using realm->context() to get the Realm's context.

Event loop

The main abstraction for an event loop inside Node.js is the uv_loop_t struct. Typically, there is one event loop per thread. This includes not only the main thread and Workers, but also helper threads that may occasionally be spawned in the course of running a Node.js program.

The current event loop can be accessed using env->event_loop() given an Environment instance. The restriction of using a single event loop is not inherent to the design of Node.js, and a sufficiently committed person could restructure Node.js to provide e.g. the ability to run parts of Node.js inside an event loop separate from the active thread's event loop.


Node.js instances are represented by the Environment class.

Currently, every Environment class is associated with:

The Environment class contains a large number of different fields for different built-in modules that can be shared across different Realm instances, for example, the inspector agent, async hooks info.

Typical ways of accessing the current Environment in the Node.js code are:

  • Given a FunctionCallbackInfo for a binding function, using Environment::GetCurrent(args).
  • Given a BaseObject, using env() or self->env().
  • Given a Context, using Environment::GetCurrent(context). This requires that context has been associated with the Environment instance, e.g. is the main Context for the Environment or one of its vm.Contexts.
  • Given an Isolate, using Environment::GetCurrent(isolate). This looks up the current Context and then uses that.


The Realm class is a container for a set of JavaScript objects and functions that are associated with a particular ECMAScript realm.

Each ECMAScript realm comes with a global object and a set of intrinsic objects. An ECMAScript realm has a [[HostDefined]] field, which represents the Node.js Realm object.

Every Realm instance is created for a particular Context. A Realm can be a principal realm or a synthetic realm. A principal realm is created for each Environment's main Context. A synthetic realm is created for the Context of each ShadowRealm constructed from the JS API. No Realm is created for the Context of a vm.Context.

Native bindings and built-in modules can be evaluated in either a principal realm or a synthetic realm.

The Realm class contains a large number of different fields for different built-in modules, for example the memory for a Uint32Array that the url module uses for storing data returned from a urlBinding.update() call.

It also provides cleanup hooks and maintains a list of BaseObject instances.

Typical ways of accessing the current Realm in the Node.js code are:

  • Given a FunctionCallbackInfo for a binding function, using Realm::GetCurrent(args).
  • Given a BaseObject, using realm() or self->realm().
  • Given a Context, using Realm::GetCurrent(context). This requires that context has been associated with the Realm instance, e.g. is the principal Realm for the Environment.
  • Given an Isolate, using Realm::GetCurrent(isolate). This looks up the current Context and then uses its Realm.


Every Node.js instance (Environment) is associated with one IsolateData instance that contains information about or associated with a given Isolate.

String table

IsolateData contains a list of strings that can be quickly accessed inside Node.js code, e.g. given an Environment instance env the JavaScript string “name” can be accessed through env->name_string() without actually creating a new JavaScript string.


Every process that uses V8 has a v8::Platform instance that provides some functionalities to V8, most importantly the ability to schedule work on background threads.

Node.js provides a NodePlatform class that implements the v8::Platform interface and uses libuv for providing background threading abilities.

The platform can be accessed through isolate_data->platform() given an IsolateData instance, although that only works when:

  • The current Node.js instance was not started by an embedder; or
  • The current Node.js instance was started by an embedder whose v8::Platform implementation also implement's the node::MultiIsolatePlatform interface and who passed this to Node.js.

Binding functions

C++ functions exposed to JS follow a specific signature. The following example is from

void ArrayBufferViewHasBuffer(const FunctionCallbackInfo<Value>& args) {

(Namespaces are usually omitted through the use of using statements in the Node.js source code.)

args[n] is a Local<Value> that represents the n-th argument passed to the function. args.This() is the this value inside this function call. args.Holder() is equivalent to args.This() in all use cases inside of Node.js.

args.GetReturnValue() is a placeholder for the return value of the function, and provides a .Set() method that can be called with a boolean, integer, floating-point number or a Local<Value> to set the return value.

Node.js provides various helpers for building JS classes in C++ and/or attaching C++ functions to the exports of a built-in module:

void Initialize(Local<Object> target,
                Local<Value> unused,
                Local<Context> context,
                void* priv) {
  Environment* env = Environment::GetCurrent(context);

  SetMethod(context, target, "getaddrinfo", GetAddrInfo);
  SetMethod(context, target, "getnameinfo", GetNameInfo);

  // 'SetMethodNoSideEffect' means that debuggers can safely execute this
  // function for e.g. previews.
  SetMethodNoSideEffect(context, target, "canonicalizeIP", CanonicalizeIP);

  // ... more code ...

  Isolate* isolate = env->isolate();
  // Building the `ChannelWrap` class for JS:
  Local<FunctionTemplate> channel_wrap =
      NewFunctionTemplate(isolate, ChannelWrap::New);
  // Allow for 1 internal field, see `BaseObject` for details on this:

  // Set various methods on the class (i.e. on the prototype):
  SetProtoMethod(isolate, channel_wrap, "queryAny", Query<QueryAnyWrap>);
  SetProtoMethod(isolate, channel_wrap, "queryA", Query<QueryAWrap>);
  // ...
  SetProtoMethod(isolate, channel_wrap, "querySoa", Query<QuerySoaWrap>);
  SetProtoMethod(isolate, channel_wrap, "getHostByAddr", Query<GetHostByAddrWrap>);

  SetProtoMethodNoSideEffect(isolate, channel_wrap, "getServers", GetServers);

  SetConstructorFunction(context, target, "ChannelWrap", channel_wrap);

// Run the `Initialize` function when loading this binding through
// `internalBinding('cares_wrap')` in Node.js's built-in JavaScript code:

If the C++ binding is loaded during bootstrap, it needs to be registered with the utilities in node_external_reference.h, like this:

namespace node {
namespace util {
void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
  // ... register all C++ functions used to create FunctionTemplates.
}  // namespace util
}  // namespace node

// The first argument passed to `NODE_BINDING_EXTERNAL_REFERENCE`,
// which is `util` here, needs to be added to the
// `EXTERNAL_REFERENCE_BINDING_LIST_BASE` list in node_external_reference.h
NODE_BINDING_EXTERNAL_REFERENCE(util, node::util::RegisterExternalReferences)

Otherwise, you might see an error message like this when building the executables:

FAILED: gen/
cd ../../; out/Release/node_mksnapshot out/Release/gen/
Unknown external reference 0x107769200.
/bin/sh: line 1:  6963 Illegal instruction: 4  out/Release/node_mksnapshot out/Release/gen/

You can try using a debugger to symbolicate the external reference. For example, with lldb's image lookup --address command (with gdb it's info symbol):

$ lldb -- out/Release/node_mksnapshot out/Release/gen/
(lldb) run
Process 7012 launched: '/Users/joyee/projects/node/out/Release/node_mksnapshot' (x86_64)
Unknown external reference 0x1004c8200.
Process 7012 stopped
(lldb) image lookup --address 0x1004c8200
      Address: node_mksnapshot[0x00000001004c8200] (node_mksnapshot.__TEXT.__text + 5009920)
      Summary: node_mksnapshot`node::util::GetHiddenValue(v8::FunctionCallbackInfo<v8::Value> const&) at

Which explains that the unregistered external reference is node::util::GetHiddenValue defined in

Per-binding state

Some internal bindings, such as the HTTP parser, maintain internal state that only affects that particular binding. In that case, one common way to store that state is through the use of Realm::AddBindingData, which gives binding functions access to an object for storing such state. That object is always a BaseObject.

In the binding, call SET_BINDING_ID() with an identifier for the binding type. For example, for http_parser::BindingData, the identifier can be http_parser_binding_data.

If the binding should be supported in a snapshot, the id and the fully-specified class name should be added to the SERIALIZABLE_BINDING_TYPES list in base_object_types.h, and the class should implement the serialization and deserialization methods. See the comments of SnapshotableObject on how to implement them. Otherwise, add the id and the class name to the UNSERIALIZABLE_BINDING_TYPES list instead.

// In base_object_types.h, add the binding to either
// The second parameter is a descriptive name of the class, which is
// usually the fully-specified class name.

#define UNSERIALIZABLE_BINDING_TYPES(V)                                         \
  V(http_parser_binding_data, http_parser::BindingData)

// In the HTTP parser source code file:
class BindingData : public BaseObject {
  BindingData(Realm* realm, Local<Object> obj) : BaseObject(realm, obj) {}


  std::vector<char> parser_buffer;
  bool parser_buffer_in_use = false;

  // ...

// Available for binding functions, e.g. the HTTP Parser constructor:
static void New(const FunctionCallbackInfo<Value>& args) {
  BindingData* binding_data = Realm::GetBindingData<BindingData>(args);
  new Parser(binding_data, args.This());

// ... because the initialization function told the Realm to store the
// BindingData object:
void InitializeHttpParser(Local<Object> target,
                          Local<Value> unused,
                          Local<Context> context,
                          void* priv) {
  Realm* realm = Realm::GetCurrent(context);
  BindingData* const binding_data =
      realm->AddBindingData<BindingData>(context, target);
  if (binding_data == nullptr) return;

  Local<FunctionTemplate> t = NewFunctionTemplate(realm->isolate(), Parser::New);

Exception handling

The V8 engine provides multiple features to work with JavaScript exceptions, as C++ exceptions are disabled inside of Node.js:

Maybe types

V8 provides the v8::Maybe<T> and v8::MaybeLocal<T> types, typically used as return values from API functions that can run JavaScript code and therefore can throw exceptions.

Conceptually, the idea is that every v8::Maybe<T> is either empty (checked through .IsNothing()) or holds a value of type T (checked through .IsJust()). If the Maybe is empty, then a JavaScript exception is pending. A typical way of accessing the value is using the .To() function, which returns a boolean indicating success of the operation (i.e. the Maybe not being empty) and taking a pointer to a T to store the value if there is one.

Checked conversion

maybe.Check() can be used to assert that the maybe is not empty, i.e. crash the process otherwise. maybe.FromJust() (aka maybe.ToChecked()) can be used to access the value and crash the process if it is not set.

This should only be performed if it is actually sure that the operation has not failed. A lot of the Node.js source code does not follow this rule, and can be brought to crash through this.

In particular, it is often not safe to assume that an operation does not throw an exception, even if it seems like it would not do that. The most common reasons for this are:

  • Calls to functions like object->Get(...) or object->Set(...) may fail on most objects, if the Object.prototype object has been modified from userland code that added getters or setters.
  • Calls that invoke any JavaScript code, including JavaScript code that is provided from Node.js internals or V8 internals, will fail when JavaScript execution is being terminated. This typically happens inside Workers when worker.terminate() is called, but it can also affect the main thread when e.g. Node.js is used as an embedded library. These exceptions can happen at any point. It is not always obvious whether a V8 call will enter JavaScript. In addition to unexpected getters and setters, accessing some types of built-in objects like Maps and Sets can also run V8-internal JavaScript code.

v8::MaybeLocal<T> is a variant of v8::Maybe<T> that is either empty or holds a value of type Local<T>. It has methods that perform the same operations as the methods of v8::Maybe, but with different names:

Maybe MaybeLocal
maybe.IsNothing() maybe_local.IsEmpty()
maybe.IsJust() !maybe_local.IsEmpty()
maybe.To(&value) maybe_local.ToLocal(&local)
maybe.ToChecked() maybe_local.ToLocalChecked()
maybe.FromJust() maybe_local.ToLocalChecked()
v8::Nothing<T>() v8::MaybeLocal<T>()
v8::Just<T>(value) v8::MaybeLocal<T>(value)
Handling empty Maybes

Usually, the best approach to encountering an empty Maybe is to just return from the current function as soon as possible, and let execution in JavaScript land resume. If the empty Maybe is encountered inside a nested function, is may be a good idea to use a Maybe or MaybeLocal for the return type of that function and pass information about pending JavaScript exceptions along that way.

Generally, when an empty Maybe is encountered, it is not valid to attempt to perform further calls to APIs that return Maybes.

A typical pattern for dealing with APIs that return Maybe and MaybeLocal is using .ToLocal() and .To() and returning early in case there is an error:

// This could also return a v8::MaybeLocal<v8::Number>, for example.
v8::Maybe<double> SumNumbers(v8::Local<v8::Context> context,
                             v8::Local<v8::Array> array_of_integers) {
  v8::Isolate* isolate = context->GetIsolate();
  v8::HandleScope handle_scope(isolate);

  double sum = 0;

  for (uint32_t i = 0; i < array_of_integers->Length(); i++) {
    v8::Local<v8::Value> entry;
    if (!array_of_integers->Get(context, i).ToLocal(&entry)) {
      // Oops, we might have hit a getter that throws an exception!
      // It's better to not continue return an empty (“nothing”) Maybe.
      return v8::Nothing<double>();

    if (!entry->IsNumber()) {
      // Let's just skip any non-numbers. It would also be reasonable to throw
      // an exception here, e.g. using the error system in src/node_errors.h,
      // and then to return an empty Maybe again.

    // This cast is valid, because we've made sure it's really a number.
    v8::Local<v8::Number> entry_as_number = entry.As<v8::Number>();

    sum += entry_as_number->Value();

  return v8::Just(sum);

// Function that is exposed to JS:
void SumNumbers(const v8::FunctionCallbackInfo<v8::Value>& args) {
  // This will crash if the first argument is not an array. Let's assume we
  // have performed type checking in a JavaScript wrapper function.

  double sum;
  if (!SumNumbers(args.GetIsolate()->GetCurrentContext(),
                  args[0].As<v8::Array>()).To(&sum)) {
    // Nothing to do, we can just return directly to JavaScript.



If there is a need to catch JavaScript exceptions in C++, V8 provides the v8::TryCatch type for doing so, which we wrap into our own node::errors::TryCatchScope in Node.js. The latter has the additional feature of providing the ability to shut down the program in the typical Node.js way (printing the exception + stack trace) if an exception is caught.

A TryCatch will catch regular JavaScript exceptions, as well as termination exceptions such as the ones thrown by worker.terminate() calls. In the latter case, the try_catch.HasTerminated() function will return true, and the exception object will not be a meaningful JavaScript value. try_catch.ReThrow() should not be used in this case.

libuv handles and requests

Two central concepts when working with libuv are handles and requests.

Handles are subclasses of the uv_handle_t “class”, and generally refer to long-lived objects that can emit events multiple times, such as network sockets or file system watchers.

In Node.js, handles are often managed through a HandleWrap subclass.

Requests are one-time asynchronous function calls on the event loop, such as file system requests or network write operations, that either succeed or fail.

In Node.js, requests are often managed through a ReqWrap subclass.

Environment cleanup

When a Node.js Environment is destroyed, it generally needs to clean up any resources owned by it, e.g. memory or libuv requests/handles.

Cleanup hooks

Cleanup hooks are provided that run before the Environment or the Realm is destroyed. They can be added and removed by using env->AddCleanupHook(callback, hint); and env->RemoveCleanupHook(callback, hint);, or realm->AddCleanupHook(callback, hint); and realm->RemoveCleanupHook(callback, hint); respectively, where callback takes a void* hint argument.

Inside these cleanup hooks, new asynchronous operations may be started on the event loop, although ideally that is avoided as much as possible.

Every BaseObject has its own cleanup hook that deletes it. For ReqWrap and HandleWrap instances, cleanup of the associated libuv objects is performed automatically, i.e. handles are closed and requests are cancelled if possible.

Closing libuv handles

If a libuv handle is not managed through a HandleWrap instance, it needs to be closed explicitly. Do not use uv_close() for that, but rather env->CloseHandle(), which works the same way but keeps track of the number of handles that are still closing.

Closing libuv requests

There is no way to abort libuv requests in general. If a libuv request is not managed through a ReqWrap instance, the env->IncreaseWaitingRequestCounter() and env->DecreaseWaitingRequestCounter() functions need to be used to keep track of the number of active libuv requests.

Calling into JavaScript

Calling into JavaScript is not allowed during cleanup. Worker threads explicitly forbid this during their shutdown sequence, but the main thread does not for backwards compatibility reasons.

When calling into JavaScript without using MakeCallback(), check the env->can_call_into_js() flag and do not proceed if it is set to false.

Classes associated with JavaScript objects


A large number of classes in the Node.js C++ codebase refer to other objects. The MemoryRetainer class is a helper for annotating C++ classes with information that can be used by the heap snapshot builder in V8, so that memory retained by C++ can be tracked in V8 heap snapshots captured in Node.js applications.

Inheriting from the MemoryRetainer class enables objects (both from JavaScript and C++) to refer to instances of that class, and in turn enables that class to point to other objects as well, including native C++ types such as std::string and track their memory usage.

This can be useful for debugging memory leaks.

The memory_tracker.h header file explains how to use this class.


A frequently recurring situation is that a JavaScript object and a C++ object need to be tied together. BaseObject is the main abstraction for that in Node.js, and most classes that are associated with JavaScript objects are subclasses of it. It is defined in base_object.h.

Every BaseObject is associated with one Realm and one v8::Object. The v8::Object needs to have at least one internal field that is used for storing the pointer to the C++ object. In order to ensure this, the V8 SetInternalFieldCount() function is usually used when setting up the class from C++.

The JavaScript object can be accessed as a v8::Local<v8::Object> by using self->object(), given a BaseObject named self.

Accessing a BaseObject from a v8::Local<v8::Object> (frequently that is args.This() or args.Holder() in a binding function) can be done using the Unwrap<T>(obj) function, where T is a subclass of BaseObject. A helper for this is the ASSIGN_OR_RETURN_UNWRAP macro that returns from the current function if unwrapping fails (typically that means that the BaseObject has been deleted earlier).

void Http2Session::Request(const FunctionCallbackInfo<Value>& args) {
  Http2Session* session;
  ASSIGN_OR_RETURN_UNWRAP(&session, args.Holder());
  Environment* env = session->env();
  Local<Context> context = env->context();
  Isolate* isolate = env->isolate();

  // ...
  // The actual function body, which can now use the `session` object.
  // ...

Lifetime management

The BaseObject class comes with a set of features that allow managing the lifetime of its instances, either associating it with the lifetime of the corresponding JavaScript object or untying the two.

The BaseObject::MakeWeak() method turns the underlying Global handle into a weak one, and makes it so that the BaseObject::OnGCCollect() virtual method is called when the JavaScript object is garbage collected. By default, that methods deletes the BaseObject instance.

BaseObject::ClearWeak() undoes this effect.

It generally makes sense to call MakeWeak() in the constructor of a BaseObject subclass, unless that subclass is referred to by e.g. the event loop, as is the case for the HandleWrap and ReqWrap classes.

In addition, there are two kinds of smart pointers that can be used to refer to BaseObjects.

BaseObjectWeakPtr<T> is similar to std::weak_ptr<T>, but holds on to an object of a BaseObject subclass T and integrates with the lifetime management of the former. When the BaseObject no longer exists, e.g. when it was garbage collected, accessing it through weak_ptr.get() will return nullptr.

BaseObjectPtr<T> is similar to std::shared_ptr<T>, but also holds on to objects of a BaseObject subclass T. While there are BaseObjectPtrs pointing to a given object, the BaseObject will always maintain a strong reference to its associated JavaScript object. This can be useful when one BaseObject refers to another BaseObject and wants to make sure it stays alive during the lifetime of that reference.

A BaseObject can be “detached” through the BaseObject::Detach() method. In this case, it will be deleted once the last BaseObjectPtr referring to it is destroyed. There must be at least one such pointer when Detach() is called. This can be useful when one BaseObject fully owns another BaseObject.


AsyncWrap is a subclass of BaseObject that additionally provides tracking functions for asynchronous calls. It is commonly used for classes whose methods make calls into JavaScript without any JavaScript stack below, i.e. more or less directly from the event loop. It is defined in async_wrap.h.

Every AsyncWrap subclass has a “provider type”. A list of provider types is maintained in src/async_wrap.h.

Every AsyncWrap instance is associated with two numbers, the “async id” and the “async trigger id”. The “async id” is generally unique per AsyncWrap instance, and only changes when the object is re-used in some way.

See the async_hooks module documentation for more information about how this information is provided to async tracking tools.


The AsyncWrap class has a set of methods called MakeCallback(), with the intention of the naming being that it is used to “make calls back into JavaScript” from the event loop, rather than making callbacks in some way. (As the naming has made its way into the Node.js public API, it's not worth the breakage of fixing it).

MakeCallback() generally calls a method on the JavaScript object associated with the current AsyncWrap, and informs async tracking code about these calls as well as takes care of running the process.nextTick() and Promise task queues once it returns.

Before calling MakeCallback(), it is typically necessary to enter both a HandleScope and a Context::Scope.

void StatWatcher::Callback(uv_fs_poll_t* handle,
                           int status,
                           const uv_stat_t* prev,
                           const uv_stat_t* curr) {
  // Get the StatWatcher instance associated with this call from libuv,
  // StatWatcher is a subclass of AsyncWrap.
  StatWatcher* wrap = ContainerOf(&StatWatcher::watcher_, handle);
  Environment* env = wrap->env();
  HandleScope handle_scope(env->isolate());
  Context::Scope context_scope(env->context());

  // Transform 'prev' and 'curr' into an array:
  Local<Value> arr = ...;

  Local<Value> argv[] = { Integer::New(env->isolate(), status), arr };
  wrap->MakeCallback(env->onchange_string(), arraysize(argv), argv);

See Callback scopes for more information.


HandleWrap is a subclass of AsyncWrap specifically designed to make working with libuv handles easier. It provides the .ref(), .unref() and .hasRef() methods as well as .close() to enable easier lifetime management from JavaScript. It is defined in handle_wrap.h.

HandleWrap instances are cleaned up automatically when the current Node.js Environment is destroyed, e.g. when a Worker thread stops.

HandleWrap also provides facilities for diagnostic tooling to get an overview over libuv handles managed by Node.js.


ReqWrap is a subclass of AsyncWrap specifically designed to make working with libuv requests easier. It is defined in req_wrap.h.

In particular, its Dispatch() method is designed to avoid the need to keep track of the current count of active libuv requests.

ReqWrap also provides facilities for diagnostic tooling to get an overview over libuv handles managed by Node.js.

Callback scopes

The public CallbackScope and the internally used InternalCallbackScope classes provide the same facilities as MakeCallback(), namely:

  • Emitting the 'before' event for async tracking when entering the scope
  • Setting the current async IDs to the ones passed to the constructor
  • Emitting the 'after' event for async tracking when leaving the scope
  • Running the process.nextTick() queue
  • Running microtasks, in particular Promise callbacks and async/await functions

Usually, using AsyncWrap::MakeCallback() or using the constructor taking an AsyncWrap* argument (i.e. used as InternalCallbackScope callback_scope(this);) suffices inside of the Node.js C++ codebase.

C++ utilities

Node.js uses a few custom C++ utilities, mostly defined in util.h.

Memory allocation

Node.js provides Malloc(), Realloc() and Calloc() functions that work like their C stdlib counterparts, but crash if memory cannot be allocated. (As V8 does not handle out-of-memory situations gracefully, it does not make sense for Node.js to attempt to do so in all cases.)

The UncheckedMalloc(), UncheckedRealloc() and UncheckedCalloc() functions return nullptr in these cases (or when size == 0).

Optional stack-based memory allocation

The MaybeStackBuffer class provides a way to allocate memory on the stack if it is smaller than a given limit, and falls back to allocating it on the heap if it is larger. This can be useful for performantly allocating temporary data if it is typically expected to be small (e.g. file paths).

The Utf8Value, TwoByteValue (i.e. UTF-16 value) and BufferValue (Utf8Value but copy data from a Buffer if one is passed) helpers inherit from this class and allow accessing the characters in a JavaScript string this way.

static void Chdir(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  // ...
  Utf8Value path(env->isolate(), args[0]);
  int err = uv_chdir(*path);
  if (err) {
    // ... error handling ...


Node.js provides a few macros that behave similar to assert():

  • CHECK(expression) aborts the process with a stack trace if expression is false.
  • CHECK_EQ(a, b) checks for a == b
  • CHECK_GE(a, b) checks for a >= b
  • CHECK_GT(a, b) checks for a > b
  • CHECK_LE(a, b) checks for a <= b
  • CHECK_LT(a, b) checks for a < b
  • CHECK_NE(a, b) checks for a != b
  • CHECK_NULL(val) checks for a == nullptr
  • CHECK_NOT_NULL(val) checks for a != nullptr
  • CHECK_IMPLIES(a, b) checks that b is true if a is true.
  • UNREACHABLE([message]) aborts the process if it is reached.

CHECKs are always enabled. For checks that should only run in debug mode, use DCHECK(), DCHECK_EQ(), etc.

Scope-based cleanup

The OnScopeLeave() function can be used to run a piece of code when leaving the current C++ scope.

static void GetUserInfo(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  uv_passwd_t pwd;
  // ...

  const int err = uv_os_get_passwd(&pwd);

  if (err) {
    // ... error handling, return early ...

  auto free_passwd = OnScopeLeave([&]() { uv_os_free_passwd(&pwd); });

  // ...
  // Turn `pwd` into a JavaScript object now; whenever we return from this
  // function, `uv_os_free_passwd()` will be called.
  // ...