为了提供诸如图像、CSS 文件和 JavaScript 文件之类的静态文件，请使用 Express 中的 express.static 内置中间件函数。
此函数特征如下：
express.static(root, [options])
The root argument specifies the root directory from which to serve static assets.
For more information on the options argument, see express.static.
例如，通过如下代码就可以将 public 目录下的图片、CSS 文件、JavaScript 文件对外开放访问了：
app.use(express.static('public'))
现在，你就可以访问 public 目录中的所有文件了：
http://localhost:3000/images/kitten.jpg
http://localhost:3000/css/style.css
http://localhost:3000/js/app.js
http://localhost:3000/images/bg.png
http://localhost:3000/hello.html
如果要使用多个静态资源目录，请多次调用 express.static 中间件函数：
app.use(express.static('public'))
app.use(express.static('files'))
访问静态资源文件时，express.static 中间件函数会根据目录的添加顺序查找所需的文件。
注意：For best results, use a reverse proxy cache to improve performance of serving static assets.
To create a virtual path prefix (where the path does not actually exist in the file system) for files that are served by the express.static function, specify a mount path for the static directory, as shown below:
app.use('/static', express.static('public'))
现在，你就可以通过带有 /static 前缀地址来访问 public 目录中的文件了。
http://localhost:3000/static/images/kitten.jpg
http://localhost:3000/static/css/style.css
http://localhost:3000/static/js/app.js
http://localhost:3000/static/images/bg.png
http://localhost:3000/static/hello.html
然而，the path that you provide to the express.static function is relative to the directory from where you launch your node process. If you run the express app from another directory, it’s safer to use the absolute path of the directory that you want to serve:
const path = require('path')
app.use('/static', express.static(path.join(__dirname, 'public')))
欲了解更多关于 serve-static 函数及其参数的知识，请参考 serve-static。