How to check the existence of a server side file on my website server without actually editing it with javascript only. -

this question has answer here:

• synchronous xmlhttprequest on main thread deprecated error 2 answers

i have found many questions on topic including stackoverflow. none of them working me. have compiled following existing answers:

function doesfileexist(urltofile) {    var xhr = new xmlhttprequest();    xhr.open('head', urltofile, false); // => error message on line    xhr.send();     if (xhr.status == "404") {      return false;    }else {      return true;    } } 

then use function call web server follows:

var thesource = "myurl"; var result = doesfileexist(thesource); if (result == true) {     // yes, file exists!     console.log('file exists:', thesource); } else {     // file not exist!     console.log('file not exist:', thesource); } 

the error message this:

synchronous xmlhttprequest on main thread deprecated because of detrimental effects end user's experience. more help, check https://xhr.spec.whatwg.org/.

the documentation here should enough started on asynchronous requests.

https://developer.mozilla.org/en-us/docs/web/api/xmlhttprequest/synchronous_and_asynchronous_requests

if explanation not sufficient, try following tutorial writing simple async functions.

http://cwbuecheler.com/web/tutorials/2013/javascript-callbacks/

here's example might help.

how can take advantage of callback functions asynchronous xmlhttprequest?

you use variation of function presented in last link, this. (edited since original posting)

function getrequeststatus(url, callback)       {          var request = new xmlhttprequest();          request.onreadystatechange = function()          {              if (request.readystate == 4)              {                  callback(request.status);               }            };           request.open('get', url);          request.send();      }     getrequeststatus("http://httpbin.org/status/404", function(status){       if(status == 404){             //it's not found here            console.log("not found")          }      });

but need read articles understand how use function.

once have read articles , run through tutorial may understand syntax below little better. if don't take time understand going on here, struggling next steps how use callbacks in loops etc.