Gulpfile.js

var gulp = require('gulp');

var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var imagemin = require('gulp-imagemin');
var sourcemaps = require('gulp-sourcemaps');
var del = require('del');

var paths = {
    scripts: ['public/javascripts/*.js'],
    images: 'public/images/**/*'
};

gulp.task('clean', function(cb) {
    del(['build'], cb);
});

gulp.task('scripts', ['clean'], function() {
    return gulp.src(paths.scripts).pipe(sourcemaps.init())
                                  .pipe(uglify())
                                  .pipe(concat('all.min.js'))
                                  .pipe(sourcemaps.write())
                                  .pipe(gulp.dest('build/js'));
});

gulp.task('images', ['clean'], function() {
    return gulp.src(paths.images).pipe(imagemin({optimizationLevel: 5}))
                                 .pipe(gulp.dest('build/img'));
});

gulp.task('watch', function() {
    gulp.watch(paths.scripts, ['scripts']);
    gulp.watch(paths.images, ['images']);
});

gulp.task('default', ['watch', 'scripts', 'images']);