new to grunt - warning: task "concat, uglify" not found

Question

As the title says I'm new to Grunt. I am following a tutorial located at: http://24ways.org/2013/grunt-is-not-weird-and-hard/. It is an older tutorial but most seems to work the same. I have installed "grunt-contrib-concat" and "grunt-contrib-uglify" and can run both individually. But when I run grunt, I get the following error: Warning: Task "concat, uglify" not found. Use --force to continue. Aborted due to errors. I've been looking around and can't seem to figure it out. My files are as follows:

Gruntfile.js:

module.exports = function(grunt) {

            // 1. All configuration goes here 
            grunt.initConfig({
                pkg: grunt.file.readJSON('package.json'),

                concat: {

                    dist: {
                        src: [
                            'js/libs/*.js', // All JS in the libs folder
                            'js/controls.js', // This specific file
                        ],
                        dest: 'dist/built.js',
                    }
                },

                uglify: {
                    build: {
                        src: 'js/build/production.js',
                        dest: 'js/build/production.min.js',
                    }
                },

            });

            // 3. Where we tell Grunt we plan to use this plug-in.
            grunt.loadNpmTasks('grunt-contrib-concat');
            grunt.loadNpmTasks('grunt-contrib-uglify');

            // 4. Where we tell Grunt what to do when we type 'grunt' into the terminal.
            grunt.registerTask('default', ['concat, uglify']);

        };

package.json:

{
  "name": "grunt_libsass_example-project",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "~0.4.1",
    "grunt-contrib-concat": "^0.5.1",
    "grunt-contrib-uglify": "^0.9.1"
  }
}

Answer 1

Your passing in only one string for the registerTask task list. It should be a array with a list of strings like:

grunt.registerTask('default', ['concat', 'uglify']);

You're getting that error because it's looking for a task named 'concat, uglify'.