javascript - E6 Ajax request not working -

i have script want fetch json data endpoint, error, script:

function httpget(url) {     return new promise(         function (resolve, reject) {             var request = new xmlhttprequest();             request.onreadystatechange = function () {                 if (this.status === 200) {                     // success                     resolve(this.response);                 } else {                     // went wrong (404 etc.)                     reject(new error(this.statustext));                 }             }             request.onerror = function () {                 reject(new error(                     'xmlhttprequest error: '+this.statustext));             };             request.open('get', url);             request.send();         }); }  var url = 'https://api.jsonplaceholder.com';  httpget(url)   .then(json.parse)   .then((r) => {     console.log(r);   }).catch(function(error) {     console.log(error);   }); 

then, in console throws error:

error @ xmlhttprequest.request.onreadystatechange (app.js:11) @ app.js:18 @ promise () @ httpget (app.js:2) @ app.js:25

you can't check status, have check readystate == 4 before can decide happened:

function httpget(url) {     return new promise(         function (resolve, reject) {             var request = new xmlhttprequest();             request.onreadystatechange = function () {                 if (this.readystate === 4) {                            // ***                     if (this.status === 200) {                          // success                         resolve(this.response);                     } else {                         // went wrong (404 etc.)                         reject(new error(this.statustext));                     }                 }                                                       // ***             }             request.onerror = function () {                 reject(new error(                     'xmlhttprequest error: '+this.statustext));             };             request.open('get', url);             request.send();         }); } 

the spec says accessing this.response before request complete should return empty string, me using chrome v57, doing gives error quoted. adding readystate check fixes it.