when I was practicing writing an interface with koa2, I encountered an ajax request for GET interface, such as the following

$.ajax({
    type: "GET",
    url:"http://localhost:3000/user_new",
    headers: {
      "Accept": "application/json",
      "Content-Type": "application/json;charset=utf-8",
      "Authorization": "JWT eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoiNTJjYjA5MjFjMjM0NDgzODhjMDQ1Mzg2N2QzZDI0NzUiLCJjb21wYW55X2lkIjoiNzM2ZDc5YjQ1NTkzNDU2NWE4ODljYjJmOTBhOTNlNzIiLCJzdGFmZl9pZCI6IjhlODM2MjFmZjQ3YTRhZGY4NjU4NGNhNWYxNDRmYzc0IiwidGVuYW50X2lkIjoiMTliMDQyMGM3ODViNGNlN2IxODNmMTFjMjY0M2I4YmUifQ.D7Mrba-lB94iSWr2pHAtS4KUkC_g06lJVHutj0MIu1Q"
    },
    success: function (data, textStatus) {
      console.log("123123")

    },
    error: function (a,b,c) {
      console.log("2222222")
    }
})

then a request from OPTIONS occurs, but it can"t be accepted all the time. The code for koa2 is as follows

const fn_usrs = async (ctx, next) => {
  let cb = ctx.request.query.callback;
  if (cb) { //ajax jsonp
    ctx.response.body = cb + "(" + JSON.stringify(jsonData) + ")";
  }else{ //
    // ctx.set("Access-Control-Allow-Origin", "*") //ajax json
    ctx.response.body = "{}"; //proxy 
  }
  
}
const fn_usrs_option = async(ctx, next) => {
  // ctx.set("Access-Control-Allow-Origin", "*");
  if (ctx.request.method == "OPTIONS") {
    ctx.response.status = 200
  }
  ctx.response.status = 200
}

module.exports = {
  "GET /user_new": fn_usrs_new,
  "OPTIONS /user_new": fn_usrs_option,
};

but the browser always shows that 404 can"t be found, and it doesn"t go to "OPTIONS / user_new": fn_usrs_option, nor to this " GET / user_new": fn_usrs_new,

the index.js of the koa2 code is as follows

const Koa = require("koa");
const app = new Koa();
const path = require("path");
const static = require("koa-static");
const bodyParser = require("koa-bodyparser");
const router = require("koa-router")();
const controller = require("./server/index.js");


// 
const staticPath = "./static";
app.use(static(
  path.join( __dirname,  staticPath)
))

// ejs 
const views = require("koa-views")
app.use(views(path.join(__dirname, "./view"), {
  extension: "ejs"
}))
// 
const handler = async (ctx, next) => {
  // log request URL:
  // ctx.set("Access-Control-Allow-Origin", "*");
  // ctx.set("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE");
  console.log(`Process ${ctx.request.method} ${ctx.request.url}`);
  try {
    await next();
    console.log("handler")
  } catch (err) {
    console.log("handler")
    ctx.response.status = err.statusCode || err.status || 500;
    ctx.response.body = {
      message: err.message
    };
  }
};
// catch. onerror ctx.app.emit("error", err, ctx);
// app.on("error", function(err) {
//   console.log("logging error ", err.message);
// });
app.use(handler)


app.use(bodyParser()); // post
app.use(controller()) //  ----

app.listen(3000, () => {
  console.log("app started at port 3000...");
});

the contents of the controller file are as follows

const fs = require("fs");

function addMapping(router, mapping) {
  for (var url in mapping) {
    if (url.startsWith("GET ")) {
      var path = url.substring(4);
      router.get(path, mapping[url]);
      // console.log(`register URL mapping: GET ${path}`);
    } else if (url.startsWith("OPTIONS ")) {
      var path = url.substring(8);
      router.get(path, mapping[url]);

    } else if (url.startsWith("POST ")) {
      var path = url.substring(5);
      router.post(path, mapping[url]);
      // console.log(`register URL mapping: POST ${path}`);
    } else if (url.startsWith("PUT ")) {
      var path = url.substring(4);
      router.put(path, mapping[url]);
      // console.log(`register URL mapping: PUT ${path}`);
    } else if (url.startsWith("DELETE ")) {
      var path = url.substring(7);
      router.del(path, mapping[url]);
      // console.log(`register URL mapping: DELETE ${path}`);
    } else {
      console.log(`invalid URL: ${url}`);
    }
  }
}

function addControllers(router, dir) {
  const files = fs.readdirSync(__dirname + "/" + dir).filter((f) => {
    return f !== "index.js";
  })
  files.forEach((f) => {
    // console.log(`process controller: ${f}...`);
    let mapping = require(__dirname + "/" + f);
    addMapping(router, mapping);
  });
}

module.exports = function(dir) {
  let controllers_dir = dir || "/",
    router = require("koa-router")();
  addControllers(router, controllers_dir);
  return router.routes();
};

is called. I don"t know why? I looked for a lot of reasons, but I couldn"t find