The use of Ajax and fetch

Traditional Ajax usage var xhr = new XMLHttpRequest (); xhr.responsetype = ' json '; xhr.timeout = 2000;console.log (xhr.readystate);//    0xhr.open (' GET ', url); Console.log (xhr.readystate);//1xhr.onloadstart = function (e) {xhr.abort (); Console.log (e);}; xhr.onprogress = function (e) {console.log (xhr.readystate);//3 Console.log (e);};    Xhr.onload = function (res) {Console.log (res); Console.log (xhr.readystate);//4};xhr.onerror = function (err) {Console.log (err);}; Xhr.ontimeout = function (e) {console.log (e);}; Xhr.onabort = function (e) {console.log (e);}; xhr.onloaded = function (e) {console.log (e);};    Xhr.send ()///Traditional Ajax usage function req (URL, data, callback) {var xhr = new XMLHttpRequest ();    Xhr.open (' POST ', url); Xhr.onreadystatechange = function () {if (xhr.readystate = = 4 && Xhr.status = =) {var res =            Json.parse (Request.responsetext);        Callback (RES);    }    }; Post Xhr.setrequestheader (' Content-type ', ' application/x-www-fOrm-urlencoded ');    Xhr.send (data);    Get Xhr.setrequestheader ("Content-type", "text/xml;charset=utf-8"); Xhr.send (null);} Req (URL, data, callback ());//Alternate Ajax Fetchfetch ('/users.json ', {method: ' POST ', body:data, mode: ' Cors ', red Irect: ' Follow ', headers:new headers ({' Content-type ': ' Text/plain '})}). Then (function (res) {Console.lo g (res);}). Then (function (res) {Console.log (res);}). catch (function (err) {Console.log (err);});        /Use Async,await's Fetchasync function req (URL, data) {try {Let response = await fetch (URL, data);        Let data = await Response.json ();    Console.log (data);    } catch (Err) {Console.log (err); }}req (' https://www.baidu.com ', data);

　　

The use of Ajax and fetch