Node-API

Node-API

Stability: 2 - Stable

Node-API (formerly N-API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (for example, V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The [ABI Stability][] guide provides a more in-depth explanation.

Addons are built/packaged with the same approach/tools outlined in the section titled [C++ Addons][]. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or [Native Abstractions for Node.js][] APIs, the functions available in Node-API are used.

APIs exposed by Node-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA-262 Language Specification. The APIs have the following properties:

All Node-API calls return a status code of type napi_status. This status indicates whether the API call succeeded or failed.
The API's return value is passed via an out parameter.
All JavaScript values are abstracted behind an opaque type named napi_value.
In case of an error status code, additional information can be obtained using napi_get_last_error_info. More information can be found in the error handling section [Error handling][].

Writing addons in various programming languages

Node-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. With this stability guarantee, it is possible to write addons in other programming languages on top of Node-API. Refer to [language and engine bindings][] for more programming languages and engines support details.

[node-addon-api][] is the official C++ binding that provides a more efficient way to write C++ code that calls Node-API. This wrapper is a header-only library that offers an inlinable C++ API. Binaries built with node-addon-api will depend on the symbols of the Node-API C-based functions exported by Node.js. The following code snippet is an example of node-addon-api:

Object obj = Object::New(env);
obj["foo"] = String::New(env, "bar");

The above node-addon-api C++ code is equivalent to the following C-based Node-API code:

napi_status status;
napi_value object, string;
status = napi_create_object(env, &object);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_set_named_property(env, object, "foo", string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

The end result is that the addon only uses the exported C APIs. Even though the addon is written in C++, it still gets the benefits of the ABI stability provided by the C Node-API.

When using node-addon-api instead of the C APIs, start with the API [docs][] for node-addon-api.

The Node-API Resource offers an excellent orientation and tips for developers just getting started with Node-API and node-addon-api. Additional media resources can be found on the [Node-API Media][] page.

Implications of ABI stability

Although Node-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:

the Node.js C++ APIs available via any of

cpp #include <node.h> #include <node_buffer.h> #include <node_version.h> #include <node_object_wrap.h>

the libuv APIs which are also included with Node.js and available via

cpp #include <uv.h>

the V8 API available via

cpp #include <v8.h>

Thus, for an addon to remain ABI-compatible across Node.js major versions, it must use Node-API exclusively by restricting itself to using

#include <node_api.h>

and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to Node-API.

Enum values in ABI stability

All enum data types defined in Node-API should be considered as a fixed size int32_t value. Bit flag enum types should be explicitly documented, and they work with bit operators like bit-OR (|) as a bit value. Unless otherwise documented, an enum type should be considered to be extensible.

A new enum value will be added at the end of the enum definition. An enum value will not be removed or renamed.

For an enum type returned from a Node-API function, or provided as an out parameter of a Node-API function, the value is an integer value and an addon should handle unknown values. New values are allowed to be introduced without a version guard. For example, when checking napi_status in switch statements, an addon should include a default branch, as new status codes may be introduced in newer Node.js versions.

For an enum type used in an in-parameter, the result of passing an unknown integer value to Node-API functions is undefined unless otherwise documented. A new value is added with a version guard to indicate the Node-API version in which it was introduced. For example, napi_get_all_property_names can be extended with new enum value of napi_key_filter.

For an enum type used in both in-parameters and out-parameters, new values are allowed to be introduced without a version guard.

Building

Unlike modules written in JavaScript, developing and deploying Node.js native addons using Node-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.

For Linux developers, the necessary C/C++ toolchain packages are readily available. [GCC][] is widely used in the Node.js community to build and test across a variety of platforms. For many developers, the [LLVM][] compiler infrastructure is also a good choice.

For Mac developers, [Xcode][] offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:

xcode-select --install

For Windows developers, [Visual Studio][] offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:

npm install --global windows-build-tools

The sections below describe the additional tools available for developing and deploying Node.js native addons.

Build tools

Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.

node-gyp

[node-gyp][] is a build system based on the [gyp-next][] tool and comes bundled with npm. node-gyp requires that Python be installed.

Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.

CMake.js

[CMake.js][] is an alternative build system based on [CMake][].

CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp. [build_with_cmake][] is an example of a CMake-based native addon project.

Uploading precompiled binaries

The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like [Travis CI][] and [AppVeyor][] to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.

node-pre-gyp

[node-pre-gyp][] is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.

prebuild

[prebuild][] is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to [GitHub releases][]. prebuild is a good choice for GitHub projects using CMake.js.

prebuildify

[prebuildify][] is a tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native addon when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native addon is installed.

Usage

In order to use the Node-API functions, include the file [node_api.h][] which is located in the src directory in the node development tree:

#include <node_api.h>

This will opt into the default NAPI_VERSION for the given release of Node.js. In order to ensure compatibility with specific versions of Node-API, the version can be specified explicitly when including the header:

#define NAPI_VERSION 3
#include <node_api.h>

This restricts the Node-API surface to just the functionality that was available in the specified (and earlier) versions.

Some of the Node-API surface is experimental and requires explicit opt-in:

#define NAPI_EXPERIMENTAL
#include <node_api.h>

In this case the entire API surface, including any experimental APIs, will be available to the module code.

Occasionally, experimental features are introduced that affect already-released and stable APIs. These features can be disabled by an opt-out:

#define NAPI_EXPERIMENTAL
#define NODE_API_EXPERIMENTAL_<FEATURE_NAME>_OPT_OUT
#include <node_api.h>

where <FEATURE_NAME> is the name of an experimental feature that affects both experimental and stable APIs.

Node-API version matrix

Up until version 9, Node-API versions were additive and versioned independently from Node.js. This meant that any version was an extension to the previous version in that it had all of the APIs from the previous version with some additions. Each Node.js version only supported a single Node-API version. For example v18.15.0 supports only Node-API version 8. ABI stability was achieved because 8 was a strict superset of all previous versions.

As of version 9, while Node-API versions continue to be versioned independently, an add-on that ran with Node-API version 9 may need code updates to run with Node-API version 10. ABI stability is maintained, however, because Node.js versions that support Node-API versions higher than 8 will support all versions between 8 and the highest version they support and will default to providing the version 8 APIs unless an add-on opts into a higher Node-API version. This approach provides the flexibility of better optimizing existing Node-API functions while maintaining ABI stability. Existing add-ons can continue to run without recompilation using an earlier version of Node-API. If an add-on needs functionality from a newer Node-API version, changes to existing code and recompilation will be needed to use those new functions anyway.

In versions of Node.js that support Node-API version 9 and later, defining NAPI_VERSION=X and using the existing add-on initialization macros will bake in the requested Node-API version that will be used at runtime into the add-on. If NAPI_VERSION is not set it will default to 8.

This table may not be up to date in older streams, the most up to date information is in the latest API documentation in: [Node-API version matrix](https://nodejs.org/docs/latest/api/n-api.html#node-api-version-matrix

<table> <tr> <th>Node-API version</th> <th scope=“col”>Supported In</th> </tr> <tr> <th scope=“row”>10</th> <td>v22.14.0+, 23.6.0+ and all later versions</td> </tr> <tr> <th scope=“row”>9</th> <td>v18.17.0+, 20.3.0+, 21.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>8</th> <td>v12.22.0+, v14.17.0+, v15.12.0+, 16.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>7</th> <td>v10.23.0+, v12.19.0+, v14.12.0+, 15.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>6</th> <td>v10.20.0+, v12.17.0+, 14.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>5</th> <td>v10.17.0+, v12.11.0+, 13.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>4</th> <td>v10.16.0+, v11.8.0+, 12.0.0 and all later versions</td> </tr> </tr> <tr> <th scope=“row”>3</th> <td>v6.14.2, 8.11.2+, v9.11.0+, 10.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>2</th> <td>v8.10.0+, v9.3.0+, 10.0.0 and all later versions</td> </tr> <tr> <th scope=“row”>1</th> <td>v8.6.0+*, v9.0.0+, 10.0.0 and all later versions</td> </tr> </table>

* Node-API was experimental.

** Node.js 8.0.0 included Node-API as experimental. It was released as Node-API version 1 but continued to evolve until Node.js 8.6.0. The API is different in versions prior to Node.js 8.6.0. We recommend Node-API version 3 or later.

Each API documented for Node-API will have a header named added in:, and APIs which are stable will have the additional header Node-API version:. APIs are directly usable when using a Node.js version which supports the Node-API version shown in Node-API version: or higher. When using a Node.js version that does not support the Node-API version: listed or if there is no Node-API version: listed, then the API will only be available if #define NAPI_EXPERIMENTAL precedes the inclusion of node_api.h or js_native_api.h. If an API appears not to be available on a version of Node.js which is later than the one shown in added in: then this is most likely the reason for the apparent absence.

The Node-APIs associated strictly with accessing ECMAScript features from native code can be found separately in js_native_api.h and js_native_api_types.h. The APIs defined in these headers are included in node_api.h and node_api_types.h. The headers are structured in this way in order to allow implementations of Node-API outside of Node.js. For those implementations the Node.js specific APIs may not be applicable.

The Node.js-specific parts of an addon can be separated from the code that exposes the actual functionality to the JavaScript environment so that the latter may be used with multiple implementations of Node-API. In the example below, addon.c and addon.h refer only to js_native_api.h. This ensures that addon.c can be reused to compile against either the Node.js implementation of Node-API or any implementation of Node-API outside of Node.js.

addon_node.c is a separate file that contains the Node.js specific entry point to the addon and which instantiates the addon by calling into addon.c when the addon is loaded into a Node.js environment.

// addon.h
#ifndef _ADDON_H_
#define _ADDON_H_
#include <js_native_api.h>
napi_value create_addon(napi_env env);
#endif  // _ADDON_H_

// addon.c
#include "addon.h"

#define NODE_API_CALL(env, call)                                  \
  do {                                                            \
    napi_status status = (call);                                  \
    if (status != napi_ok) {                                      \
      const napi_extended_error_info* error_info = NULL;          \
      napi_get_last_error_info((env), &error_info);               \
      const char* err_message = error_info->error_message;        \
      bool is_pending;                                            \
      napi_is_exception_pending((env), &is_pending);              \
      /* If an exception is already pending, don't rethrow it */  \
      if (!is_pending) {                                          \
        const char* message = (err_message == NULL)               \
            ? "empty error message"                               \
            : err_message;                                        \
        napi_throw_error((env), NULL, message);                   \
      }                                                           \
      return NULL;                                                \
    }                                                             \
  } while(0)

static napi_value
DoSomethingUseful(napi_env env, napi_callback_info info) {
  // Do something useful.
  return NULL;
}

napi_value create_addon(napi_env env) {
  napi_value result;
  NODE_API_CALL(env, napi_create_object(env, &result));

  napi_value exported_function;
  NODE_API_CALL(env, napi_create_function(env,
                                          "doSomethingUseful",
                                          NAPI_AUTO_LENGTH,
                                          DoSomethingUseful,
                                          NULL,
                                          &exported_function));

  NODE_API_CALL(env, napi_set_named_property(env,
                                             result,
                                             "doSomethingUseful",
                                             exported_function));

  return result;
}

// addon_node.c
#include <node_api.h>
#include "addon.h"

NAPI_MODULE_INIT(/* napi_env env, napi_value exports */) {
  // This function body is expected to return a `napi_value`.
  // The variables `napi_env env` and `napi_value exports` may be used within
  // the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
  return create_addon(env);
}

Environment life cycle APIs

[Section Agents][] of the [ECMAScript Language Specification][] defines the concept of an “Agent” as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.

A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as [worker threads][]. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.

From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.

Native addons may need to allocate global state which they use during their life cycle of an Node.js environment such that the state can be unique to each instance of the addon.

To this end, Node-API provides a way to associate data such that its life cycle is tied to the life cycle of a Node.js environment.

`napi_set_instance_data`

napi_status napi_set_instance_data(node_api_basic_env env,
                                   void* data,
                                   napi_finalize finalize_cb,
                                   void* finalize_hint);

[in] env: The environment that the Node-API call is invoked under.
[in] data: The data item to make available to bindings of this instance.
[in] finalize_cb: The function to call when the environment is being torn down. The function receives data so that it might free it. [napi_finalize][] provides more details.
[in] finalize_hint: Optional hint to pass to the finalize callback during collection.

Returns napi_ok if the API succeeded.

This API associates data with the currently running Node.js environment. data can later be retrieved using napi_get_instance_data(). Any existing data associated with the currently running Node.js environment which was set by means of a previous call to napi_set_instance_data() will be overwritten. If a finalize_cb was provided by the previous call, it will not be called.

`napi_get_instance_data`

napi_status napi_get_instance_data(node_api_basic_env env,
                                   void** data);

[in] env: The environment that the Node-API call is invoked under.
[out] data: The data item that was previously associated with the currently running Node.js environment by a call to napi_set_instance_data().

Returns napi_ok if the API succeeded.

This API retrieves data that was previously associated with the currently running Node.js environment via napi_set_instance_data(). If no data is set, the call will succeed and data will be set to NULL.

Basic Node-API data types

Node-API exposes the following fundamental data types as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other Node-API calls.

`napi_status`

Integral status code indicating the success or failure of a Node-API call. Currently, the following status codes are supported.

typedef enum {
  napi_ok,
  napi_invalid_arg,
  napi_object_expected,
  napi_string_expected,
  napi_name_expected,
  napi_function_expected,
  napi_number_expected,
  napi_boolean_expected,
  napi_array_expected,
  napi_generic_failure,
  napi_pending_exception,
  napi_cancelled,
  napi_escape_called_twice,
  napi_handle_scope_mismatch,
  napi_callback_scope_mismatch,
  napi_queue_full,
  napi_closing,
  napi_bigint_expected,
  napi_date_expected,
  napi_arraybuffer_expected,
  napi_detachable_arraybuffer_expected,
  napi_would_deadlock,  /* unused */
  napi_no_external_buffers_allowed,
  napi_cannot_run_js
} napi_status;

If additional information is required upon an API returning a failed status, it can be obtained by calling napi_get_last_error_info.

`napi_extended_error_info`

typedef struct {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
} napi_extended_error_info;

error_message: UTF8-encoded string containing a VM-neutral description of the error.
engine_reserved: Reserved for VM-specific error details. This is currently not implemented for any VM.
engine_error_code: VM-specific error code. This is currently not implemented for any VM.
error_code: The Node-API status code that originated with the last error.

See the [Error handling][] section for additional information.

`napi_env`

napi_env is used to represent a context that the underlying Node-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making Node-API calls. Specifically, the same napi_env that was passed in when the initial native function was called must be passed to any subsequent nested Node-API calls. Caching the napi_env for the purpose of general reuse, and passing the napi_env between instances of the same addon running on different [Worker][] threads is not allowed. The napi_env becomes invalid when an instance of a native addon is unloaded. Notification of this event is delivered through the callbacks given to [napi_add_env_cleanup_hook][] and [napi_set_instance_data][].

`node_api_basic_env`

Stability: 1 - Experimental

This variant of napi_env is passed to synchronous finalizers ([node_api_basic_finalize][]). There is a subset of Node-APIs which accept a parameter of type node_api_basic_env as their first argument. These APIs do not access the state of the JavaScript engine and are thus safe to call from synchronous finalizers. Passing a parameter of type napi_env to these APIs is allowed, however, passing a parameter of type node_api_basic_env to APIs that access the JavaScript engine state is not allowed. Attempting to do so without a cast will produce a compiler warning or an error when add-ons are compiled with flags which cause them to emit warnings and/or errors when incorrect pointer types are passed into a function. Calling such APIs from a synchronous finalizer will ultimately result in the termination of the application.

`napi_value`

This is an opaque pointer that is used to represent a JavaScript value.

`napi_threadsafe_function`

This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via napi_call_threadsafe_function().

`napi_threadsafe_function_release_mode`

A value to be given to napi_release_threadsafe_function() to indicate whether the thread-safe function is to be closed immediately (napi_tsfn_abort) or merely released (napi_tsfn_release) and thus available for subsequent use via napi_acquire_threadsafe_function() and napi_call_threadsafe_function().

typedef enum {
  napi_tsfn_release,
  napi_tsfn_abort
} napi_threadsafe_function_release_mode;

`napi_threadsafe_function_call_mode`

A value to be given to napi_call_threadsafe_function() to indicate whether the call should block whenever the queue associated with the thread-safe function is full.

typedef enum {
  napi_tsfn_nonblocking,
  napi_tsfn_blocking
} napi_threadsafe_function_call_mode;

Node-API memory management types

`napi_handle_scope`

This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, Node-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, Node-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.

Handle scopes are created using [napi_open_handle_scope][] and are destroyed using [napi_close_handle_scope][]. Closing the scope can indicate to the GC that all napi_values created during the lifetime of the handle scope are no longer referenced from the current stack frame.

For more details, review the [Object lifetime management][].

`napi_escapable_handle_scope`

Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.

`napi_ref`

This is the abstraction to use to reference a napi_value. This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly.

For more details, review the [Object lifetime management][].

`napi_type_tag`

A 128-bit value stored as two unsigned 64-bit integers. It serves as a UUID with which JavaScript objects or [externals][] can be “tagged” in order to ensure that they are of a certain type. This is a stronger check than [napi_instanceof][], because the latter can report a false positive if the object's prototype has been manipulated. Type-tagging is most useful in conjunction with [napi_wrap][] because it ensures that the pointer retrieved from a wrapped object can be safely cast to the native type corresponding to the type tag that had been previously applied to the JavaScript object.

typedef struct {
  uint64_t lower;
  uint64_t upper;
} napi_type_tag;

`napi_async_cleanup_hook_handle`

An opaque value returned by [napi_add_async_cleanup_hook][]. It must be passed to [napi_remove_async_cleanup_hook][] when the chain of asynchronous cleanup events completes.

Node-API callback types

`napi_callback_info`

Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.

`napi_callback`

Function pointer type for user-provided native functions which are to be exposed to JavaScript via Node-API. Callback functions should satisfy the following signature:

typedef napi_value (*napi_callback)(napi_env, napi_callback_info);

Unless for reasons discussed in [Object Lifetime Management][], creating a handle and/or callback scope inside a napi_callback is not necessary.

`node_api_basic_finalize`

Stability: 1 - Experimental

Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object it was associated with has been garbage-collected. The user must provide a function satisfying the following signature which would get called upon the object's collection. Currently, node_api_basic_finalize can be used for finding out when objects that have external data are collected.

typedef void (*node_api_basic_finalize)(node_api_basic_env env,
                                      void* finalize_data,
                                      void* finalize_hint);

Unless for reasons discussed in [Object Lifetime Management][], creating a handle and/or callback scope inside the function body is not necessary.

Since these functions may be called while the JavaScript engine is in a state where it cannot execute JavaScript code, only Node-APIs which accept a node_api_basic_env as their first parameter may be called. [node_api_post_finalizer][] can be used to schedule Node-API calls that require access to the JavaScript engine's state to run after the current garbage collection cycle has completed.

In the case of [node_api_create_external_string_latin1][] and [node_api_create_external_string_utf16][] the env parameter may be null, because external strings can be collected during the latter part of environment shutdown.

Change History:

experimental (NAPI_EXPERIMENTAL):

Only Node-API calls that accept a node_api_basic_env as their first parameter may be called, otherwise the application will be terminated with an appropriate error message. This feature can be turned off by defining NODE_API_EXPERIMENTAL_BASIC_ENV_OPT_OUT.

`napi_finalize`

Function pointer type for add-on provided function that allow the user to schedule a group of calls to Node-APIs in response to a garbage collection event, after the garbage collection cycle has completed. These function pointers can be used with [node_api_post_finalizer][].

typedef void (*napi_finalize)(napi_env env,
                              void* finalize_data,
                              void* finalize_hint);

Change History:

experimental (NAPI_EXPERIMENTAL is defined):

A function of this type may no longer be used as a finalizer, except with [node_api_post_finalizer][]. [node_api_basic_finalize][] must be used instead. This feature can be turned off by defining NODE_API_EXPERIMENTAL_BASIC_ENV_OPT_OUT.

`napi_async_execute_callback`

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_execute_callback)(napi_env env, void* data);

Implementations of this function must avoid making Node-API calls that execute JavaScript or interact with JavaScript objects. Node-API calls should be in the napi_async_complete_callback instead. Do not use the napi_env parameter as it will likely result in execution of JavaScript.

`napi_async_complete_callback`

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

Unless for reasons discussed in [Object Lifetime Management][], creating a handle and/or callback scope inside the function body is not necessary.

`napi_threadsafe_function_call_js`

Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via napi_call_function, and then make the call into JavaScript.

The data arriving from the secondary thread via the queue is given in the data parameter and the JavaScript function to call is given in the js_callback parameter.

Node-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via napi_call_function rather than via napi_make_callback.

Callback functions must satisfy the following signature:

typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                 napi_value js_callback,
                                                 void* context,
                                                 void* data);

[in] env: The environment to use for API calls, or NULL if the thread-safe function is being torn down and data may need to be freed.
[in] js_callback: The JavaScript function to call, or NULL if the thread-safe function is being torn down and data may need to be freed. It may also be NULL if the thread-safe function was created without js_callback.
[in] context: The optional data with which the thread-safe function was created.
[in] data: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with Node-API functions) that can be passed as parameters when js_callback is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.

Unless for reasons discussed in [Object Lifetime Management][], creating a handle and/or callback scope inside the function body is not necessary.

`napi_cleanup_hook`

Function pointer used with [napi_add_env_cleanup_hook][]. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

typedef void (*napi_cleanup_hook)(void* data);

[in] data: The data that was passed to [napi_add_env_cleanup_hook][].

`napi_async_cleanup_hook`

Function pointer used with [napi_add_async_cleanup_hook][]. It will be called when the environment is being torn down.

Callback functions must satisfy the following signature:

typedef void (*napi_async_cleanup_hook)(napi_async_cleanup_hook_handle handle,
                                        void* data);

[in] handle: The handle that must be passed to [napi_remove_async_cleanup_hook][] after completion of the asynchronous cleanup.
[in] data: The data that was passed to [napi_add_async_cleanup_hook][].

The body of the function should initiate the asynchronous cleanup actions at the end of which handle must be passed in a call to [napi_remove_async_cleanup_hook][].

Error handling

Node-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.

Return values

All of the Node-API functions share the same error handling pattern. The return type of all API functions is napi_status.

The return value will be napi_ok if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the napi_status value for the error will be returned. If an exception was thrown, and no error occurred, napi_pending_exception will be returned.

In cases where a return value other than napi_ok or napi_pending_exception is returned, [napi_is_exception_pending][] must be called to check if an exception is pending. See the section on exceptions for more details.

The full set of possible napi_status values is defined in napi_api_types.h.

The napi_status return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information.

In order to retrieve this information [napi_get_last_error_info][] is provided which returns a napi_extended_error_info structure. The format of the napi_extended_error_info structure is as follows:

typedef struct napi_extended_error_info {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
};

error_message: Textual representation of the error that occurred.
engine_reserved: Opaque handle reserved for engine use only.
engine_error_code: VM specific error code.
error_code: Node-API status code for the last error.

[napi_get_last_error_info][] returns the information for the last Node-API call that was made.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

`napi_get_last_error_info`

napi_status
napi_get_last_error_info(node_api_basic_env env,
                         const napi_extended_error_info** result);

[in] env: The environment that the API is invoked under.
[out] result: The napi_extended_error_info structure with more information about the error.

Returns napi_ok if the API succeeded.

This API retrieves a napi_extended_error_info structure with information about the last error that occurred.

The content of the napi_extended_error_info returned is only valid up until a Node-API function is called on the same env. This includes a call to napi_is_exception_pending so it may often be necessary to make a copy of the information so that it can be used later. The pointer returned in error_message points to a statically-defined string so it is safe to use that pointer if you have copied it out of the error_message field (which will be overwritten) before another Node-API function was called.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

This API can be called even if there is a pending JavaScript exception.

Exceptions

Any Node-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.

If the napi_status returned by a function is napi_ok then no exception is pending and no additional action is required. If the napi_status returned is anything other than napi_ok or napi_pending_exception, in order to try to recover and continue instead of simply returning immediately, [napi_is_exception_pending][] must be called in order to determine if an exception is pending or not.

In many cases when a Node-API function is called and an exception is already pending, the function will return immediately with a napi_status of napi_pending_exception. However, this is not the case for all functions. Node-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, napi_status will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call.

When an exception is pending one of two approaches can be employed.

The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript, the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most Node-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception, so do as little as possible and then return to JavaScript where the exception can be handled.

The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases [napi_get_and_clear_last_exception][] can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript Object thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with [napi_throw][] where error is the JavaScript value to be thrown.

The following utility functions are also available in case native code needs to throw an exception or determine if a napi_value is an instance of a JavaScript Error object: [napi_throw_error][], [napi_throw_type_error][], [napi_throw_range_error][], [node_api_throw_syntax_error][] and [napi_is_error][].

The following utility functions are also available in case native code needs to create an Error object: [napi_create_error][], [napi_create_type_error][], [napi_create_range_error][] and [node_api_create_syntax_error][], where result is the napi_value that refers to the newly created JavaScript Error object.

The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with Node-API, both in internal functionality and for module specific functionality (as its good practice), the throw_ and create_ functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is NULL then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be:

originalName [code]

where originalName is the original name associated with the error and code is the code that was provided. For example, if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be:

TypeError [ERR_ERROR_1]

`napi_throw`

NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);

[in] env: The environment that the API is invoked under.
[in] error: The JavaScript value to be thrown.

Returns napi_ok if the API succeeded.

This API throws the JavaScript value provided.

`napi_throw_error`

NAPI_EXTERN napi_status napi_throw_error(napi_env env,
                                         const char* code,
                                         const char* msg);

[in] env: The environment that the API is invoked under.
[in] code: Optional error code to be set on the error.
[in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript Error with the text provided.

`napi_throw_type_error`

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