watchUntil.js

var Promise = require('bluebird');
var fs = require('fs');
var noop = function(){};
var DEFAULT_TIMEOUT = 60 * 2 * 1000;

module.exports = function watch(fileName, until, timeout) {
  var watcher;
    
  if (typeof until !== 'function') until = noop;
  timeout = Number(timeout) || DEFAULT_TIMEOUT;
    
  return new Promise(function(resolve, reject) {
    if (until()) return resolve();
    console.log('watching %s for changes...', fileName);
    watcher = fs.watch(fileName, function(event) {
      if (event !== 'change') return;
      console.log(fileName, 'changed');
      if (until()) resolve();
    });
  }).timeout(timeout).finally(function(e) {
    if (watcher) watcher.close();
  });
};