允许确定流是否已经或即将被读取。 如果已触发 'data'、'end'、'error' 或 'close',则返回 true。
'data'
'end'
'error'
'close'
Allows determining if the stream has been or is about to be read. Returns true if 'data', 'end', 'error' or 'close' has been emitted.