process.exitCode
当进程正常退出或通过 process.exit() 退出而不指定代码时，将作为进程退出码的数字。
将代码指定为 process.exit(code) 将覆盖 process.exitCode 的任何先前设置。
A number which will be the process exit code, when the process either
exits gracefully, or is exited via process.exit() without specifying
a code.
Specifying a code to process.exit(code) will override any
previous setting of process.exitCode.