How to react to a NS ERROR FAILURE error in the browser's Javascript

Question

In Firefox' JavaScript engine I do a (deprecated) synchronous XMLHttpRequest like this:

var request = new XMLHttpRequest();
request.open('GET', 'http://localhost:8780/BoilerData/invention.html', false);
request.send();

if (request.status === 200) {
  ...
} else {
  ...
}

Problem is when the server myserver.com is not running I get in the browser console a NS ERROR FAILURE error and the else part is not executed.

However I would like to react to this error in JavaScript. How can I catch this error and react to it (e.g. with using another server) while still doing a a synchronous XMLHttpRequest.

Update

Here more details to the error message. In the console of my browser (Firefox 78.4.1esr) I see the following:

Not sure why the use of the old XMLHttpRequest instead of await fetch() is so popular, and even less sure why forcing asynchronous things to be synchronous and blocking the UI is such a trend these days... Synchronous requests were never a thing. It's not really deprecated because it's never been "precated"(?) in the first place. — Jeremy Thille
– Jeremy Thille, Commented Apr 19, 2021 at 6:30

coderLMN · Accepted Answer · 2021-04-19 07:11:35Z

1

If you prefer a synchronous request, try catch the net::ERR_CONNECTION_TIMED_OUT exception:

var request = new XMLHttpRequest();
request.open('GET', 'http:/myserver.com/bar/foo.txt', false);
try{
  request.send();
  if (request.status === 200) {
    //...
  }
}catch(e){
  console.log('Got an exception: ', e);
  //else ...
}

Synchronous XMLHttpRequest on the main thread is deprecated, you may want to do request.open asynchronously, and put the else logic in onerror event processor:

var request = new XMLHttpRequest();
request.open('GET', 'http:/myserver.com/bar/foo.txt', true);

request.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
    //    ...
    } 
}
request.onerror = function() {
   console.log('An error occured! status = ', request.status);
   // what else {
   // ...
}

request.send();

edited Apr 19, 2021 at 7:11

answered Apr 19, 2021 at 6:36

coderLMN

3,0861 gold badge23 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

halloleo Over a year ago

Thanks for the code sample. You use a asynchronous request, right? Does this mean reacting to the error is not possible with a synchronous_ request?

coderLMN Over a year ago

It's possible, but not recommended. See my update.

halloleo Over a year ago

I tried your try ...catch solution, but the browser still quits the JS execution with a NS ERROR FAILURE and does not go through the catch part.

coderLMN Over a year ago

Which kind of browser did you use? I tested it in Edge and Chrome，where it worked fine.

halloleo Over a year ago

Using Firefox 78.4.1esr. Maybe it is a Firefox issue or a version specific problem...

|

Collectives™ on Stack Overflow

How to react to a NS ERROR FAILURE error in the browser's Javascript

Update

1 Answer 1

10 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Update

1 Answer 1

10 Comments

Your Answer

Sign up or log in

Post as a guest

Related