三、跨域服务端代码

const koa = require("koa");
const app =  new koa();
const router  =  require("koa-router")();
router.get("/jsonp",async ctx=>{
    let callbackName = ctx.query.callback || 'callback'
  // 返回jsonp
  var obj ={
      name:"cheng",
      age:18
  }
  ctx.body = callbackName + `(${JSON.stringify(obj)})`
})
app.use(router.routes()).use(router.allowedMethods());
app.listen(7000)