node/doc/api/embedding.md

C++ embedder API

Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.

The documentation for these APIs can be found in src/node.h in the Node.js source tree. In addition to the APIs exposed by Node.js, some required concepts are provided by the V8 embedder API.

Because using Node.js as an embedded library is different from writing code that is executed by Node.js, breaking changes do not follow typical Node.js deprecation policy and may occur on each semver-major release without prior warning.

Example embedding application

The following sections will provide an overview over how to use these APIs to create an application from scratch that will perform the equivalent of node -e <code>, i.e. that will take a piece of JavaScript and run it in a Node.js-specific environment.

The full code can be found in the Node.js source tree.

Setting up a per-process state

Node.js requires some per-process state management in order to run:

• Arguments parsing for Node.js CLI options,
• V8 per-process requirements, such as a v8::Platform instance.

The following example shows how these can be set up. Some class names are from the node and v8 C++ namespaces, respectively.

int main(int argc, char** argv) {
  argv = uv_setup_args(argc, argv);
  std::vector<std::string> args(argv, argv + argc);
  // Parse Node.js CLI options, and print any errors that have occurred while
  // trying to parse them.
  std::unique_ptr<node::InitializationResult> result =
      node::InitializeOncePerProcess(args, {
        node::ProcessInitializationFlags::kNoInitializeV8,
        node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
      });

  for (const std::string& error : result->errors())
    fprintf(stderr, "%s: %s\n", args[0].c_str(), error.c_str());
  if (result->early_return() != 0) {
    return result->exit_code();
  }

  // Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way
  // to create a v8::Platform instance that Node.js can use when creating
  // Worker threads. When no `MultiIsolatePlatform` instance is present,
  // Worker threads are disabled.
  std::unique_ptr<MultiIsolatePlatform> platform =
      MultiIsolatePlatform::Create(4);
  V8::InitializePlatform(platform.get());
  V8::Initialize();

  // See below for the contents of this function.
  int ret = RunNodeInstance(
      platform.get(), result->args(), result->exec_args());

  V8::Dispose();
  V8::DisposePlatform();

  node::TearDownOncePerProcess();
  return ret;
}

Restricting access to environment variables

When the arguments passed to node::InitializeOncePerProcess() enable the Permission Model without --allow-env=*, the process environment must not contain any variable that --allow-env does not grant access to. node::InitializeOncePerProcess() fails otherwise. Unlike the node executable, embedders own the process environment, so Node.js does not remove these variables itself.

node::ScrubProcessEnvironment() removes them. Because it modifies the process environment without any locking that native code calling getenv() participates in, it must be called before starting any thread that may read the environment, and before node::InitializeOncePerProcess():

int main(int argc, char** argv) {
  argv = uv_setup_args(argc, argv);
  std::vector<std::string> args(argv, argv + argc);

  // Keep the variables the embedder itself reads, in addition to the ones
  // Node.js reads (see node::GetRuntimeEnvironmentDefaults()).
  node::ProcessEnvironmentScrubOptions scrub_options;
  scrub_options.allow = {"PORT", "APP_*"};
  if (node::ScrubProcessEnvironment(scrub_options).IsNothing()) {
    return 1;
  }

  // args contains, for example, --permission --allow-env=PORT
  std::unique_ptr<node::InitializationResult> result =
      node::InitializeOncePerProcess(args, {
        node::ProcessInitializationFlags::kNoInitializeV8,
        node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
      });
  // ...
}

process.permission.drop('env', name) removes a variable from the process environment, so it throws when called from a node::Environment created without node::EnvironmentFlags::kOwnsProcessState.

Setting up a per-instance state

Node.js has a concept of a “Node.js instance”, that is commonly being referred to as node::Environment. Each node::Environment is associated with:

• Exactly one v8::Isolate, i.e. one JS Engine instance,
• Exactly one uv_loop_t, i.e. one event loop,
• A number of v8::Contexts, but exactly one main v8::Context, and
• One node::IsolateData instance that contains information that could be shared by multiple node::Environments. The embedder should make sure that node::IsolateData is shared only among node::Environments that use the same v8::Isolate, Node.js does not perform this check.

node::Environments that share a node::IsolateData also share its uv_loop_t. node::FreeEnvironment() runs that loop until the handles of the node::Environment being freed have closed. Timers, I/O callbacks and thread pool completions of the other node::Environments that become due in those loop iterations run normally, including their JavaScript; only the node::Environment being freed can no longer call into JavaScript.

In order to set up a v8::Isolate, an v8::ArrayBuffer::Allocator needs to be provided. One possible choice is the default Node.js allocator, which can be created through node::ArrayBufferAllocator::Create(). Using the Node.js allocator allows minor performance optimizations when addons use the Node.js C++ Buffer API, and is required in order to track ArrayBuffer memory in process.memoryUsage().

Additionally, each v8::Isolate that is used for a Node.js instance needs to be registered and unregistered with the MultiIsolatePlatform instance, if one is being used, in order for the platform to know which event loop to use for tasks scheduled by the v8::Isolate.

The node::NewIsolate() helper function creates a v8::Isolate, sets it up with some Node.js-specific hooks (e.g. the Node.js error handler), and registers it with the platform automatically.

int RunNodeInstance(MultiIsolatePlatform* platform,
                    const std::vector<std::string>& args,
                    const std::vector<std::string>& exec_args) {
  int exit_code = 0;

  // Set up a libuv event loop, v8::Isolate, and Node.js Environment.
  std::vector<std::string> errors;
  std::unique_ptr<CommonEnvironmentSetup> setup =
      CommonEnvironmentSetup::Create(platform, &errors, args, exec_args);
  if (!setup) {
    for (const std::string& err : errors)
      fprintf(stderr, "%s: %s\n", args[0].c_str(), err.c_str());
    return 1;
  }

  Isolate* isolate = setup->isolate();
  Environment* env = setup->env();

  {
    Locker locker(isolate);
    Isolate::Scope isolate_scope(isolate);
    HandleScope handle_scope(isolate);
    // The v8::Context needs to be entered when node::CreateEnvironment() and
    // node::LoadEnvironment() are being called.
    Context::Scope context_scope(setup->context());

    // Set up the Node.js instance for execution, and run code inside of it.
    // There is also a variant that takes a callback and provides it with
    // the `require` and `process` objects, so that it can manually compile
    // and run scripts as needed.
    // The `require` function inside this script does *not* access the file
    // system, and can only load built-in Node.js modules.
    // `module.createRequire()` is being used to create one that is able to
    // load files from the disk, and uses the standard CommonJS file loader
    // instead of the internal-only `require` function.
    MaybeLocal<Value> loadenv_ret = node::LoadEnvironment(
        env,
        "const publicRequire ="
        "  require('node:module').createRequire(process.cwd() + '/');"
        "globalThis.require = publicRequire;"
        "require('node:vm').runInThisContext(process.argv[1]);");

    if (loadenv_ret.IsEmpty())  // There has been a JS exception.
      return 1;

    exit_code = node::SpinEventLoop(env).FromMaybe(1);

    // node::Stop() can be used to explicitly stop the event loop and keep
    // further JavaScript from running. It can be called from any thread,
    // and will act like worker.terminate() if called from another thread.
    node::Stop(env);
  }

  return exit_code;
}