doc: add function and function reference doc

PR-URL: nodejs/node-addon-api#299
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/function.md

@@ -1,5 +1,282 @@
 # Function
 
-You are reading a draft of the next documentation and it's in continuous update so
-if you don't find what you need please refer to:
-[C++ wrapper classes for the ABI-stable C APIs for Node.js](https://nodejs.github.io/node-addon-api/)
+The `Napi::Function` class provides a set of methods for creating a function object in
+native code that can later be called from JavaScript. The created function is not
+automatically visible from JavaScript. Instead it needs to be part of the add-on's
+module exports or be returned by one of the module's exported functions.
+
+In addition the `Function` class also provides methods that can be used to call
+functions that were created in JavaScript and passed to the native add-on.
+
+The `Napi::Function` class inherits its behavior from the `Napi::Object` class (for more info
+see: [`Napi::Object`](object.md)).
+
+## Example
+
+```cpp
+#include <napi.h>
+
+using namespace Napi;
+
+Value Fn(const CallbackInfo& info) {
+  Env env = info.Env();
+  // ...
+  return String::New(env, "Hello World");
+}
+
+Object Init(Env env, Object exports) {
+  exports.Set(String::New(env, "fn"), Function::New(env, Fn));
+}
+
+NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)
+```
+
+The above code can be used from JavaScript as follows:
+
+```js
+const addon = require('./addon');
+addon.fn();
+```
+
+With the `Napi::Function` class it is possible to call a JavaScript function object
+from a native add-on with two different methods: `Call` and `MakeCallback`.
+The API of these two methods is very similar, but they are used in different
+contexts. The `MakeCallback` method is used to call from native code back into
+JavaScript after returning from an [asynchronous operation](async_operations.md)
+and in general in situations which don't have an existing JavaScript function on
+the stack. The `Call` method is used when there is already a JavaScript function
+on the stack (for example when running a native method called from JavaScript).
+
+## Methods
+
+### Constructor
+
+Creates a new empty instance of `Napi::Function`.
+
+```cpp
+Function();
+```
+
+### Constructor
+
+Creates a new instance of the `Napi::Function` object.
+
+```cpp
+Function(napi_env env, napi_value value);
+```
+
+- `[in] env`: The `napi_env` environment in which to construct the `Napi::Function` object.
+- `[in] value`: The `napi_value` which is a handle for a JavaScript function.
+
+Returns a non-empty `Napi::Function` instance.
+
+### New
+
+Creates an instance of a `Napi::Function` object.
+
+```cpp
+template <typename Callable>
+static Function New(napi_env env, Callable cb, const char* utf8name = nullptr, void* data = nullptr);
+```
+
+- `[in] env`: The `napi_env` environment in which to construct the `Napi::Function` object.
+- `[in] cb`: Object that implements `Callable`.
+- `[in] utf8name`: Null-terminated string to be used as the name of the function.
+- `[in] data`: User-provided data context. This will be passed back into the
+function when invoked later.
+
+Returns an instance of a `Napi::Function` object.
+
+### New
+
+```cpp
+template <typename Callable>
+static Function New(napi_env env, Callable cb, const std::string& utf8name, void* data = nullptr);
+```
+
+- `[in] env`: The `napi_env` environment in which to construct the `Napi::Function` object.
+- `[in] cb`: Object that implements `Callable`.
+- `[in] utf8name`: String to be used as the name of the function.
+- `[in] data`: User-provided data context. This will be passed back into the
+function when invoked later.
+
+Returns an instance of a `Napi::Function` object.
+
+### New
+
+Creates a new JavaScript value from one that represents the constructor for the
+object.
+
+```cpp
+Napi::Object New(const std::initializer_list<napi_value>& args) const;
+```
+
+- `[in] args`: Initializer list of JavaScript values as `napi_value` representing
+the arguments of the contructor function.
+
+Returns a new JavaScript object.
+
+### New
+
+Creates a new JavaScript value from one that represents the constructor for the
+object.
+
+```cpp
+Napi::Object New(const std::vector<napi_value>& args) const;
+```
+
+- `[in] args`: Vector of JavaScript values as `napi_value` representing the
+arguments of the constructor function.
+
+Returns a new JavaScript object.
+
+### New
+
+Creates a new JavaScript value from one that represents the constructor for the
+object.
+
+```cpp
+Napi::Object New(size_t argc, const napi_value* args) const;
+```
+
+- `[in] argc`: The number of the arguments passed to the contructor function.
+- `[in] args`: Array of JavaScript values as `napi_value` representing the
+arguments of the constructor function.
+
+Returns a new JavaScript object.
+
+### Call
+
+Calls a Javascript function from a native add-on.
+
+```cpp
+Napi::Value Call(const std::initializer_list<napi_value>& args) const;
+```
+
+- `[in] args`: Initializer list of JavaScript values as `napi_value` representing
+the arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### Call
+
+Calls a JavaScript function from a native add-on.
+
+```cpp
+Napi::Value Call(const std::vector<napi_value>& args) const;
+```
+
+- `[in] args`: Vector of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### Call
+
+Calls a Javascript function from a native add-on.
+
+```cpp
+Napi::Value Call(size_t argc, const napi_value* args) const;
+```
+
+- `[in] argc`: The number of the arguments passed to the function.
+- `[in] args`: Array of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### Call
+
+Calls a Javascript function from a native add-on.
+
+```cpp
+Napi::Value Call(napi_value recv, const std::initializer_list<napi_value>& args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] args`: Initializer list of JavaScript values as `napi_value` representing
+the arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### Call
+
+Calls a Javascript function from a native add-on.
+
+```cpp
+Napi::Value Call(napi_value recv, const std::vector<napi_value>& args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] args`: Vector of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### Call
+
+Calls a Javascript function from a native add-on.
+
+```cpp
+Napi::Value Call(napi_value recv, size_t argc, const napi_value* args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] argc`: The number of the arguments passed to the function.
+- `[in] args`: Array of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### MakeCallback
+
+Calls a Javascript function from a native add-on after an asynchronous operation.
+
+```cpp
+Napi::Value MakeCallback(napi_value recv, const std::initializer_list<napi_value>& args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] args`: Initializer list of JavaScript values as `napi_value` representing
+the arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### MakeCallback
+
+Calls a Javascript function from a native add-on after an asynchronous operation.
+
+```cpp
+Napi::Value MakeCallback(napi_value recv, const std::vector<napi_value>& args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] args`: List of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+### MakeCallback
+
+Calls a Javascript function from a native add-on after an asynchronous operation.
+
+```cpp
+Napi::Value MakeCallback(napi_value recv, size_t argc, const napi_value* args) const;
+```
+
+- `[in] recv`: The `this` object passed to the called function.
+- `[in] argc`: The number of the arguments passed to the function.
+- `[in] args`: Array of JavaScript values as `napi_value` representing the
+arguments of the function.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.
+
+## Operator
+
+```cpp
+Napi::Value operator ()(const std::initializer_list<napi_value>& args) const;
+```
+
+- `[in] args`: Initializer list of JavaScript values as `napi_value`.
+
+Returns a `Napi::Value` representing the JavaScript value returned by the function.