koa接口跨域報錯

用koa+mongodb寫了幾個接口,用ajax請求時總是跨域報錯,koa中用了cors也不行,谷歌了好多還是沒有解決問題

const cors = require('koa2-cors');
var MongoClient = require("mongodb").MongoClient;
var DB_URL = "mongodb://localhost:27017/test";
// app.use(cors());
app.use(cors({
    origin: function (ctx) {
            return "*"; // 允許來自所有域名請求
    },
    exposeHeaders: ['WWW-Authenticate', 'Server-Authorization'],
    maxAge: 5,
    credentials: true,
    allowMethods: ['GET', 'POST', 'DELETE'],
    allowHeaders: ['Content-Type', 'Authorization', 'Accept'],
}))
//省略封裝的函數(shù)代碼
app.use(router['routes']());
router.get('/',findFn)
router.get('/add',insertFn)
app.listen(3000);

讓我的localhost:1234/index.html頁面里請求localhost:3000的接口就會報如上圖的錯誤(Failed to load localhost:3000: Cross origin requests are only supported for protocol schemes: http, data, chrome, chrome-extension, https.)

望大家指點一二,謝謝