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<details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#c-embedder-api">C++ embedder API</a>
<ul>
<li><a href="#example-embedding-application">Example embedding application</a>
<ul>
<li><a href="#setting-up-a-per-process-state">Setting up a per-process state</a></li>
<li><a href="#setting-up-a-per-instance-state">Setting up a per-instance state</a></li>
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<div role="main" id="apicontent">
<h2>C++ embedder API<span><a class="mark" href="#c-embedder-api" id="c-embedder-api">#</a></span><a aria-hidden="true" class="legacy" id="embedding_c_embedder_api"></a></h2>
<p>Node.js provides a number of C++ APIs that can be used to execute JavaScript
in a Node.js environment from other C++ software.</p>
<p>The documentation for these APIs can be found in <a href="https://github.com/nodejs/node/blob/HEAD/src/node.h">src/node.h</a> in the Node.js
source tree. In addition to the APIs exposed by Node.js, some required concepts
are provided by the V8 embedder API.</p>
<p>Because using Node.js as an embedded library is different from writing code
that is executed by Node.js, breaking changes do not follow typical Node.js
<a href="deprecations.html">deprecation policy</a> and may occur on each semver-major release without prior
warning.</p>
<section><h3>Example embedding application<span><a class="mark" href="#example-embedding-application" id="example-embedding-application">#</a></span><a aria-hidden="true" class="legacy" id="embedding_example_embedding_application"></a></h3>
<p>The following sections will provide an overview over how to use these APIs
to create an application from scratch that will perform the equivalent of
<code>node -e <code></code>, i.e. that will take a piece of JavaScript and run it in
a Node.js-specific environment.</p>
<p>The full code can be found <a href="https://github.com/nodejs/node/blob/HEAD/test/embedding/embedtest.cc">in the Node.js source tree</a>.</p>
<h4>Setting up a per-process state<span><a class="mark" href="#setting-up-a-per-process-state" id="setting-up-a-per-process-state">#</a></span><a aria-hidden="true" class="legacy" id="embedding_setting_up_a_per_process_state"></a></h4>
<p>Node.js requires some per-process state management in order to run:</p>
<ul>
<li>Arguments parsing for Node.js <a href="cli.html">CLI options</a>,</li>
<li>V8 per-process requirements, such as a <code>v8::Platform</code> instance.</li>
</ul>
<p>The following example shows how these can be set up. Some class names are from
the <code>node</code> and <code>v8</code> C++ namespaces, respectively.</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">int</span> <span class="hljs-title">main</span><span class="hljs-params">(<span class="hljs-type">int</span> argc, <span class="hljs-type">char</span>** argv)</span> </span>{
argv = <span class="hljs-built_in">uv_setup_args</span>(argc, argv);
<span class="hljs-function">std::vector<std::string> <span class="hljs-title">args</span><span class="hljs-params">(argv, argv + argc)</span></span>;
<span class="hljs-comment">// Parse Node.js CLI options, and print any errors that have occurred while</span>
<span class="hljs-comment">// trying to parse them.</span>
std::unique_ptr<node::InitializationResult> result =
node::<span class="hljs-built_in">InitializeOncePerProcess</span>(args, {
node::ProcessInitializationFlags::kNoInitializeV8,
node::ProcessInitializationFlags::kNoInitializeNodeV8Platform
});
<span class="hljs-keyword">for</span> (<span class="hljs-type">const</span> std::string& error : result-><span class="hljs-built_in">errors</span>())
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: %s\n"</span>, args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>(), error.<span class="hljs-built_in">c_str</span>());
<span class="hljs-keyword">if</span> (result-><span class="hljs-built_in">early_return</span>() != <span class="hljs-number">0</span>) {
<span class="hljs-keyword">return</span> result-><span class="hljs-built_in">exit_code</span>();
}
<span class="hljs-comment">// Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way</span>
<span class="hljs-comment">// to create a v8::Platform instance that Node.js can use when creating</span>
<span class="hljs-comment">// Worker threads. When no `MultiIsolatePlatform` instance is present,</span>
<span class="hljs-comment">// Worker threads are disabled.</span>
std::unique_ptr<MultiIsolatePlatform> platform =
MultiIsolatePlatform::<span class="hljs-built_in">Create</span>(<span class="hljs-number">4</span>);
V8::<span class="hljs-built_in">InitializePlatform</span>(platform.<span class="hljs-built_in">get</span>());
V8::<span class="hljs-built_in">Initialize</span>();
<span class="hljs-comment">// See below for the contents of this function.</span>
<span class="hljs-type">int</span> ret = <span class="hljs-built_in">RunNodeInstance</span>(
platform.<span class="hljs-built_in">get</span>(), result-><span class="hljs-built_in">args</span>(), result-><span class="hljs-built_in">exec_args</span>());
V8::<span class="hljs-built_in">Dispose</span>();
V8::<span class="hljs-built_in">DisposePlatform</span>();
node::<span class="hljs-built_in">TearDownOncePerProcess</span>();
<span class="hljs-keyword">return</span> ret;
}</code> <button class="copy-button">copy</button></pre>
<h4>Setting up a per-instance state<span><a class="mark" href="#setting-up-a-per-instance-state" id="setting-up-a-per-instance-state">#</a></span><a aria-hidden="true" class="legacy" id="embedding_setting_up_a_per_instance_state"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v15.0.0</td>
<td><p>The <code>CommonEnvironmentSetup</code> and <code>SpinEventLoop</code> utilities were added.</p></td></tr>
</tbody></table>
</details>
</div>
<p>Node.js has a concept of a “Node.js instance”, that is commonly being referred
to as <code>node::Environment</code>. Each <code>node::Environment</code> is associated with:</p>
<ul>
<li>Exactly one <code>v8::Isolate</code>, i.e. one JS Engine instance,</li>
<li>Exactly one <code>uv_loop_t</code>, i.e. one event loop,</li>
<li>A number of <code>v8::Context</code>s, but exactly one main <code>v8::Context</code>, and</li>
<li>One <code>node::IsolateData</code> instance that contains information that could be
shared by multiple <code>node::Environment</code>s. The embedder should make sure
that <code>node::IsolateData</code> is shared only among <code>node::Environment</code>s that
use the same <code>v8::Isolate</code>, Node.js does not perform this check.</li>
</ul>
<p>In order to set up a <code>v8::Isolate</code>, an <code>v8::ArrayBuffer::Allocator</code> needs
to be provided. One possible choice is the default Node.js allocator, which
can be created through <code>node::ArrayBufferAllocator::Create()</code>. Using the Node.js
allocator allows minor performance optimizations when addons use the Node.js
C++ <code>Buffer</code> API, and is required in order to track <code>ArrayBuffer</code> memory in
<a href="process.html#processmemoryusage"><code>process.memoryUsage()</code></a>.</p>
<p>Additionally, each <code>v8::Isolate</code> that is used for a Node.js instance needs to
be registered and unregistered with the <code>MultiIsolatePlatform</code> instance, if one
is being used, in order for the platform to know which event loop to use
for tasks scheduled by the <code>v8::Isolate</code>.</p>
<p>The <code>node::NewIsolate()</code> helper function creates a <code>v8::Isolate</code>,
sets it up with some Node.js-specific hooks (e.g. the Node.js error handler),
and registers it with the platform automatically.</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">int</span> <span class="hljs-title">RunNodeInstance</span><span class="hljs-params">(MultiIsolatePlatform* platform,
<span class="hljs-type">const</span> std::vector<std::string>& args,
<span class="hljs-type">const</span> std::vector<std::string>& exec_args)</span> </span>{
<span class="hljs-type">int</span> exit_code = <span class="hljs-number">0</span>;
<span class="hljs-comment">// Setup up a libuv event loop, v8::Isolate, and Node.js Environment.</span>
std::vector<std::string> errors;
std::unique_ptr<CommonEnvironmentSetup> setup =
CommonEnvironmentSetup::<span class="hljs-built_in">Create</span>(platform, &errors, args, exec_args);
<span class="hljs-keyword">if</span> (!setup) {
<span class="hljs-keyword">for</span> (<span class="hljs-type">const</span> std::string& err : errors)
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: %s\n"</span>, args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>(), err.<span class="hljs-built_in">c_str</span>());
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
}
Isolate* isolate = setup-><span class="hljs-built_in">isolate</span>();
Environment* env = setup-><span class="hljs-built_in">env</span>();
{
<span class="hljs-function">Locker <span class="hljs-title">locker</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-function">Isolate::Scope <span class="hljs-title">isolate_scope</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-function">HandleScope <span class="hljs-title">handle_scope</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-comment">// The v8::Context needs to be entered when node::CreateEnvironment() and</span>
<span class="hljs-comment">// node::LoadEnvironment() are being called.</span>
<span class="hljs-function">Context::Scope <span class="hljs-title">context_scope</span><span class="hljs-params">(setup->context())</span></span>;
<span class="hljs-comment">// Set up the Node.js instance for execution, and run code inside of it.</span>
<span class="hljs-comment">// There is also a variant that takes a callback and provides it with</span>
<span class="hljs-comment">// the `require` and `process` objects, so that it can manually compile</span>
<span class="hljs-comment">// and run scripts as needed.</span>
<span class="hljs-comment">// The `require` function inside this script does *not* access the file</span>
<span class="hljs-comment">// system, and can only load built-in Node.js modules.</span>
<span class="hljs-comment">// `module.createRequire()` is being used to create one that is able to</span>
<span class="hljs-comment">// load files from the disk, and uses the standard CommonJS file loader</span>
<span class="hljs-comment">// instead of the internal-only `require` function.</span>
MaybeLocal<Value> loadenv_ret = node::<span class="hljs-built_in">LoadEnvironment</span>(
env,
<span class="hljs-string">"const publicRequire ="</span>
<span class="hljs-string">" require('node:module').createRequire(process.cwd() + '/');"</span>
<span class="hljs-string">"globalThis.require = publicRequire;"</span>
<span class="hljs-string">"require('node:vm').runInThisContext(process.argv[1]);"</span>);
<span class="hljs-keyword">if</span> (loadenv_ret.<span class="hljs-built_in">IsEmpty</span>()) <span class="hljs-comment">// There has been a JS exception.</span>
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
exit_code = node::<span class="hljs-built_in">SpinEventLoop</span>(env).<span class="hljs-built_in">FromMaybe</span>(<span class="hljs-number">1</span>);
<span class="hljs-comment">// node::Stop() can be used to explicitly stop the event loop and keep</span>
<span class="hljs-comment">// further JavaScript from running. It can be called from any thread,</span>
<span class="hljs-comment">// and will act like worker.terminate() if called from another thread.</span>
node::<span class="hljs-built_in">Stop</span>(env);
}
<span class="hljs-keyword">return</span> exit_code;
}</code> <button class="copy-button">copy</button></pre></section>
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