node-v16.4.2.tar.gz

# Node.js C++ codebase Hi! 👋 You’ve found the C++ code backing Node.js. This README aims to help you get started working on it and document some idioms you may encounter while doing so. ## Coding style Node.js has a document detailing its [C++ coding style][] that can be helpful as a reference for stylistic issues. ## V8 API documentation A lot of the Node.js codebase is around what the underlying JavaScript engine, V8, provides through its API for embedders. Knowledge of this API can also be useful when working with native addons for Node.js written in C++, although for new projects [N-API][] is typically the better alternative. V8 does not provide much public API documentation beyond what is available in its C++ header files, most importantly `v8.h`, which can be accessed online in the following locations: * On GitHub: [`v8.h` in Node.js master][] * On GitHub: [`v8.h` in V8 master][] * On the Chromium project’s Code Search application: [`v8.h` in Code Search][] V8 also provides an [introduction for V8 embedders][], which can be useful for understanding some of the concepts it uses in its embedder API. Important concepts when using V8 are the ones of [`Isolate`][]s and [JavaScript value handles][]. ## libuv API documentation The other major dependency of Node.js is [libuv][], providing the [event loop][] and other operation system abstractions to Node.js. There is a [reference documentation for the libuv API][]. ## File structure The Node.js C++ files follow this structure: The `.h` header files contain declarations, and sometimes definitions that don’t require including other headers (e.g. getters, setters, etc.). They should only include other `.h` header files and nothing else. The `-inl.h` header files contain definitions of inline functions from the corresponding `.h` header file (e.g. functions marked `inline` in the declaration or `template` functions). They always include the corresponding `.h` header file, and can include other `.h` and `-inl.h` header files as needed. It is not mandatory to split out the definitions from the `.h` file into an `-inl.h` file, but it becomes necessary when there are multiple definitions and contents of other `-inl.h` files start being used. Therefore, it is recommended to split a `-inl.h` file when inline functions become longer than a few lines to keep the corresponding `.h` file readable and clean. All visible definitions from the `-inl.h` file should be declared in the corresponding `.h` header file. The `.cc` files contain definitions of non-inline functions from the corresponding `.h` header file. They always include the corresponding `.h` header file, and can include other `.h` and `-inl.h` header files as needed. ## Helpful concepts A number of concepts are involved in putting together Node.js on top of V8 and libuv. This section aims to explain some of them and how they work together. <a id="isolate"></a> ### `Isolate` The `v8::Isolate` class represents a single JavaScript engine instance, in particular a set of JavaScript objects that can refer to each other (the “heap”). The `v8::Isolate` is often passed to other V8 API functions, and provides some APIs for managing the behaviour of the JavaScript engine or querying about its current state or statistics such as memory usage. V8 APIs are not thread-safe unless explicitly specified. In a typical Node.js application, the main thread and any `Worker` threads each have one `Isolate`, and JavaScript objects from one `Isolate` cannot refer to objects from another `Isolate`. Garbage collection, as well as other operations that affect the entire heap, happen on a per-`Isolate` basis. Typical ways of accessing the current `Isolate` in the Node.js code are: * Given a `FunctionCallbackInfo` for a [binding function][], using `args.GetIsolate()`. * Given a [`Context`][], using `context->GetIsolate()`. * Given a [`Environment`][], using `env->isolate()`. ### V8 JavaScript values V8 provides classes that mostly correspond to JavaScript types; for example, `v8::Value` is a class representing any kind of JavaScript type, with subclasses such as `v8::Number` (which in turn has subclasses like `v8::Int32`), `v8::Boolean` or `v8::Object`. Most types are represented by subclasses of `v8::Object`, e.g. `v8::Uint8Array` or `v8::Date`. <a id="internal-fields"></a> ### Internal fields V8 provides the ability to store data in so-called “internal fields” inside `v8::Object`s that were created as instances of C++-backed classes. The number of fields needs to be defined when creating that class. Both JavaScript values and `void*` pointers may be stored in such fields. In most native Node.js objects, the first internal field is used to store a pointer to a [`BaseObject`][] subclass, which then contains all relevant information associated with the JavaScript object. Typical ways of working with internal fields are: * `obj->InternalFieldCount()` to look up the number of internal fields for an object (`0` for regular JavaScript objects). * `obj->GetInternalField(i)` to get a JavaScript value from an internal field. * `obj->SetInternalField(i, v)` to store a JavaScript value in an internal field. * `obj->GetAlignedPointerFromInternalField(i)` to get a `void*` pointer from an internal field. * `obj->SetAlignedPointerInInternalField(i, p)` to store a `void*` pointer in an internal field. [`Context`][]s provide the same feature under the name “embedder data”. <a id="js-handles"></a> ### JavaScript value handles All JavaScript values are accessed through the V8 API through so-called handles, of which there are two types: [`Local`][]s and [`Global`][]s. <a id="local-handles"></a> #### `Local` handles A `v8::Local` handle is a temporary pointer to a JavaScript object, where “temporary” usually means that is no longer needed after the current function is done executing. `Local` handles can only be allocated on the C++ stack. Most of the V8 API uses `Local` handles to work with JavaScript values or return them from functions. Whenever a `Local` handle is created, a `v8::HandleScope` or `v8::EscapableHandleScope` object must exist on the stack. The `Local` is then added to that scope and deleted along with it. When inside a [binding function][], a `HandleScope` already exists outside of it, so there is no need to explicitly create one. `EscapableHandleScope`s can be used to allow a single `Local` handle to be passed to the outer scope. This is useful when a function returns a `Local`. The following JavaScript and C++ functions are mostly equivalent: ```js function getFoo(obj) { return obj.foo; } ``` ```cpp v8::Local<v8::Value> GetFoo(v8::Local<v8::Context> context, v8::Local<v8::Object> obj) { v8::Isolate* isolate = context->GetIsolate(); v8::EscapableHandleScope handle_scope(isolate); // The 'foo_string' handle cannot be returned from this function because // it is not “escaped” with `.Escape()`. v8::Local<v8::String> foo_string = v8::String::NewFromUtf8(isolate, "foo").ToLocalChecked(); v8::Local<v8::Value> return_value; if (obj->Get(context, foo_string).ToLocal(&return_value)) { return handle_scope.Escape(return_value); } else { // There was a JS exception! Handle it somehow. return v8::Local<v8::Value>(); } } ``` See [exception handling][] for more information about the usage of `.To()`, `.ToLocalChecked()`, `v8::Maybe` and `v8::MaybeLocal` usage. ##### Casting local handles If it is known that a `Local<Value>` refers to a more specific type, it can be cast to that type using `.As<...>()`: ```cpp v8::Local<v8::Value> some_value; // CHECK() is a Node.js utilitity that works similar to assert(). CHECK(some_value->IsUint8Array()); v8::Local<v8::Uint8Array> as_uint8 = some_value.As<v8::Uint8Array>(); ``` Generally, using `val.As<v8::X>()` is only valid if `val->IsX()` is true, and failing to follow that rule may lead to crashes. #####