Ajax原理一篇就够了:https://segmentfault.com/a/1190000017396192(简单明了)
XMLHttpRequest—必知必会:https://www.jianshu.com/p/918c63045bc3/
fetch 如何请求常见数据格式:https://juejin.im/post/6844903619356000263
fetch使用的常见问题及解决办法:https://www.cnblogs.com/wonyun/p/fetch_polyfill_timeout_jsonp_cookie_progress.html
axios和ajax及fetch原理浅析:https://blog.csdn.net/qq_39096319/article/details/82347033
九种跨域方式实现原理(完整版):https://juejin.im/post/6844903767226351623(此文章的参考文章也是可以查看)