gulpfile.js

'use strict';

var gulp = require('gulp');
var rename = require('gulp-rename');
var uglify = require('gulp-uglify');

gulp.task('build', function() {
    gulp.src('js/script.js')
        .pipe(uglify())
        .pipe(rename({
            suffix: '.min'
        }))
        .pipe(gulp.dest('js'));
});

gulp.task('default', function(){
    gulp.start('build');
});