【fs.watchFile】
fs.watchFile(filename[, options], listener)
Watch for changes on filename. The callback listener will be called each time the file is accessed.
The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. Theoptions object may specify an interval property indicating how often the target should be polled in milliseconds. The default is { persistent: true, interval: 5007 }.
The listener gets two arguments the current stat object and the previous stat object:
These stat objects are instances of fs.Stat.