访问主模块

¥Accessing the main module

当文件直接从 Node.js 运行时，则 require.main 被设置为其 module。这意味着可以通过测试 require.main === module 来确定文件是否被直接运行。

¥When a file is run directly from Node.js, require.main is set to its module. That means that it is possible to determine whether a file has been run directly by testing require.main === module.

对于文件 foo.js，如果通过 node foo.js 运行，则为 true，如果通过 require('./foo') 运行，则为 false。

¥For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').

当入口点不是 CommonJS 模块时，则 require.main 为 undefined，且主模块不可达。

¥When the entry point is not a CommonJS module, require.main is undefined, and the main module is out of reach.