grunt-watch-event.js

// watch config
watch: {
      options: {
        spawn: false // <--- important
      },
      js: {
        files: '*.js',
        tasks: []
      }
}
// watch event 
grunt.event.on('watch', function(action, filepath, target) {
  grunt.config(['log'], filepath);
  grunt.task.run('log');
});
// your task
grunt.registerTask('log', 'Log stuff.', function() {
 var filepath = grunt.config.get('log'));
 // do stuff with filepath...
});