105 KiB
N-API
Stability: 1 - Experimental
N-API (pronounced N as in the letter, followed by API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (ex 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 version to run on later versions of Node.js without recompilation.
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 the N-API are used.
APIs exposed by N-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA262 Language Specification. The APIs have the following properties:
- All N-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.
The documentation for N-API is structured as follows:
- Basic N-API Data Types
- Error Handling
- Object Lifetime Management
- Module Registration
- Working with JavaScript Values
- Working with JavaScript Values - Abstract Operations
- Working with JavaScript Properties
- Working with JavaScript Functions
- Object Wrap
- Aynchronous Operations
The N-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. However, we also understand that a C++ API can be easier to use in many cases. To support these cases we expect there to be one or more C++ wrapper modules that provide an inlineable C++ API. Binaries built with these wrapper modules will depend on the symbols for the N-API C based functions exported by Node.js. These wrappers are not part of N-API, nor will they be maintained as part of Node.js. One such example is: node-api.
In order to use the N-API functions, include the file node_api.h which is located in the src directory in the node development tree. For example:
#include <node_api.h>
Basic N-API Data Types
N-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other N-API calls.
napi_status
Integral status code indicating the success or failure of a N-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_status_last
} 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 N-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 N-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 N-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 N-API calls. Caching the napi_env for the purpose of general reuse is
not allowed.
napi_value
This is an opaque pointer that is used to represent a JavaScript value.
N-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, N-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, N-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.
N-API Callback types
napi_callback_info
Opaque datatype that is passed to a callback function. It can be used for two purposes:
- Get additional information about the context in which the callback was invoked.
- Set the return value of the callback.
napi_callback
Function pointer type for user-provided native functions which are to be exposed to JavaScript via N-API. Callback functions should satisfy the following signature:
typedef void (*napi_callback)(napi_env, napi_callback_info);
napi_finalize
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 with which 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, napi_finalize can be used for
finding out when objects that have external data are collected.
napi_async_execute_callback
Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:
typedef void (*napi_async_execute_callback)(napi_env env, void* data);
napi_async_complete_callback
Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature:
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
Error Handling
N-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 N-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 occured.engine_reserved: Opaque handle reserved for engine use only.engine_error_code: VM specific error code.error_code: n-api status code for the last error.
napi_get_last_error_info returns the information for the last
N-API call that was made.
Note: 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_EXTERN napi_status
napi_get_last_error_info(napi_env env,
const napi_extended_error_info** result);
[in] env: The environment that the API is invoked under.[out] result: Thenapi_extended_error_infostructure 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 occured.
Note: 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.
Exceptions
Any N-API function call may result in a pending JavaScript exception. This is obviously the case for any function that may cause the execution of JavaScript, but N-API specifies that an exception may be pending on return from any of the API functions.
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.
When an exception is pending one of two approaches can be employed.
The first appoach 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 N-API calls
is unspecified while an exception is pending, and many will simply return
napi_pending_exception, so it is important to 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 Error object 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 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, and napi_create_range_error.
where result is the napi_value that refers to the newly created
JavaScript Error object.
napi_throw
NODE_EXTERN napi_status napi_throw(napi_env env, napi_value error);
[in] env: The environment that the API is invoked under.[in] error: Thenapi_valuefor the Error to be thrown.
Returns napi_ok if the API succeeded.
This API throws the JavaScript Error provided.
napi_throw_error
NODE_EXTERN napi_status napi_throw_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[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
NODE_EXTERN napi_status napi_throw_type_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[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 TypeError with the text provided.
napi_throw_range_error
NODE_EXTERN napi_status napi_throw_range_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[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 RangeError with the text provided.
napi_is_error
NODE_EXTERN napi_status napi_is_error(napi_env env,
napi_value value,
bool* result);
[in] env: The environment that the API is invoked under.[in] msg: Thenapi_valueto be checked.[out] result: Boolean value that is set to true ifnapi_valuerepresents an error, false otherwise.
Returns napi_ok if the API succeeded.
This API queries a napi_value to check if it represents an error object.
napi_create_error
NODE_EXTERN napi_status napi_create_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[in] msg: C string representing the text to be associated with.[out] result:napi_valuerepresenting the error created.
Returns napi_ok if the API succeeded.
This API returns a JavaScript Error with the text provided.
napi_create_type_error
NODE_EXTERN napi_status napi_create_type_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[in] msg: C string representing the text to be associated with.[out] result:napi_valuerepresenting the error created.
Returns napi_ok if the API succeeded.
This API returns a JavaScript TypeError with the text provided.
napi_create_range_error
NODE_EXTERN napi_status napi_create_range_error(napi_env env, const char* msg);
[in] env: The environment that the API is invoked under.[in] msg: C string representing the text to be associated with.[out] result:napi_valuerepresenting the error created.
Returns napi_ok if the API succeeded.
This API returns a JavaScript RangeError with the text provided.
napi_get_and_clear_last_exception
NAPI_EXTERN napi_status napi_get_and_clear_last_exception(napi_env env,
napi_value* result);
[in] env: The environment that the API is invoked under.[out] result: The exception if one is pending, NULL otherwise.
Returns napi_ok if the API succeeded.
This API returns true if an exception is pending.
napi_is_exception_pending
NAPI_EXTERN napi_status napi_is_exception_pending(napi_env env, bool* result);
[in] env: The environment that the API is invoked under.[out] result: Boolean value that is set to true if an exception is pending.
Returns napi_ok if the API succeeded.
This API returns true if an exception is pending.
Object Lifetime management
As N-API calls are made, handles to objects in the heap for the underlying
VM may be returned as napi_values. These handles must hold the
objects 'live' until they are no longer required by the native code,
otherwise the objects could be collected before the native code was
finished using them.
As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.
In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the N-API functions than can be used to change the handle lifespan from the default.
Making handle lifespan shorter than that of the native method
It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:
for (int i = 0; i < 1000000; i++) {
napi_value result;
napi_status status = napi_get_element(e object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
}
This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.
To handle this case, N-API provides the ability to establish a new 'scope' to
which newly created handles will be associated. Once those handles
are no longer required, the scope can be 'closed' and any handles associated
with the scope are invalidated. The methods available to open/close scopes are
napi_open_handle_scope and napi_close_handle_scope.
N-API only supports a single nested hiearchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.
Taking the earlier example, adding calls to napi_open_handle_scope and
napi_close_handle_scope would ensure that at most a single handle
is valid throughout the execution of the loop:
for (int i = 0; i < 1000000; i++) {
napi_handle_scope scope;
napi_status status = napi_open_handle_scope(env, &scope);
if (status != napi_ok) {
break;
}
napi_value result;
status = napi_get_element(e object, i, &result);
if (status != napi_ok) {
break;
}
// do something with element
status = napi_close_handle_scope(env, scope);
if (status != napi_ok) {
break;
}
}
When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. N-API supports an 'escapable scope' in order to support this case. An escapable scope allows one or more handles to be 'promoted' so that they 'escape' the current scope and the lifespan of the handle(s) changes from the current scope to that of the outer scope.
The methods available to open/close escapable scopes are
napi_open_escapable_handle_scope and napi_close_escapable_handle_scope.
The request to promote a handle is made through the napi_escape_handle.
napi_open_handle_scope
NODE_EXTERN napi_status napi_open_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env: The environment that the API is invoked under.[out] result:napi_valuerepresenting the new scope.
Returns napi_ok if the API succeeded.
This API open a new scope.
napi_close_handle_scope
NODE_EXTERN napi_status napi_close_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env: The environment that the API is invoked under.[in] scope:napi_valuerepresenting the scope to be closed.
Returns napi_ok if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
napi_open_escapable_handle_scope
NODE_EXTERN napi_status
napi_open_escapable_handle_scope(napi_env env,
napi_handle_scope* result);
[in] env: The environment that the API is invoked under.[out] result:napi_valuerepresenting the new scope.
Returns napi_ok if the API succeeded.
This API open a new scope from which objects can be promoted to the outer scope.
napi_close_escapable_handle_scope
NODE_EXTERN napi_status
napi_close_escapable_handle_scope(napi_env env,
napi_handle_scope scope);
[in] env: The environment that the API is invoked under.[in] scope:napi_valuerepresenting the scope to be closed.
Returns napi_ok if the API succeeded.
This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.
napi_escape_handle
NAPI_EXTERN napi_status napi_escape_handle(napi_env env,
napi_escapable_handle_scope scope,
napi_value escapee,
napi_value* result);
[in] env: The environment that the API is invoked under.[in] scope:napi_valuerepresenting the current scope.[in] escapee:napi_valuerepresenting the JavaScript Object to be escaped.[out] result:napi_valuerepresenting the handle to the escaped Object in the outer scope.
Returns napi_ok if the API succeeded.
This API promotes the handle to the JavaScript object so that it valid for the lifetime of the outer scope.
References to objects with a lifespan longer than that of the native method
In some cases an addon will need to be able to create and reference objects
with a lifespan longer than that of a single native method invocation. For
example, to create a constructor and later use that constructor
in a request to creates instances, it must be possible to reference
the constructor object across many different instance creation requests. This
would not be possible with a normal handle returned as a napi_value as
described in the earlier section. The lifespan of a normal handle is
managed by scopes and all scopes must be closed before the end of a native
method.
N-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.
References can be created with an initial reference count. The count can
then be modified through napi_reference_ref and
napi_reference_unref. If an object is collected while the count
for a reference is 0, all subsequent calls to
get the object associated with the reference napi_get_reference_value
will return NULL for the returned napi_value. An attempt to call
napi_reference_ref for a reference whose object has been collected
will result in an error.
References must be deleted once they are no longer required by the addon. When a reference is deleted it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference will result in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.
There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count.
napi_create_reference
NODE_EXTERN napi_status napi_create_reference(napi_env env,
napi_value value,
int initial_refcount,
ndapi_ref* result);
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting the Object to which we want a reference to.[in] initial_refcount: Initial reference count for the new reference.[out] result:napi_refpointing to the new reference.
Returns napi_ok if the API succeeded.
This API create a new reference with the specified reference count to the Object passed in.
napi_delete_reference
NODE_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
[in] env: The environment that the API is invoked under.[in] ref:napi_refto be deleted.
Returns napi_ok if the API succeeded.
This API deletes the reference passed in.
napi_reference_ref
NODE_EXTERN napi_status napi_reference_ref(napi_env env,
napi_ref ref,
int* result);
[in] env: The environment that the API is invoked under.[in] ref:napi_reffor which the reference count will be incremented.[out] result: The new reference count.
Returns napi_ok if the API succeeded.
This API increments the reference count for the reference passed in and returns the resulting reference count.
napi_reference_unref
NODE_EXTERN napi_status napi_reference_unref(napi_env env,
napi_ref ref,
int* result);
[in] env: The environment that the API is invoked under.[in] ref:napi_reffor which the reference count will be decremented.[out] result: The new reference count.
Returns napi_ok if the API succeeded.
This API decrements the reference count for the reference passed in and returns the resulting reference count.
napi_get_reference_value
NODE_EXTERN napi_status napi_get_reference_value(napi_env env,
napi_ref ref,
napi_value* result);
the napi_value passed in or out of these methods is a handle to the
object to which the reference is related.
[in] env: The environment that the API is invoked under.[in] ref:napi_reffor which we requesting the corresponding Object.[out] result: Thenapi_valuefor the Object referenced by thenapi_ref.
Returns napi_ok if the API succeeded.
If still valid, this API returns the napi_value representing the
JavaScript Object associated with the napi_ref. Otherise, result
will be NULL.
Module registration
N-API modules are registered in the same manner as other modules
except that instead of using the NODE_MODULE macro the following
is used:
NAPI_MODULE(addon, Init)
The next difference is the signature for the Init method. For a N-API
module it is as follows:
void Init(napi_env env, napi_value exports, napi_value module, void* priv);
As with any other module, functions are exported by either adding them to
the exports or module objects passed to the Init method.
For example, to add the method hello as a function so that it can be called
as a method provided by the addon:
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_property_descriptor desc =
{"hello", Method, 0, 0, 0, napi_default, 0};
status = napi_define_properties(env, exports, 1, &desc);
}
For example, to set a function to be returned by the require() for the addon:
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_property_descriptor desc =
{"exports", Method, 0, 0, 0, napi_default, 0};
status = napi_define_properties(env, module, 1, &desc);
}
For example, to define a class so that new instances can be created (often used with Object Wrap):
// NOTE: partial example, not all referenced code is included
napi_status status;
napi_property_descriptor properties[] = {
{ "value", nullptr, GetValue, SetValue, 0, napi_default, 0 },
DECLARE_NAPI_METHOD("plusOne", PlusOne),
DECLARE_NAPI_METHOD("multiply", Multiply),
};
napi_value cons;
status =
napi_define_class(env, "MyObject", New, nullptr, 3, properties, &cons);
if (status != napi_ok) return;
status = napi_create_reference(env, cons, 1, &constructor);
if (status != napi_ok) return;
status = napi_set_named_property(env, exports, "MyObject", cons);
if (status != napi_ok) return;
For more details on setting properties on either the exports or module
objects, see the section on
Working with JavaScript Properties.
For more details on building addon modules in general, refer to the existing API
Working with JavaScript Values
N-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.
Fundamentally, these APIs are used to do one of the following:
- Create a new JavaScript object
- Convert from a primitive C type to an N-API value
- Convert from N-API value to a primitive C type
- Get global instances including
undefinedandnull
N-API values are represented by the type napi_value.
Any N-API call that requires a JavaScript value takes in a napi_value.
In some cases, the API does check the type of the napi_value up-front.
However, for better performance, it's better for the caller to make sure that
the napi_value in question is of the JavaScript type expected by the API.
Enum types
napi_valuetype
typedef enum {
// ES6 types (corresponds to typeof)
napi_undefined,
napi_null,
napi_boolean,
napi_number,
napi_string,
napi_symbol,
napi_object,
napi_function,
napi_external,
} napi_valuetype;
Describes the type of a napi_value. This generally corresponds to the types
described in
Section 6.1 of
the ECMAScript Language Specification.
In addition to types in that section, napi_valuetype can also represent
Functions and Objects with external data.
napi_typedarray_type
typedef enum {
napi_int8_array,
napi_uint8_array,
napi_uint8_clamped_array,
napi_int16_array,
napi_uint16_array,
napi_int32_array,
napi_uint32_array,
napi_float32_array,
napi_float64_array,
} napi_typedarray_type;
This represents the underlying binary scalar datatype of the TypedArray. Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification.
Object Creation Functions
napi_create_array
napi_status napi_create_array(napi_env env, napi_value* result)
[in] env: The environment that the N-API call is invoked under.[out] result: Anapi_valuerepresenting a JavaScript Array.
Returns napi_ok if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.
napi_create_array_with_length
napi_status napi_create_array_with_length(napi_env env,
size_t length,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] length: The initial length of the Array.[out] result: Anapi_valuerepresenting a JavaScript Array.
Returns napi_ok if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Array type.
The Array's length property is set to the passed-in length parameter.
However, the underlying buffer is not guaranteed to be pre-allocated by the VM
when the array is created - that behavior is left to the underlying VM
implementation.
If the buffer must be a contiguous block of memory that can be
directly read and/or written via C, consider using
napi_create_external_arraybuffer.
JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.
napi_create_arraybuffer
napi_status napi_create_arraybuffer(napi_env env,
size_t byte_length,
void** data,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] length: The length in bytes of the array buffer to create.[out] data: Pointer to the underlying byte buffer of the ArrayBuffer.[out] result: Anapi_valuerepresenting a JavaScript ArrayBuffer.
Returns napi_ok if the API succeeded.
This API returns an N-API value corresponding to a JavaScript ArrayBuffer.
ArrayBuffers are used to represent fixed-length binary data buffers. They are
normally used as a backing-buffer for TypedArray objects.
The ArrayBuffer allocated will have an underlying byte buffer whose size is
determined by the length parameter that's passed in.
The underlying buffer is optionally returned back to the caller in case the
caller wants to directly manipulate the buffer. This buffer can only be
written to directly from native code. To write to this buffer from JavaScript,
a typed array or DataView object would need to be created.
JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification.
napi_create_buffer
napi_status napi_create_buffer(napi_env env,
size_t size,
void** data,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] size: Size in bytes of the underlying buffer.[out] data: Raw pointer to the underlying buffer.[out] result: Anapi_valuerepresenting anode::Buffer.
Returns napi_ok if the API succeeded.
This API allocates a node::Buffer object. While this is still a
fully-supported data structure, in most cases using a TypedArray will suffice.
napi_create_buffer_copy
napi_status napi_create_buffer_copy(napi_env env,
size_t length,
const void* data,
void** result_data,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] size: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data: Raw pointer to the underlying buffer to copy from.[out] result_data: Pointer to the new Buffer's underlying data buffer.[out] result: Anapi_valuerepresenting anode::Buffer.
Returns napi_ok if the API succeeded.
This API allocates a node::Buffer object and initializes it with data copied
from the passed-in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray will suffice.
napi_create_external
napi_status napi_create_external(napi_env env,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] data: Raw pointer to the external data.[in] finalize_cb: Optional callback to call when the external value is being collected.[in] finalize_hint: Optional hint to pass to the finalize callback during collection.[out] result: Anapi_valuerepresenting an external value.
Returns napi_ok if the API succeeded.
This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code. The API allows the caller to pass in a finalize callback, in case the underlying native resource needs to be cleaned up when the external JavaScript value gets collected.
Note: The created value is not an object, and therefore does not support
additional properties. It is considered a distinct value type: calling
napi_typeof() with an external value yields napi_external.
napi_create_external_arraybuffer
napi_status
napi_create_external_arraybuffer(napi_env env,
void* external_data,
size_t byte_length,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] external_data: Pointer to the underlying byte buffer of the ArrayBuffer.[in] byte_length: The length in bytes of the underlying buffer.[in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected.[in] finalize_hint: Optional hint to pass to the finalize callback during collection.[out] result: Anapi_valuerepresenting a JavaScript ArrayBuffer.
Returns napi_ok if the API succeeded.
This API returns an N-API value corresponding to a JavaScript ArrayBuffer. The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.
JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.
napi_create_external_buffer
napi_status napi_create_external_buffer(napi_env env,
size_t length,
void* data,
napi_finalize finalize_cb,
void* finalize_hint,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] length: Size in bytes of the input buffer (should be the same as the size of the new buffer).[in] data: Raw pointer to the underlying buffer to copy from.[in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected.[in] finalize_hint: Optional hint to pass to the finalize callback during collection.[out] result: Anapi_valuerepresenting anode::Buffer.
Returns napi_ok if the API succeeded.
This API allocates a node::Buffer object and initializes it with data
backed by the passed in buffer. While this is still a fully-supported data
structure, in most cases using a TypedArray will suffice.
Note: For Node.js >=4 Buffers are Uint8Arrays.
napi_create_function
napi_status napi_create_function(napi_env env,
const char* utf8name,
napi_callback cb,
void* data,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] utf8name: A string representing the name of the function encoded as UTF8.[in] cb: A function pointer to the native function to be invoked when the created function is invoked from JavaScript.[in] data: Optional arbitrary context data to be passed into the native function when it is invoked.[out] result: Anapi_valuerepresenting a JavaScript function.
Returns napi_ok if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Function object. It's used to wrap native functions so that they can be invoked from JavaScript.
JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.
napi_create_object
napi_status napi_create_object(napi_env env, napi_value* result)
[in] env: The environment that the API is invoked under.[out] result: Anapi_valuerepresenting a JavaScript Object.
Returns napi_ok if the API succeeded.
This API allocates a default JavaScript Object.
It is the equivalent of doing new Object() in JavaScript.
The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification.
napi_create_symbol
napi_status napi_create_symbol(napi_env env,
const char* description,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] description: Null-terminated character buffer representing a UTF8-encoded string to describe the symbol.[out] result: Anapi_valuerepresenting a JavaScript Symbol.
Returns napi_ok if the API succeeded.
This API creates a JavaScript Symbol object from a UTF8-encoded C string
The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification.
napi_create_typedarray
napi_status napi_create_typedarray(napi_env env,
napi_typedarray_type type,
size_t length,
napi_value arraybuffer,
size_t byte_offset,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] type: Scalar datatype of the elements within the TypedArray.[in] length: Number of elements in the TypedArray.[in] arraybuffer: ArrayBuffer underlying the typed array.[in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the TypedArray.[out] result: Anapi_valuerepresenting a JavaScript TypedArray.
Returns napi_ok if the API succeeded.
This API creates a JavaScript TypedArray object over an existing ArrayBuffer. TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.
It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised.
JavaScript TypedArray Objects are described in Section 22.2 of the ECMAScript Language Specification.
Functions to convert from C types to N-API
napi_create_number
napi_status napi_create_number(napi_env env, double value, napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: Double-precision value to be represented in JavaScript.[out] result: Anapi_valuerepresenting a JavaScript Number.
Returns napi_ok if the API succeeded.
This API is used to convert from the C double type to the JavaScript Number type.
The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.
napi_create_string_utf16
napi_status napi_create_string_utf16(napi_env env,
const char16_t* str,
size_t length,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] str: Character buffer representing a UTF16-LE-encoded string.[in] length: The length of the string in characters, or -1 if it is null-terminated.[out] result: Anapi_valuerepresenting a JavaScript String.
Returns napi_ok if the API succeeded.
This API creates a JavaScript String object from a UTF16-LE-encoded C string
The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.
napi_create_string_utf8
napi_status napi_create_string_utf8(napi_env env,
const char* str,
size_t length,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] s: Character buffer representing a UTF8-encoded string.[in] length: The length of the string in characters, or -1 if it is null-terminated.[out] result: Anapi_valuerepresenting a JavaScript String.
Returns napi_ok if the API succeeded.
This API creates a JavaScript String object from a UTF8-encoded C string
The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.
Functions to convert from N-API to C types
napi_get_array_length
napi_status napi_get_array_length(napi_env env,
napi_value value,
uint32_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting the JavaScript Array whose length is being queried.[out] result:uint32representing length of the array.
Returns napi_ok if the API succeeded.
This API returns the length of an array.
Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification.
napi_get_arraybuffer_info
napi_status napi_get_arraybuffer_info(napi_env env,
napi_value arraybuffer,
void** data,
size_t* byte_length)
[in] env: The environment that the API is invoked under.[in] arraybuffer:napi_valuerepresenting the ArrayBuffer being queried.[out] data: The underlying data buffer of the ArrayBuffer.[out] byte_length: Length in bytes of the underlying data buffer.
Returns napi_ok if the API succeeded.
This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length.
WARNING: Use caution while using this API. The lifetime of the underlying data
buffer is managed by the ArrayBuffer even after it's returned. A
possible safe way to use this API is in conjunction with napi_create_reference,
which can be used to guarantee control over the lifetime of the
ArrayBuffer. It's also safe to use the returned data buffer within the same
callback as long as there are no calls to other APIs that might trigger a GC.
napi_get_buffer_info
napi_status napi_get_buffer_info(napi_env env,
napi_value value,
void** data,
size_t* length)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting thenode::Bufferbeing queried.[out] data: The underlying data buffer of thenode::Buffer.[out] length: Length in bytes of the underlying data buffer.
Returns napi_ok if the API succeeded.
This API is used to retrieve the underlying data buffer of a node::Buffer
and it's length.
Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.
napi_get_prototype
napi_status napi_get_prototype(napi_env env,
napi_value object,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] object:napi_valuerepresenting JavaScript Object whose prototype to return. This returns the equivalent ofObject.getPrototypeOf(which is not the same as the function'sprototypeproperty).[out] result:napi_valuerepresenting prototype of the given object.
Returns napi_ok if the API succeeded.
napi_get_typedarray_info
napi_status napi_get_typedarray_info(napi_env env,
napi_value typedarray,
napi_typedarray_type* type,
size_t* length,
void** data,
napi_value* arraybuffer,
size_t* byte_offset)
[in] env: The environment that the API is invoked under.[in] typedarray:napi_valuerepresenting the TypedArray whose properties to query.[out] type: Scalar datatype of the elements within the TypedArray.[out] length: Number of elements in the TypedArray.[out] data: The data buffer underlying the typed array.[out] byte_offset: The byte offset within the data buffer from which to start projecting the TypedArray.
Returns napi_ok if the API succeeded.
This API returns various properties of a typed array.
Warning: Use caution while using this API since the underlying data buffer is managed by the VM
napi_get_value_bool
napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript Boolean.[out] result: C boolean primitive equivalent of the given JavaScript Boolean.
Returns napi_ok if the API succeeded. If a non-boolean napi_value is
passed in it returns napi_boolean_expected.
This API returns the C boolean primitive equivalent of the given JavaScript Boolean.
napi_get_value_double
napi_status napi_get_value_double(napi_env env,
napi_value value,
double* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript Number.[out] result: C double primitive equivalent of the given JavaScript Number.
Returns napi_ok if the API succeeded. If a non-number napi_value is passed
in it returns napi_number_expected.
This API returns the C double primitive equivalent of the given JavaScript Number.
napi_get_value_external
napi_status napi_get_value_external(napi_env env,
napi_value value,
void** result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript external value.[out] result: Pointer to the data wrapped by the JavaScript external value.
Returns napi_ok if the API succeeded. If a non-external napi_value is
passed in it returns napi_invalid_arg.
This API retrieves the external data pointer that was previously passed to
napi_create_external().
napi_get_value_int32
napi_status napi_get_value_int32(napi_env env,
napi_value value,
int32_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript Number.[out] result: C int32 primitive equivalent of the given JavaScript Number.
Returns napi_ok if the API succeeded. If a non-number napi_value
is passed in `napi_number_expected .
This API returns the C int32 primitive equivalent of the given JavaScript Number. If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the the value is > 2^31 -1.
napi_get_value_int64
napi_status napi_get_value_int64(napi_env env,
napi_value value,
int64_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript Number.[out] result: C int64 primitive equivalent of the given JavaScript Number.
Returns napi_ok if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected.
This API returns the C int64 primitive equivalent of the given JavaScript Number
napi_get_value_string_length
napi_status napi_get_value_string_length(napi_env env,
napi_value value,
int* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript string.[out] result: Number of characters in the given JavaScript string.
Returns napi_ok if the API succeeded. If a non-String napi_value
is passed in it returns napi_string_expected.
This API returns the number of characters in the given JavaScript string.
napi_get_value_string_utf8
napi_status napi_get_value_string_utf8(napi_env env,
napi_value value,
char* buf,
size_t bufsize,
size_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript string.[in] buf: Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned.[in] bufsize: Size of the destination buffer.[out] result: Number of bytes copied into the buffer including the null. terminator. If the buffer size is insufficient, the string will be truncated including a null terminator.
Returns napi_ok if the API succeeded. Ifa non-String napi_value
x is passed in it returns napi_string_expected.
This API returns the UTF8-encoded string corresponding the value passed in.
napi_get_value_string_utf16
napi_status napi_get_value_string_utf16(napi_env env,
napi_value value,
char16_t* buf,
size_t bufsize,
size_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript string.[in] buf: Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string (in 2-byte code units) is returned.[in] bufsize: Size of the destination buffer.[out] result: Number of 2-byte code units copied into the buffer including the null terminateor. If the buffer size is insufficient, the string will be truncated including a null terminator.
Returns napi_ok if the API succeeded. If a non-String napi_value
is passed in it returns napi_string_expected.
This API returns the UTF16-encoded string corresponding the value passed in.
napi_get_value_uint32
napi_status napi_get_value_uint32(napi_env env,
napi_value value,
uint32_t* result)
[in] env: The environment that the API is invoked under.[in] value:napi_valuerepresenting JavaScript Number.[out] result: C primitive equivalent of the givennapi_valueas auint32_t.
Returns napi_ok if the API succeeded. If a non-number napi_value
is passed in it returns napi_number_expected.
This API returns the C primitive equivalent of the given napi_value as a
uint32_t.
Functions to get global instances
napi_get_boolean
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: The value of the boolean to retrieve.[out] result:napi_valuerepresenting JavaScript Boolean singleton to retrieve.
Returns napi_ok if the API succeeded.
This API is used to return the JavaScript singleton object that is used to represent the given boolean value
napi_get_global
napi_status napi_get_global(napi_env env, napi_value* result)
[in] env: The environment that the API is invoked under.[out] result:napi_valuerepresenting JavaScript Global Object.
Returns napi_ok if the API succeeded.
This API returns the global Object.
napi_get_null
napi_status napi_get_null(napi_env env, napi_value* result)
[in] env: The environment that the API is invoked under.[out] result:napi_valuerepresenting JavaScript Null Object.
Returns napi_ok if the API succeeded.
This API returns the null Object.
napi_get_undefined
napi_status napi_get_undefined(napi_env env, napi_value* result)
[in] env: The environment that the API is invoked under.[out] result:napi_valuerepresenting JavaScript Undefined value.
Returns napi_ok if the API succeeded.
This API returns the Undefined object.
Working with JavaScript Values - Abstract Operations
N-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.
These APIs support doing one of the following:
- Coerce JavaScript values to specific JavaScript types (such as Number or String)
- Check the type of a JavaScript value
- Check for equality between two JavaScript values
napi_coerce_to_bool
napi_status napi_coerce_to_bool(napi_env env,
napi_value value,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to coerce.[out] result:napi_valuerepresenting the coerced JavaScript Boolean.
Returns napi_ok if the API succeeded.
This API implements the abstract operation ToBoolean as defined in Section 7.1.2 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_number
napi_status napi_coerce_to_number(napi_env env,
napi_value value,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to coerce.[out] result:napi_valuerepresenting the coerced JavaScript Number.
Returns napi_ok if the API succeeded.
This API implements the abstract operation ToNumber as defined in Section 7.1.3 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_object
napi_status napi_coerce_to_object(napi_env env,
napi_value value,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to coerce.[out] result:napi_valuerepresenting the coerced JavaScript Object.
Returns napi_ok if the API succeeded.
This API implements the abstract operation ToObject as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_coerce_to_string
napi_status napi_coerce_to_string(napi_env env,
napi_value value,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to coerce.[out] result:napi_valuerepresenting the coerced JavaScript String.
Returns napi_ok if the API succeeded.
This API implements the abstract operation ToString as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.
napi_typeof
napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value whose type to query.[out] result: The type of the JavaScript value.
Returns napi_ok if the API succeeded.
napi_invalid_argif the type ofvalueis not a known ECMAScript type andvalueis not an External value.
This API represents behavior similar to invoking the typeof Operator on
the object as defined in Section 12.5.5 of the ECMAScript Language
Specification. However, it has support for detecting an External value.
If value has a type that is invalid, an error is returned.
napi_instanceof
napi_status napi_instanceof(napi_env env,
napi_value object,
napi_value constructor,
bool* result)
[in] env: The environment that the API is invoked under.[in] object: The JavaScript value to check.[in] constructor: The JavaScript function object of the constructor function to check against.[out] result: Boolean that is set to true ifobject instanceof constructoris true.
Returns napi_ok if the API succeeded.
This API represents invoking the instanceof Operator on the object as
defined in
Section 12.10.4
of the ECMAScript Language Specification.
napi_is_array
napi_status napi_is_array(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to check.[out] result: Whether the given object is an array.
Returns napi_ok if the API succeeded.
This API represents invoking the IsArray operation on the object
as defined in Section 7.2.2
of the ECMAScript Language Specification.
napi_is_arraybuffer
napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to check.[out] result: Whether the given object is an ArrayBuffer.
Returns napi_ok if the API succeeded.
This API checks if the Object passsed in is an array buffer.
napi_is_buffer
napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to check.[out] result: Whether the givennapi_valuerepresents anode::Bufferobject.
Returns napi_ok if the API succeeded.
This API checks if the Object passsed in is a buffer.
napi_is_error
napi_status napi_is_error(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to check.[out] result: Whether the givennapi_valuerepresents an Error object.
Returns napi_ok if the API succeeded.
This API checks if the Object passsed in is an Error.
napi_is_typedarray
napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
[in] env: The environment that the API is invoked under.[in] value: The JavaScript value to check.[out] result: Whether the givennapi_valuerepresents a TypedArray.
Returns napi_ok if the API succeeded.
This API checks if the Object passsed in is a typed array.
napi_strict_equals
napi_status napi_strict_equals(napi_env env,
napi_value lhs,
napi_value rhs,
bool* result)
[in] env: The environment that the API is invoked under.[in] lhs: The JavaScript value to check.[in] rhs: The JavaScript value to check against.[out] result: Whether the twonapi_valueobjects are equal.
Returns napi_ok if the API succeeded.
This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.
Working with JavaScript Properties
N-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.
Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in N-API can be represented in one of the following forms:
- Named: a simple UTF8-encoded string
- Integer-Indexed: an index value represented by
uint32_t - JavaScript value: these are represented in N-API by
napi_value. This can be anapi_valuerepresenting a String, Number or Symbol.
N-API values are represented by the type napi_value.
Any N-API call that requires a JavaScript value takes in a napi_value.
However, it's the caller's responsibility to make sure that the
napi_value in question is of the JavaScript type expected by the API.
The APIs documented in this section provide a simple interface to
get and set properties on arbitrary JavaScript objects represented by
napi_value.
For instance, consider the following JavaScript code snippet:
const obj = {};
obj.myProp = 123;
The equivalent can be done using N-API values with the following snippet:
napi_status status = napi_generic_failure;
// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;
// Create a napi_value for 123
status = napi_create_number(env, 123, &value);
if (status != napi_ok) return status;
// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;
Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:
const arr = [];
arr[123] = 'hello';
The equivalent can be done using N-API values with the following snippet:
napi_status status = napi_generic_failure;
// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", -1, &value);
if (status != napi_ok) return status;
// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;
Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:
const arr = [];
const value = arr[123];
The following is the approximate equivalent of the N-API counterpart:
napi_status status = napi_generic_failure;
// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;
// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;
Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:
const obj = {};
Object.defineProperties(obj, {
'foo': { value: 123, writable: true, configurable: true, enumerable: true },
'bar': { value: 456, writable: true, configurable: true, enumerable: true }
});
The following is the approximate equivalent of the N-API counterpart:
napi_status status = napi_status_generic_failure;
// const obj = {};
napi_value obj;
status = napi_create_obj(env, &obj);
if (status != napi_ok) return status;
// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_number(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_number(env, 456, &barValue);
if (status != napi_ok) return status;
// Set the properties
napi_property_descriptors descriptors[] = {
{ "foo", fooValue, 0, 0, 0, napi_default, 0 },
{ "bar", barValue, 0, 0, 0, napi_default, 0 }
}
status = napi_define_properties(env,
obj,
sizeof(descriptors) / sizeof(descriptors[0]),
descriptors);
if (status != napi_ok) return status;
Structures
napi_property_attributes
typedef enum {
napi_default = 0,
napi_read_only = 1 << 0,
napi_dont_enum = 1 << 1,
napi_dont_delete = 1 << 2,
napi_static_property = 1 << 10,
} napi_property_attributes;
napi_property_attributes are flags used to control the behavior of properties
set on a JavaScript object. They roughly correspond to the attributes listed in
Section 6.1.7.1 of the
ECMAScript Language Specification. They can
be one or more of the following bitflags:
napi_default- Used to indicate that no explicit attributes are set on the given property. By default, a property is Writable, Enumerable, and Configurable. This is a deviation from the ECMAScript specification, where generally the values for a property descriptor attribute default to false if they're not provided.napi_read_only- Used to indicate that a given property is not Writable.napi_dont_enum- Used to indicate that a given property is not Enumerable.napi_dont_delete- Used to indicate that a given property is not. Configurable, as defined in Section 6.1.7.1 of the ECMAScript Language Specification.napi_static_property- Used to indicate that the property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only bynapi_define_class. It is ignored bynapi_define_properties.
napi_property_descriptor
typedef struct {
const char* utf8name;
napi_callback method;
napi_callback getter;
napi_callback setter;
napi_value value;
napi_property_attributes attributes;
void* data;
} napi_property_descriptor;
utf8name: String describing the key for the property, encoded as UTF8.value: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, setgetter,setter,methodanddatatoNULL(since these members won't be used).getter: A function to call when a get access of the property is performed. If this is passed in, setvalueandmethodtoNULL(since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a N-API call).setter: A function to call when a set access of the property is performed. If this is passed in, setvalueandmethodtoNULL(since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a N-API call).method: Set this to make the property descriptor object'svalueproperty to be a JavaScript function represented bymethod. If this is passed in, setvalue,getterandsettertoNULL(since these members won't be used).data: The callback data passed intomethod,getterandsetterif this function is invoked.attributes: The attributes associated with the particular property. Seenapi_property_attributes.
Functions
napi_get_property_names
napi_status napi_get_property_names(napi_env env,
napi_value object,
napi_value* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to retrieve the properties.[out] result: Anapi_valuerepresenting an array of JavaScript values that represent the property names of the object. The API can be used to iterate overresultusingnapi_get_array_lengthandnapi_get_element.
Returns napi_ok if the API succeeded.
This API returns the array of propertys for the Object passed in
napi_set_property
napi_status napi_set_property(napi_env env,
napi_value object,
napi_value key,
napi_value value);
[in] env: The environment that the N-API call is invoked under.[in] object: The object on which to set the property.[in] key: The name of the property to set.[in] value: The property value.
Returns napi_ok if the API succeeded.
This API set a property on the Object passed in.
napi_get_property
napi_status napi_get_property(napi_env env,
napi_value object,
napi_value key,
napi_value* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to retrieve the property.[in] key: The name of the property to retrieve.[out] result: The value of the property.
Returns napi_ok if the API succeeded.
This API gets the requested property from the Object passed in.
napi_has_property
napi_status napi_has_property(napi_env env,
napi_value object,
napi_value key,
bool* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object to query.[in] key: The name of the property whose existence to check.[out] result: Whether the property exists on the object or not.
Returns napi_ok if the API succeeded.
This API checks if the Object passed in has the named property.
napi_set_named_property
napi_status napi_set_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value value);
[in] env: The environment that the N-API call is invoked under.[in] object: The object on which to set the property.[in] utf8Name: The name of the property to set.[in] value: The property value.
Returns napi_ok if the API succeeded.
This method is equivalent to calling napi_set_property with a napi_value
created from the string passed in as utf8Name
napi_get_named_property
napi_status napi_get_named_property(napi_env env,
napi_value object,
const char* utf8Name,
napi_value* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to retrieve the property.[in] utf8Name: The name of the property to get.[out] result: The value of the property.
Returns napi_ok if the API succeeded.
This method is equivalent to calling napi_get_property with a napi_value
created from the string passed in as utf8Name
napi_has_named_property
napi_status napi_has_named_property(napi_env env,
napi_value object,
const char* utf8Name,
bool* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object to query.[in] utf8Name: The name of the property whose existence to check.[out] result: Whether the property exists on the object or not.
Returns napi_ok if the API succeeded.
This method is equivalent to calling napi_has_property with a napi_value
created from the string passed in as utf8Name
napi_set_element
napi_status napi_set_element(napi_env env,
napi_value object,
uint32_t index,
napi_value value);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to set the properties.[in] index: The index of the property to set.[in] value: The property value.
Returns napi_ok if the API succeeded.
This API sets and element on the Object passed in.
napi_get_element
napi_status napi_get_element(napi_env env,
napi_value object,
uint32_t index,
napi_value* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to retrieve the property.[in] index: The index of the property to get.[out] result: The value of the property.
Returns napi_ok if the API succeeded.
This API gets the element at the requested index.
napi_has_element
napi_status napi_has_element(napi_env env,
napi_value object,
uint32_t index,
bool* result);
[in] env: The environment that the N-API call is invoked under.[in] object: The object to query.[in] index: The index of the property whose existence to check.[out] result: Whether the property exists on the object or not.
Returns napi_ok if the API succeeded.
This API returns if the Object passed in has an element at the requested index.
napi_define_properties
napi_status napi_define_properties(napi_env env,
napi_value object,
size_t property_count,
const napi_property_descriptor* properties);
[in] env: The environment that the N-API call is invoked under.[in] object: The object from which to retrieve the properties.[in] property_count: The number of elements in thepropertiesarray.[in] properties: The array of property descriptors.
Returns napi_ok if the API succeeded.
This method allows the efficient definition of multiple properties on a given
object. The properties are defined using property descriptors (See
napi_property_descriptor). Given an array of such property descriptors, this
API will set the properties on the object one at a time, as defined by
DefineOwnProperty (described in Section 9.1.6 of the ECMA262 specification).
Working with JavaScript Functions
N-API provides a set of APIs that allow JavaScript code to
call back into native code. N-API APIs that support calling back
into native code take in a callback functions represented by
the napi_callback type. When the JavaScript VM calls back to
native code, the napi_callback function provided is invoked. The APIs
documented in this section allow the callback function to do the
following:
- Get information about the context in which the callback was invoked.
- Get the arguments passed into the callback.
- Return a
napi_valueback from the callback.
Additionally, N-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.
napi_call_function
napi_status napi_call_function(napi_env env,
napi_value recv,
napi_value func,
int argc,
const napi_value* argv,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] recv: Thethisobject passed to the called function.[in] func:napi_valuerepresenting the JavaScript function to be invoked.[in] argc: The count of elements in theargvarray.[in] argv: Array ofnapi_valuesrepresenting JavaScript values passed in as arguments to the function.[out] result:napi_valuerepresenting the JavaScript object returned.
Returns napi_ok if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This is an primary mechanism of calling back from the add-on's
native code into JavaScript. For special cases like calling into JavaScript
after an async operation, see napi_make_callback.
A sample use case might look as follows. Consider the following JavaScript snippet:
function AddTwo(num) {
return num + 2;
}
Then, the above function can be invoked from a native add-on using the following code:
// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;
// const arg = 1337
status = napi_create_number(env, 1337, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;
// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;
napi_create_function
napi_status napi_create_function(napi_env env,
const char* utf8name,
napi_callback cb,
void* data,
napi_value* result);
[in] env: The environment that the API is invoked under.[in] utf8Name: The name of the function encoded as UTF8. This is visible within JavaScript as the new function object'snameproperty.[in] cb: The native function which should be called when this function object is invoked.[in] data: User-provided data context. This will be passed back into the function when invoked later.[out] result:napi_valuerepresenting the JavaScript function object for the newly created function.
Returns napi_ok if the API succeeded.
This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from Javascript.
Note: The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.
In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:
void SayHello(napi_env env, napi_callback_info info) {
printf("Hello\n");
}
void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
napi_status status;
napi_value fn;
status = napi_create_function(env, NULL, SayHello, NULL, &fn);
if (status != napi_ok) return;
status = napi_set_named_property(env, exports, "sayHello", fn);
if (status != napi_ok) return;
}
NAPI_MODULE(addon, Init)
Given the above code, the add-on can be used from JavaScript as follows:
const myaddon = require('./addon');
myaddon.sayHello();
Note: The string passed to require is not necessarily the name passed into
NAPI_MODULE in the earlier snippet but the name of the target in binding.gyp
responsible for creating the .node file.
napi_get_cb_info
napi_status napi_get_cb_info(napi_env env,
napi_callback_info cbinfo,
size_t* argc,
napi_value* argv,
napi_value* thisArg,
void** data)
[in] env: The environment that the API is invoked under.[in] cbinfo: The callback info passed into the callback function.[in-out] argc: Specifies the size of the providedargvarray and receives the actual count of arguments.[out] argv: Buffer to which thenapi_valuerepresenting the arguments are copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest ofargvis filled withnapi_valuevalues that representundefined.[out] this: Receives the JavaScriptthisargument for the call.[out] data: Receives the data pointer for the callback.
Returns napi_ok if the API succeeded.
This method is used within a callback function to retrieve details about the
call like the arguments and the this pointer from a given callback info.
napi_is_construct_call
napi_status napi_is_construct_call(napi_env env,
napi_callback_info cbinfo,
bool* result)
[in] env: The environment that the API is invoked under.[in] cbinfo: The callback info passed into the callback function.[out] result: Whether the native function is being invoked as a constructor call.
Returns napi_ok if the API succeeded.
This API checks if the the current callback was due to a consructor call.
napi_new_instance
napi_status napi_new_instance(napi_env env,
napi_value cons,
size_t argc,
napi_value* argv,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] cons:napi_valuerepresenting the JavaScript function to be invoked as a constructor.[in] argc: The count of elements in theargvarray.[in] argv: Array of JavaScript values asnapi_valuerepresenting the arguments to the constructor.[out] result:napi_valuerepresenting the JavaScript object returned, which in this case is the constructed object.
This method is used to instantiate a new JavaScript value using a given
napi_value that represents the constructor for the object. For example,
consider the following snippet:
function MyObject(param) {
this.param = param;
}
const arg = 'hello';
const value = new MyObject(arg);
The following can be approximated in N-API using the following snippet:
// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;
status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;
// const arg = "hello"
status = napi_create_string_utf8(env, "hello", -1, &arg);
if (status != napi_ok) return;
napi_value* argv = &arg;
size_t argc = 1;
// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);
Returns napi_ok if the API succeeded.
napi_make_callback
napi_status napi_make_callback(napi_env env,
napi_value recv,
napi_value func,
int argc,
const napi_value* argv,
napi_value* result)
[in] env: The environment that the API is invoked under.[in] recv: Thethisobject passed to the called function.[in] func:napi_valuerepresenting the JavaScript function to be invoked.[in] argc: The count of elements in theargvarray.[in] argv: Array of JavaScript values asnapi_valuerepresenting the arguments to the function.[out] result:napi_valuerepresenting the JavaScript object returned.
Returns napi_ok if the API succeeded.
This method allows a JavaScript function object to be called from a native
add-on. This API is similar to napi_call_function. However, it is used to call
from native code back into JavaScript after returning from an async
operation (when there is no other script on the stack). It is a fairly simple
wrapper around node::MakeCallback.
For an example on how to use napi_make_callback, see the section on
[Asynchronous Operations][].
Object Wrap
N-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.
- The
napi_define_classAPI defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the The C++ class. - When JavaScript code invokes the constructor, the constructor callback
uses
napi_wrapto wrap a new C++ instance in a JavaScript object, then returns the wrapper object. - When JavaScript code invokes a method or property accessor on the class,
the corresponding
napi_callbackC++ function is invoked. For an instance callback,napi_unwrapobtains the C++ instance that is the target of the call.
napi_define_class
napi_status napi_define_class(napi_env env,
const char* utf8name,
napi_callback constructor,
void* data,
size_t property_count,
const napi_property_descriptor* properties,
napi_value* result);
[in] env: The environment that the API is invoked under.[in] utf8name: Name of the JavaScript constructor function; this is not required to be the same as the C++ class name, though it is recommended for clarity.[in] constructor: Callback function that handles constructing instances of the class. (This should be a static method on the class, not an actual C++ constructor function.)[in] data: Optional data to be passed to the constructor callback as thedataproperty of the callback info.[in] property_count: Number of items in thepropertiesarray argument.[in] properties: Array of property descriptors describing static and instance data properties, accessors, and methods on the class Seenapi_property_descriptor.[out] result: Anapi_valuerepresenting the constructor function for the class.
Returns napi_ok if the API succeeded.
Defines a JavaScript class that corresponds to a C++ class, including:
- A JavaScript constructor function that has the class name and invokes the provided C++ constructor callback.
- Properties on the constructor function corresponding to static data
properties, accessors, and methods of the C++ class (defined by
property descriptors with the
napi_staticattribute). - Properties on the constructor function's
prototypeobject corresponding to non-static data properties, accessors, and methods of the C++ class (defined by property descriptors without thenapi_staticattribute).
The C++ constructor callback should be a static method on the class that calls
the actual class constructor, then wraps the new C++ instance in a JavaScript
object, and returns the wrapper object. See napi_wrap() for details.
The JavaScript constructor function returned from napi_define_class is
often saved and used later, to construct new instances of the class from native
code, and/or check whether provided values are instances of the class. In that
case, to prevent the function value from being garbage-collected, create a
persistent reference to it using napi_create_reference and ensure the
reference count is kept >= 1.
napi_wrap
napi_status napi_wrap(napi_env env,
napi_value js_object,
void* native_object,
napi_finalize finalize_cb,
void* finalize_hint,
napi_ref* result);
[in] env: The environment that the API is invoked under.[in] js_object: The JavaScript object that will be the wrapper for the native object. This object must have been created from theprototypeof a constructor that was created usingnapi_define_class().[in] native_object: The native instance that will be wrapped in the JavaScript object.[in] finalize_cb: Optional native callback that can be used to free the native instance when the JavaScript object is ready for garbage-collection.[in] finalize_hint: Optional contextual hint that is passed to the finalize callback.[out] result: Optional reference to the wrapped object.
Returns napi_ok if the API succeeded.
Wraps a native instance in JavaScript object of the corresponding type.
The native instance can be retrieved later using napi_unwrap().
When JavaScript code invokes a constructor for a class that was defined using
napi_define_class(), the napi_callback for the constructor is invoked.
After constructing an instance of the native class, the callback must then call
napi_wrap() to wrap the newly constructed instance in the already-created
JavaScript object that is the this argument to the constructor callback.
(That this object was created from the constructor function's prototype,
so it already has definitions of all the instance properties and methods.)
Typically when wrapping a class instance, a finalize callback should be
provided that simply deletes the native instance that is received as the data
argument to the finalize callback.
The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.
Caution: The optional returned reference (if obtained) should be deleted via
napi_delete_reference ONLY in response to the finalize callback
invocation. (If it is deleted before then, then the finalize callback may never
be invoked.) Therefore, when obtaining a reference a finalize callback is also
required in order to enable correct proper of the reference.
Note: This API may modify the prototype chain of the wrapper object.
Afterward, additional manipulation of the wrapper's prototype chain may cause
napi_unwrap() to fail.
napi_unwrap
napi_status napi_unwrap(napi_env env,
napi_value js_object,
void** result);
[in] env: The environment that the API is invoked under.[in] js_object: The object associated with the C++ class instance.[out] result: Pointer to the wrapped C++ class instance.
Returns napi_ok if the API succeeded.
Retrieves a native instance that was previously wrapped in a JavaScript
object using napi_wrap().
When JavaScript code invokes a method or property accessor on the class, the
corresponding napi_callback is invoked. If the callback is for an instance
method or accessor, then the this argument to the callback is the wrapper
object; the wrapped C++ instance that is the target of the call can be obtained
then by calling napi_unwrap() on the wrapper object.
Asynchronous Operations
Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This is important in order to allow them to avoid blocking overall execution of the Node.js application.
N-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.
N-API defines the napi_work structure which is used to manage
asynchronous workers. Instances are created/deleted with
napi_create_async_work and napi_delete_async_work.
The execute and complete callbacks are functions that will be
invoked when the executor is ready to execute and when it completes its
task respectively. These functions implement the following interfaces:
typedef void (*napi_async_execute_callback)(napi_env env,
void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
napi_status status,
void* data);
When these methods are invoked, the data parameter passed will be the
addon-provided void* data that was passed into the
napi_create_async_work call.
Once created the async worker can be queued
for execution using the napi_queue_async_work function:
NAPI_EXTERN napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
napi_cancel_async_work can be used if the work needs
to be cancelled before the work has started execution.
After calling napi_cancel_async_work, the complete callback
will be invoked with a status value of napi_cancelled.
The work should not be deleted before the complete
callback invocation, even when it was cancelled.
napi_create_async_work
NAPI_EXTERN
napi_status napi_create_async_work(napi_env env,
napi_async_execute_callback execute,
napi_async_complete_callback complete,
void* data,
napi_async_work* result);
[in] env: The environment that the API is invoked under.[in] execute: The native function which should be called to excute the logic asynchronously.[in] complete: The native function which will be called when the asynchronous logic is comple or is cancelled.[in] data: User-provided data context. This will be passed back into the execute and complete functions.[out] result:napi_async_work*which is the handle to the newly created async work.
Returns napi_ok if the API succeeded.
This API allocates a work object that is used to execute logic asynchronously.
It should be freed using napi_delete_async_work once the work is no longer
required.
napi_delete_async_work
NAPI_EXTERN napi_status napi_delete_async_work(napi_env env,
napi_async_work work);
[in] env: The environment that the API is invoked under.[in] work: The handle returned by the call tonapi_create_async_work.
Returns napi_ok if the API succeeded.
This API frees a previously allocated work object.
napi_queue_async_work
NAPI_EXTERN napi_status napi_queue_async_work(napi_env env,
napi_async_work work);
[in] env: The environment that the API is invoked under.[in] work: The handle returned by the call tonapi_create_async_work.
Returns napi_ok if the API succeeded.
This API requests that the previously allocated work be scheduled for execution.
napi_cancel_async_work
NAPI_EXTERN napi_status napi_cancel_async_work(napi_env env,
napi_async_work work);
[in] env: The environment that the API is invoked under.[in] work: The handle returned by the call tonapi_create_async_work.
Returns napi_ok if the API succeeded.
This API cancels queued work if it has not yet
been started. If it has already started executing, it cannot be
cancelled and napi_generic_failure will be returned. If successful,
the complete callback will be invoked with a status value of
napi_cancelled. The work should not be deleted before the complete
callback invocation, even if it has been successfully cancelled.
Version Management
napi_get_version
NAPI_EXTERN napi_status napi_get_version(napi_env env,
uint32_t* result);
[in] env: The environment that the API is invoked under.[out] result: The highest version of N-API supported.
Returns napi_ok if the API succeeded.
This API returns the highest N-API version supported by the Node.js runtime. N-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:
- Call
napi_get_version()to determine if the API is available. - If available, dynamically load a pointer to the function using
uv_dlsym(). - Use the dynamically loaded pointer to invoke the function.
- If the function is not available, provide an alternate implementation that does not use the function.