demo2
 -----dist
 -----src
     -css
     -img
     -js
 -----gulpfile.js
 -----package.json

 var Oper = {
   add : function( n1, n2 ){
     return n1 + n2;
   },
   sbb : function( n1, n2 ){
     return n1 - n2;
   }
 }

 var gulp = require( 'gulp' );
 var uglify = require( 'gulp-uglify' );
 
 gulp.task('min-js', function() {
   gulp.src('src/js/*.js')
     .pipe( uglify() )
     .pipe( gulp.dest('dist/js') );
 });

var Oper={add:function(n,r){return n+r},sbb:function(n,r){return n-r}};

 <script src="./dist/js/lib.js"></script>
   <script>
     alert( Oper.add( 100, 200 ) );
   </script>

html,body {
  margin:0;
  padding:0;
}
li {
  list-style-type:none;
}
a{
  text-decoration: none;
  color:#666;
}

var gulp = require( 'gulp' );
var uglify = require( 'gulp-uglify' );
var cleanCSS = require( 'gulp-clean-css' );

gulp.task('min-js', function() {
  gulp.src('src/js/*.js')
    .pipe( uglify() )
    .pipe( gulp.dest('dist/js') );
});

gulp.task('min-css', function() {
  gulp.src('src/css/*.css')
    .pipe( cleanCSS() )
    .pipe( gulp.dest('dist/css') );
});

 body,html{margin:0;padding:0}li{list-style-type:none}a{text-decoration:none;color:#666}

@c1 : green;
@c2 : red;
div {
  width:200px;
  height:200px;
  background:@c1;
  div {
    background:@c2;
    transition:all ease 1s;
  }
}

var gulp = require( 'gulp' );
var uglify = require( 'gulp-uglify' );
var cleanCSS = require( 'gulp-clean-css' );
var less = require('gulp-less');

gulp.task('min-js', function() {
  gulp.src('src/js/*.js')
    .pipe( uglify() )
    .pipe( gulp.dest('dist/js') );
});

gulp.task('min-css', function() {
  gulp.src('src/css/*.css')
    .pipe( cleanCSS() )
    .pipe( gulp.dest('dist/css') );
});

gulp.task( 'compile-less', function(){
  gulp.src( 'src/css/*.less' )
    .pipe( less() )
    .pipe( gulp.dest( 'dist/less' ) );
} );

 div {
  width: 200px;
  height: 200px;
  background: green;
 }
 div div {
  background: red;
  transition: all ease 1s;
 }

 function $( id ){
   return document.getElementById( id );
 }

var gulp = require( 'gulp' );
var uglify = require( 'gulp-uglify' );
var cleanCSS = require( 'gulp-clean-css' );
var less = require('gulp-less');
var concat = require('gulp-concat');

gulp.task('min-js', function() {
  gulp.src('src/js/*.js')
    .pipe( uglify() )
    .pipe( gulp.dest('dist/js') );
});

gulp.task( 'concat-file', function(){
  gulp.src( 'src/js/*.js' )
    .pipe( concat('all.min.js') )
    .pipe( uglify() )
    .pipe( gulp.dest( 'dist/js' ) );
} );

gulp.task('min-css', function() {
  gulp.src('src/css/*.css')
    .pipe( cleanCSS() )
    .pipe( gulp.dest('dist/css') );
});

gulp.task( 'compile-less', function(){
  gulp.src( 'src/css/*.less' )
    .pipe( less() )
    .pipe( gulp.dest( 'dist/less' ) );
} );

 function $(n){return document.getElementById(n)}var Oper={add:function(n,t){return n+t},sbb:function(n,t){return n-t}};

 gulp.task( 'auto', function(){
   gulp.watch( 'src/js/*.js', ['min-js'] );
   gulp.watch( 'src/css/*.css', ['min-css'] );
 } );
 gulp.task( 'default', ['auto'] );