函数参数

【Function arguments】

插件通常会暴露可以从在 Node.js 中运行的 JavaScript 访问的对象和函数。当函数从 JavaScript 中被调用时，输入参数和返回值必须在 C/C++ 代码和 JavaScript 之间进行映射。

【Addons will typically expose objects and functions that can be accessed from JavaScript 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.】

下面的示例演示了如何读取从 JavaScript 传递的函数参数以及如何返回结果：

【The following example illustrates how to read function arguments passed from JavaScript and how to return a result:】

// addon.cc
#include <node.h>

namespace demo {

using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    // Throw an Error that is passed back to JavaScript
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong number of arguments").ToLocalChecked()));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong arguments").ToLocalChecked()));
    return;
  }

  // Perform the operation
  double value =
      args[0].As<Number>()->Value() + args[1].As<Number>()->Value();
  Local<Number> num = Number::New(isolate, value);

  // Set the return value (using the passed in
  // FunctionCallbackInfo<Value>&)
  args.GetReturnValue().Set(num);
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo 拷贝

编译后，可以在 Node.js 中加载和使用示例插件：

【Once compiled, the example addon can be required and used from within Node.js:】

// test.js
const addon = require('./build/Release/addon');

console.log('This should be eight:', addon.add(3, 5)); 拷贝