grunt watch

//package.json
{
  "name": "xyz",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "1.0.0",
    "grunt-contrib-jshint": "latest",
    "jshint-stylish": "latest",
    "grunt-contrib-uglify": "latest",
    "grunt-contrib-sass": "latest",
    "grunt-contrib-cssmin": "latest",
    "grunt-contrib-watch": "latest"
  }
}

//gruntfile.js
module.exports = function (grunt) {
    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),
        jshint: {
            options: {
                reporter: require('jshint-stylish')
            },
            build: ['Gruntfile.js', 'src/**/*.js']
        },
        uglify: {
            options: {
                banner: '/*\n <%= pkg.name %> <%= grunt.template.today("yyyy-mm-dd") %> \n*/\n'
            },
            build: {
                files: {
                    'dist/js/script.min.js': 'src/js/script.js'
                }
            }
        },
        sass: {
            build: {
                files: {
                    'src/css/style.css': 'src/css/style.scss'
                }
            }
        },
        cssmin: {
            options: {
                banner: '/*\n <%= pkg.name %> <%= grunt.template.today("yyyy-mm-dd") %> \n*/\n'
            },
            build: {
                files: {
                    'dist/css/style.min.css': 'src/css/style.css'
                }
            }
        },
        watch: {
            css: {
                files: 'src/*.scss',
                tasks: ['sass', 'cssmin']
            },
            scripts: {
                files: 'src/**/*.js',
                tasks: ['jshint', 'uglify']
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-jshint');
    grunt.loadNpmTasks('grunt-contrib-uglify');
    grunt.loadNpmTasks('grunt-contrib-sass');
    grunt.loadNpmTasks('grunt-contrib-cssmin');
    grunt.loadNpmTasks('grunt-contrib-watch');

    grunt.registerTask('default', ['jshint', 'uglify', 'cssmin', 'sass']);
};