Why fs.watchFile called twice in Node?

Question

Why fs.watchFile called twice in Node?

Ubuntu 12.04 Node v0.6.14 CoffeeScript 1.3.1

fs.watchFile coffee_eval, (e) ->
  console.log e
  result = spawn 'coffee', ['-bc', coffee_eval]
  msg = ''
  result.stderr.on 'data', (str) ->
    msg+= str
  result.stderr.on 'end', ->
    console.log 'msg: ', msg
  print "!! #{coffee_eval}\n"

Whole code on gist: https://gist.github.com/2621576

Every time I save a file which is watched, the main function is called twitce rather than once.
My Editor is Sumlime Text 2.

the output words can be see :
enter image description here

node.js

Answer 1

fs.watchFile is unstable. From the node docs:

fs.watchFile(filename, [options], listener)#

Stability: 2 - Unstable. Use fs.watch instead, if available.

You can try fs.watch, but unfortunately it may suffer from the same problem. I had the same issue with fs.watch on windows, when trying to create a similar monitor script.

The workaround was to log the time when the modification occurs and ignore the second change if it was triggered withing a few milliseconds. A bit ugly but it worked.

Answer 2

I would suggest trying node-inotify-plusplus (https://github.com/coolaj86/node-inotify-plusplus) which has worked much better for me than fs.watchFile or fs.watch.

Answer 3

If you are using underscore or lodash, you could consider using throttle and discard the calls on the trailing edge. A basic example would be

var fs = require('fs');
var _ = require("lodash");

function FileWatcher (fileName)
{
    this.file = fileName;
    this.onChange = _.throttle(this.trigger, 100, {leading: false});
}
FileWatcher.prototype.observe = function ()
{
    fs.watch(this.file, this.onChange);
}
FileWatcher.prototype.trigger = function ()
{
    console.log("file changed :)");
}

var fileToWatch = __dirname + "/package.json";
new FileWatcher(fileToWatch).observe();

Answer 4

I had the same problem with fs.watch on Windows. The funny thing is that using fs.watchFile (which they recommend against in the docs) worked fine.

Answer 5

To solve this problem, I keep track of the previous "file modified" timestamp and don't run my normal callback code if the value is the same.

var filename = "/path/to/file";
var previousMTime = new Date(0);
var watcher = fs.watch(filename, {
    persistent: false
}, function(){
    fs.stat(filename, function(err, stats){
        if(stats.mtime.valueOf() === previousMTime.valueOf()){
            console.log("File Update Callback Stopped (same revision time)");
            return;
        }
        previousMTime = stats.mtime;

        // do your interesting stuff down here
    });
});