There are three options for implementing addons: Node-API, nan, or directuse of internal V8, libuv, and Node.js libraries. Unless there is a need fordirect access to functionality which is not exposed by Node-API, use Node-API.Refer to C/C++ addons with Node-API for more information onNode-API.
V8: the C++ library Node.js uses to provide theJavaScript implementation. V8 provides the mechanisms for creating objects,calling functions, etc. V8's API is documented mostly in thev8.h header file (deps/v8/include/v8.h in the Node.js sourcetree), which is also available online.
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libuv: The C library that implements the Node.js event loop, its workerthreads and all of the asynchronous behaviors of the platform. It alsoserves as a cross-platform abstraction library, giving easy, POSIX-likeaccess across all major operating systems to many common system tasks, suchas interacting with the file system, sockets, timers, and system events. libuvalso provides a threading abstraction similar to POSIX threads formore sophisticated asynchronous addons that need to move beyond thestandard event loop. Addon authors shouldavoid blocking the event loop with I/O or other time-intensive tasks byoffloading work via libuv to non-blocking system operations, worker threads,or a custom use of libuv threads.
Node.js includes other statically linked libraries including OpenSSL. Theseother libraries are located in the deps/ directory in the Node.js sourcetree. Only the libuv, OpenSSL, V8, and zlib symbols are purposefullyre-exported by Node.js and may be used to various extents by addons. SeeLinking to libraries included with Node.js for additional information.
There are environments in which Node.js addons may need to be loaded multipletimes in multiple contexts. For example, the Electron runtime runs multipleinstances of Node.js in a single process. Each instance will have its ownrequire() cache, and thus each instance will need a native addon to behavecorrectly when loaded via require(). This means that the addonmust support multiple initializations.
A context-aware addon can be constructed by using the macroNODE_MODULE_INITIALIZER, which expands to the name of a function which Node.jswill expect to find when it loads an addon. An addon can thus be initialized asin the following example:
Another option is to use the macro NODE_MODULE_INIT(), which will alsoconstruct a context-aware addon. Unlike NODE_MODULE(), which is used toconstruct an addon around a given addon initializer function,NODE_MODULE_INIT() serves as the declaration of such an initializer to befollowed by a function body.
The choice to build a context-aware addon carries with it the responsibility ofcarefully managing global static data. Since the addon may be loaded multipletimes, potentially even from different threads, any global static data storedin the addon must be properly protected, and must not contain any persistentreferences to JavaScript objects. The reason for this is that JavaScriptobjects are only valid in one context, and will likely cause a crash whenaccessed from the wrong context or from a different thread than the one on whichthey were created.
This will ensure that the per-addon-instance data reaches each binding that canbe called from JavaScript. The per-addon-instance data must also be passed intoany asynchronous callbacks the addon may create.
In order to support Worker threads, addons need to clean up any resourcesthey may have allocated when such a thread exists. This can be achieved throughthe usage of the AddEnvironmentCleanupHook() function:
This function adds a hook that will run before a given Node.js instance shutsdown. If necessary, such hooks can be removed before they are run usingRemoveEnvironmentCleanupHook(), which has the same signature. Callbacks arerun in last-in first-out order.
If necessary, there is an additional pair of AddEnvironmentCleanupHook()and RemoveEnvironmentCleanupHook() overloads, where the cleanup hook takes acallback function. This can be used for shutting down asynchronous resources,such as any libuv handles registered by the addon.
Once the source code has been written, it must be compiled into the binaryaddon.node file. To do so, create a file called binding.gyp in thetop-level of the project describing the build configuration of the moduleusing a JSON-like format. This file is used by node-gyp, a tool writtenspecifically to compile Node.js addons.
A version of the node-gyp utility is bundled and distributed withNode.js as part of npm. This version is not made directly available fordevelopers to use and is intended only to support the ability to use thenpm install command to compile and install addons. Developers who wish touse node-gyp directly can install it using the commandnpm install -g node-gyp. See the node-gyp installation instructions formore information, including platform-specific requirements.
Once the binding.gyp file has been created, use node-gyp configure togenerate the appropriate project build files for the current platform. Thiswill generate either a Makefile (on Unix platforms) or a vcxproj file(on Windows) in the build/ directory.
When using npm install to install a Node.js addon, npm uses its own bundledversion of node-gyp to perform this same set of actions, generating acompiled version of the addon for the user's platform on demand.
Because the exact path to the compiled addon binary can vary depending on howit is compiled (i.e. sometimes it may be in ./build/Debug/), addons can usethe bindings package to load the compiled module.
Node.js uses statically linked libraries such as V8, libuv, and OpenSSL. Alladdons are required to link to V8 and may link to any of the other dependenciesas well. Typically, this is as simple as including the appropriate#include statements (e.g. #include ) and node-gyp will locatethe appropriate headers automatically. However, there are a few caveats to beaware of:
When node-gyp runs, it will detect the specific release version of Node.jsand download either the full source tarball or just the headers. If the fullsource is downloaded, addons will have complete access to the full set ofNode.js dependencies. However, if only the Node.js headers are downloaded,then only the symbols exported by Node.js will be available.
The filename extension of the compiled addon binary is .node (as opposedto .dll or .so). The require() function is written to look forfiles with the .node file extension and initialize those as dynamically-linkedlibraries.
When calling require(), the .node extension can usually beomitted and Node.js will still find and initialize the addon. One caveat,however, is that Node.js will first attempt to locate and load modules orJavaScript files that happen to share the same base name. For instance, ifthere is a file addon.js in the same directory as the binary addon.node,then require('addon') will give precedence to the addon.js fileand load it instead.
Each of the examples illustrated in this document directly use theNode.js and V8 APIs for implementing addons. The V8 API can, and has, changeddramatically from one V8 release to the next (and one major Node.js release tothe next). With each change, addons may need to be updated and recompiled inorder to continue functioning. The Node.js release schedule is designed tominimize the frequency and impact of such changes but there is little thatNode.js can do to ensure stability of the V8 APIs.
The Native Abstractions for Node.js (or nan) provide a set of tools thataddon developers are recommended to use to keep compatibility between past andfuture releases of V8 and Node.js. See the nan examples for anillustration of how it can be used.
Node-API is an API for building native addons. It is independent fromthe underlying JavaScript runtime (e.g. V8) and is maintained as part ofNode.js itself. This API will be Application Binary Interface (ABI) stableacross versions of Node.js. It is intended to insulate addons fromchanges in the underlying JavaScript engine and allow modulescompiled for one version to run on later versions of Node.js withoutrecompilation. Addons are built/packaged with the same approach/toolsoutlined in this document (node-gyp, etc.). The only difference is theset of APIs that are used by the native code. Instead of using the V8or Native Abstractions for Node.js APIs, the functions availablein the Node-API are used.
Following are some example addons intended to help developers get started. Theexamples use the V8 APIs. Refer to the online V8 referencefor help with the various V8 calls, and V8's Embedder's Guide for anexplanation of several concepts used such as handles, scopes, functiontemplates, etc.
Addons will typically expose objects and functions that can be accessed fromJavaScript running within Node.js. When functions are invoked from JavaScript,the input arguments and return value must be mapped to and from the C/C++code.
This example uses a two-argument form of Init() that receives the fullmodule object as the second argument. This allows the addon to completelyoverwrite exports with a single function instead of adding the function as aproperty of exports.
Addons can create and return new objects from within a C++ function asillustrated in the following example. An object is created and returned with aproperty msg that echoes the string passed to createObject():
The destructor for a wrapper object will run when the object isgarbage-collected. For destructor testing, there are command-line flags thatcan be used to make it possible to force garbage collection. These flags areprovided by the underlying V8 JavaScript engine. They are subject to changeor removal at any time. They are not documented by Node.js or V8, and theyshould never be used outside of testing.
During shutdown of the process or worker threads destructors are not calledby the JS engine. Therefore it's the responsibility of the user to trackthese objects and ensure proper destruction to avoid resource leaks.
In addition to wrapping and returning C++ objects, it is possible to passwrapped objects around by unwrapping them with the Node.js helper functionnode::ObjectWrap::Unwrap. The following examples shows a function add()that can take two MyObject objects as input arguments: 152ee80cbc
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