@andrewkoung thanks for the update. Here are a few thoughts, reading your code above:

The way you're dealing with interval will fail in some cases. I would suggest you create another line that basically does const interval = options.interval || 5007; after you deal with the options object. You can probably ignore persistent for now.
The code you have for the watcher can be removed. Instead, we'll use your interval.
You will need to create an object that uses filenames as keys, and stores interval return values, so we can use that to clearInterval() in fs.unwatch(path). This object will need to be shared with the fs.unwatch code. Some work started on unwatch in Issue 551-Add unwatchfile method #553, which you might decide to take over.
You'll need to store the current and prev result of calling stat. I would declare these above your setInterval call. Then, when you do a stat you will need to compare the two objects, and see if anything has changed. If so, you'll need to call the listener function that was passed to you, passing along the current and prev values.

If you want to open a PR when you do that, I can start commenting directly in the code to help you more.

Add support for fs.watchFile() #654

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