A grunt-copy task filter option function. Enables the copying of only changed files to the destination.

grunt.copy.onlyNew.js

/**
 * Returns a function that can be used with grunt's copy
 * task 'filter' option. Checks if file being copied
 * is newer than that destination file.  A local file can be
 * used as the "destination" timestampt - a useful method for
 * when the destination file is really in a remote system
 * (ex. linux server, or a WEBDav folder). 
 *
 * @param {Array} target
 *      The grunt Copy configurtaion task that this instance
 *      will apply to.
 * @param {String} timestampFile
 *      A timestamp file. Will be used instead of accessing the
 *      destination file when detemining if file should be copied.
 *
 * @return {Boolean}
 *      True - yes, its new
 *      false - no, its not new
 * 
 * @see https://gist.github.com/purtuga/e0c466eee74f9c411f54
 * 
 * @example
 *
 *  grunt.initConfig({
 *      copy: {
 *          build: {
 *              src:    "src/**,
 *              dest:   "build/",
 *              expand: true,
 *              filter: onlyNew(['copy', 'build'])
 *          }
 *      }
 *  })
 *
 */
function onlyNew(target, timestampFile) {

    var newTimestamp,
        taskCreated     = false;


    if (!onlyNew.isTaskCreated) {
        onlyNew.isTaskCreated = true;
        grunt.registerTask('onlyNewPostRun', function(f){
            var file = Array.prototype.slice.call(arguments, 0).join(':');
            grunt.log.writeln("onlyNewPostRun Task RUNNING for file: " + file);
            fs.writeFileSync(file, 'temp file');
        });
        onlyNew.timestampFiles = {};
    }


    // Return the callback function for each file check - used in the task
    return function(src) {

        var dest    = grunt.config(target.concat('dest')),
            cwd     = grunt.config(target.concat('cwd')),
            dstat, stat, response;

        if (!timestampFile) {
            dest = cwd ?
                   path.join(dest, path.relative(cwd, src)) :
                   path.join(dest, src);
        } else {
            dest = timestampFile;
        }

        if (timestampFile && !onlyNew.timestampFiles[timestampFile]) {
            onlyNew.timestampFiles[timestampFile] = true;
            grunt.task.run("onlyNewPostRun:" + timestampFile);
        }

        grunt.verbose.writeln("Src  File: " + src);
        grunt.verbose.writeln("Dest File: " + dest);

        try {
            dstat   = fs.statSync(dest);
            stat    = fs.statSync(src);
        } catch (e) {
            return true;
        }

        response = ( stat.isFile() && stat.mtime.getTime() > dstat.mtime.getTime() );
        return response;

    };

}