解决跨域

const app = Koa()
app.use((req, res, next) => {
  // res.header("Access-Control-Allow-Origin", "*")
  res.header("Access-Control-Allow-Origin", "http://localhost:8080")
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Content-Length, Accept, Authorization")
  res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS,HEAD")
  res.header("X-Powered-By",' 3.2.1')
  res.header("Content-Type", "application/json;charset=utf-8")
  // 支持 cookie
  res.header('Access-Control-Allow-Credentials', true)
  // 屏蔽 OPTIONS
  if (req.method.toLowerCase() === 'options') {
    return res.send()
  }
  next()
})