res.type(type)

将 Content-Type HTTP 标头设置为由指定 type 确定的 MIME 类型。如果 type 包含 "/" 字符，则它将 Content-Type 设置为 type 的确切值，否则假定它是文件扩展名并使用 express.static.mime.lookup() 方法在映射中查找 MIME 类型。

Sets the Content-Type HTTP header to the MIME type as determined by the specified type. If type contains the "/" character, then it sets the Content-Type to the exact value of type, otherwise it is assumed to be a file extension and the MIME type is looked up in a mapping using the express.static.mime.lookup() method.

res.type('.html') // => 'text/html'
res.type('html') // => 'text/html'
res.type('json') // => 'application/json'
res.type('application/json') // => 'application/json'
res.type('png') // => image/png: