C++ embedder API
Added in: v12.19.0
Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.
The documentation for these APIs can be found in src/node.h 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.
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 deprecation policy and may occur on each semver-major release without prior warning.
Example embedding application
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
node -e <code>
, i.e. that will take a piece of JavaScript and run it in
a Node.js-specific environment.
The full code can be found in the Node.js source tree.
Setting up per-process state
Node.js requires some per-process state management in order to run:
- Arguments parsing for Node.js CLI options,
- V8 per-process requirements, such as a
v8::Platform
instance.
The following example shows how these can be set up. Some class names are from
the node
and v8
C++ namespaces, respectively.
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Per-instance state
Node.js has a concept of a “Node.js instance”, that is commonly being referred
to as node::Environment
. Each node::Environment
is associated with:
- Exactly one
v8::Isolate
, i.e. one JS Engine instance, - Exactly one
uv_loop_t
, i.e. one event loop, and - A number of
v8::Context
s, but exactly one mainv8::Context
. - One
node::IsolateData
instance that contains information that could be shared by multiplenode::Environment
s that use the samev8::Isolate
. Currently, no testing if performed for this scenario.
In order to set up a v8::Isolate
, an v8::ArrayBuffer::Allocator
needs
to be provided. One possible choice is the default Node.js allocator, which
can be created through node::ArrayBufferAllocator::Create()
. Using the Node.js
allocator allows minor performance optimizations when addons use the Node.js
C++ Buffer
API, and is required in order to track ArrayBuffer
memory in
process.memoryUsage()
.
Additionally, each v8::Isolate
that is used for a Node.js instance needs to
be registered and unregistered with the MultiIsolatePlatform
instance, if one
is being used, in order for the platform to know which event loop to use
for tasks scheduled by the v8::Isolate
.
The node::NewIsolate()
helper function creates a v8::Isolate
,
sets it up with some Node.js-specific hooks (e.g. the Node.js error handler),
and registers it with the platform automatically.
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