NODE_V8_COVERAGE=dir

设置后，Node.js 将开始将 V8 JavaScript 代码覆盖率 和 源映射 数据输出到作为参数提供的目录（覆盖信息以 JSON 格式写入具有 coverage 前缀的文件）。

¥When set, Node.js will begin outputting V8 JavaScript code coverage and Source Map data to the directory provided as an argument (coverage information is written as JSON to files with a coverage prefix).

NODE_V8_COVERAGE 将自动传播到子进程，从而更容易检测调用 child_process.spawn() 系列函数的应用。NODE_V8_COVERAGE 可以设置为空字符串，防止传播。

¥NODE_V8_COVERAGE will automatically propagate to subprocesses, making it easier to instrument applications that call the child_process.spawn() family of functions. NODE_V8_COVERAGE can be set to an empty string, to prevent propagation.