新增于: v8.5.0
Node.js 事件循环退出时的高解析度毫秒时间戳。
如果事件循环尚未退出，则该属性的值为 -1。
它只能在 'exit' 事件的句柄中具有非 -1 的值。
Added in: v8.5.0
The high resolution millisecond timestamp at which the Node.js event loop
exited. If the event loop has not yet exited, the property has the value of -1.
It can only have a value of not -1 in a handler of the 'exit' event.