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3 min read Coding

Using V8 Fast API in Node.js core

Node.js uses V8 as the JavaScript engine. Embedder functions implemented in C++ incur a high overhead, so V8 provides an API to implement fast-path C functions which may be invoked directly from JITted code. These functions also come with additional constraints, for example, they may not trigger garbage collection.


  • Fast API functions may not trigger garbage collection. This means by proxy that JavaScript execution and heap allocation are also forbidden, including v8::Array::Get() or v8::Number::New()
  • Throwing errors is not available on fast API but can be done through the fallback to slow API.
  • Not all parameter and return types are supported in fast API calls. For a full list, please look into v8-fast-api-calls.h .


  • Each unique fast API function signature should be defined inside the node_external_reference.h file.
  • To test fast APIs, run the tests in a loop with a decent iterations count to trigger V8 for optimization and to prefer the fast API over the slow one.
  • The fast callback must be idempotent up to the point where error and fallback conditions are checked because otherwise, executing the slow callback might produce visible side effects twice.

Fallback to the slow path

Fast APIs support fallback to the slow path (implementation that uses V8 internals) in case logically it is wise to do so, for example, when providing a more detailed error. The fallback mechanism can be enabled and changed from the caller JavaScript function and the fast API function declaration.

Passing a true value to fallback option will force V8 to run the slow path with the same arguments.

In V8, the options fallback struct is defined as FastApiCallbackOptions under the v8-fast-api-calls.h file.

Example of C++ fallback

// Anywhere in the execution flow, you can set fallback and stop the execution.
static double divide(const int32_t a,
                     const int32_t b,
                     v8::FastApiCallbackOptions& options) {
  if (b == 0) {
    options.fallback = true;
    return 0;
  } else {
    return a / b;

Anywhere in the execution flow, you can set fallback and stop the execution.

Example usage

  • On JavaScript side:
Example usage of a javascript user-land function
const { divide } = internalBinding('custom_namespace');
  • On the C++ side:
Example module declaration in C++ with V8
#include "v8-fast-api-calls.h"
namespace node {
namespace custom_namespace {
static void divide(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  CHECK_GE(args.Length(), 2);
  auto a = args[0].As<v8::Int32>();
  auto b = args[1].As<v8::Int32>();
  if (b->Value() == 0) {
    return node::THROW_ERR_INVALID_STATE(env, "Error");
  double result = a->Value() / b->Value();
static double FastDivide(const int32_t a,
                         const int32_t b,
                         v8::FastApiCallbackOptions& options) {
  if (b == 0) {
    options.fallback = true;
    return 0;
  } else {
    return a / b;
CFunction fast_divide_(CFunction::Make(FastDivide));
static void Initialize(Local<Object> target,
                       Local<Value> unused,
                       Local<Context> context,
                       void* priv) {
  SetFastMethod(context, target, "divide", Divide, &fast_divide_);
void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
} // namespace custom_namespace
} // namespace node
  • Update node_external_reference.h

Since our implementation used int(const v8::FastApiCallbackOptions& options) signature, we need to add it to external references if it is not available and in ALLOWED_EXTERNAL_REFERENCE_TYPES.

Example external reference declaration
using CFunctionCallbackReturningDouble = double (*)(const int32_t a,
                                                    const int32_t b,
                                                    v8::FastApiCallbackOptions& options);