gruntfile.js

var fs = require('fs');
var path = require('path');
var sweet = require('sweet.js');

module.exports = function(grunt) {
    grunt.initConfig({
        sweetjs: {
            options: {
                modules: ['es6-macros'],
                sourceMap: true,
                nodeSourceMapSupport: true
            },
            server: {
                files: [{
                  expand: true,
                  cwd: 'src/',
                  src: ['**/*.js'],
                  dest: 'build/'
                }]
            },
            client: {
                options: {
                    nodeSourceMapSupport: false
                },
                files: [{
                  expand: true,
                  cwd: 'static/js/',
                  src: ['**/*.js', '!lib/**/*.js'],
                  dest: 'static/js-build/'
                }]
            }
        },
        copy: {
            client: {
                files: [{
                    expand: true,
                    cwd: 'static/js/',
                    src: ['lib/**/*.js'],
                    dest: 'static/js-build/'
                }]
            }
        },
        watch: {
            options: {
                nospawn: true
            },
            server_js: {
                files: ['src/**/*.js'],
                tasks: ['sweetjs:server']
            },
            client_js: {
                files: ['static/js/**/*.js'],
                tasks: ['sweetjs:client']
            }
        }
    });

    grunt.event.on('watch', function(action, filepath, target) {
        if(action == 'changed') {
            var dest, cmd;
            if(target == 'server_js') {
                dest = filepath.replace(/^src/, 'build');
                cmd = 'sweetjs.server';
            }
            else {
                dest = filepath.replace(/^static\/js/, 'static/js-build');
                cmd = 'sweetjs.client';
            }

            grunt.config.set(cmd + '.src', [filepath]);
            grunt.config.set(cmd + '.dest', dest);
        }
        else if(action == 'deleted') {
            // remove files in build
        }
    });
    
    grunt.loadNpmTasks('grunt-contrib-watch');
    grunt.loadNpmTasks('grunt-contrib-copy');
    grunt.loadNpmTasks('grunt-sweet.js');

    grunt.registerTask('default', ['sweetjs', 'copy']);
};