模块封装器

在执行模块代码之前，Node.js 将使用如下所示的函数封装器对其进行封装：

(function(exports, require, module, __filename, __dirname) {
// 模块代码实际存在于此处
});

通过这样做，Node.js 实现了以下几点：

• 它将顶层变量（用 var、const 或 let 定义）保持在模块而不是全局对象的范围内。
• 它有助于提供一些实际特定于模块的全局变量，例如：
  • module 和 exports 对象，实现者可以用来从模块中导出值。
  • 便利变量 __filename 和 __dirname，包含模块的绝对文件名和目录路径。

Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:

(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});

By doing this, Node.js achieves a few things:

• It keeps top-level variables (defined with var, const or let) scoped to the module rather than the global object.
• It helps to provide some global-looking variables that are actually specific to the module, such as:
  • The module and exports objects that the implementor can use to export values from the module.
  • The convenience variables __filename and __dirname, containing the module's absolute filename and directory path.