How to tell if a <script> tag failed to load

Question

I'm dynamically adding <script> tags to a page's <head>, and I'd like to be able to tell whether the loading failed in some way -- a 404, a script error in the loaded script, whatever.

In Firefox, this works:

var script_tag = document.createElement('script');
script_tag.setAttribute('type', 'text/javascript');
script_tag.setAttribute('src', 'http://fail.org/nonexistant.js');
script_tag.onerror = function() { alert("Loading failed!"); }
document.getElementsByTagName('head')[0].appendChild(script_tag);

However, this doesn't work in IE or Safari.

Does anyone know of a way to make this work in browsers other than Firefox?

(I don't think a solution that requires placing special code within the .js files is a good one. It's inelegant and inflexible.)

Big website, automatic loading of dependencies for content loaded through ajax. if+polling is annoying cruft that I don't want to have to put into all JS. — David, Commented Feb 11, 2009 at 21:49
You might also want to check for load failure when making a JSONP request using script tag injection... — schellsan, Commented Jan 23, 2012 at 17:57

andrerom · Accepted Answer · 2021-12-09 12:19:38Z

47

UPDATE 2021: All browsers today support onerror="" on script tags, examples:

Building script tag in js on MDN
Html example by @Rudey in comments: <script src="nonexistent.js" onerror="alert('error!')"></script>

Original comment from 2010:

If you only care about html5 browsers you can use error event.

From the spec:

If the src attribute's value is the empty string or if it could not be resolved, then the user agent must queue a task to fire a simple event named error at the element, and abort these steps.

(...)

If the load resulted in an error (for example a DNS error, or an HTTP 404 error) Executing the script block must just consist of firing a simple event named error at the element.

This means you don't have to do any error prone polling and can combine it with async and defer attribute to make sure the script is not blocking page rendering:

The defer attribute may be specified even if the async attribute is specified, to cause legacy Web browsers that only support defer (and not async) to fall back to the defer behavior instead of the synchronous blocking behavior that is the default.

More on http://www.w3.org/TR/html5/scripting-1.html#script

edited Dec 9, 2021 at 12:19

answered Nov 12, 2010 at 10:08

andrerom

5634 silver badges5 bronze badges

1

Any idea of JSONP implementations, like that in jQuery support this? Everything I've read here about detecting that JSONP failures to load says it can't be detected but a timeout can be a workaround, and a timeout was added to a recent version of JSONP. It seems that this answer provides something better than the timeout though - is that correct?
– hippietrail
Commented Dec 26, 2011 at 21:29
1

example possible please?
– rogerdpack
Commented Oct 27, 2016 at 20:23
18

<script src="nonexistent.js" onerror="alert('error!')"></script> fiddle
– Rudey
Commented Feb 17, 2017 at 10:48
1

@Rudey Uncaught SyntaxError: Unexpected token '<' (in latest Chrome)
– daleyjem
Commented Feb 29, 2020 at 5:25
@daleyjem sounds like you've attempted to put a HTML tag in JavaScript. <script src="nonexistent.js" onerror="alert('error!')"></script> should go in your HTML file, not JS.
– Rudey
Commented Mar 3, 2020 at 16:18

Add a comment |

Diodeus - James MacFarlane · Accepted Answer · 2009-02-11 21:17:54Z

35

There is no error event for the script tag. You can tell when it is successful, and assume that it has not loaded after a timeout:

<script type="text/javascript" onload="loaded=1" src="....js"></script>

edited Feb 11, 2009 at 21:17

answered Feb 11, 2009 at 21:12

Diodeus - James MacFarlane

114k33 gold badges160 silver badges179 bronze badges

8

The "onload" listener will be fired even if there's a javascript error.
– Luca Matteis
Commented Feb 11, 2009 at 21:38
44

Well, yes if the file LOADS and there is an error in the file itself, but if the file is not served up, the onload will never fire.
– Diodeus - James MacFarlane
Commented Feb 11, 2009 at 21:49
1

yes, true, that's why I asked to be more specific of what kind of error he's looking for.
– Luca Matteis
Commented Feb 11, 2009 at 21:51
18

There is onerror evnt for script tag. It will fire when the resource not found.
– Nguyen Tran
Commented Dec 22, 2017 at 8:59
2

@Diodeus-JamesMacFarlane as others have commented, there is an error event, at least in all modern browsers - down voted for incorrectly stating "There is no error event for the script tag". Please update or remove this outdated/wrong answer.
– mindplay.dk
Commented Oct 28, 2021 at 13:11

| Show 4 more comments

Gibin Ealias · Accepted Answer · 2022-12-10 02:26:57Z

30

my working clean solution (2017)

function loaderScript(scriptUrl){
   return new Promise(function (res, rej) {
    let script = document.createElement('script');
    script.src = scriptUrl;
    script.type = 'text/javascript';
    script.onerror = rej;
    script.async = true;
    script.onload = res;
    script.addEventListener('error',rej);
    script.addEventListener('load',res);
    document.head.appendChild(script);
 })

}

As Martin pointed, used like that:

const event = loaderScript("myscript.js")
  .then(() => { console.log("loaded"); })
  .catch(() => { console.log("error"); });

OR

try{
 await loaderScript("myscript.js")
 console.log("loaded"); 
}catch{
 console.log("error");
}

edited Dec 10, 2022 at 2:26

Gibin Ealias

2,8315 gold badges23 silver badges41 bronze badges

answered Jun 2, 2017 at 9:45

pery mimon

8,1456 gold badges55 silver badges64 bronze badges

1

Use it as follows: loaderScript("myscript.js").then(() => { console.log("loaded"); }).catch(() => { console.log("error"); });
– Martin Wantke
Commented Sep 20, 2017 at 17:28

Add a comment |

Aram Kocharyan · Accepted Answer · 2013-05-11 13:15:48Z

The script from Erwinus works great, but isn't very clearly coded. I took the liberty to clean it up and decipher what it was doing. I've made these changes:

Meaningful variable names
Use of prototype.
require() uses an argument variable
No alert() messages are returned by default
Fixed some syntax errors and scope issues I was getting

Thanks again to Erwinus, the functionality itself is spot on.

function ScriptLoader() {
}

ScriptLoader.prototype = {

    timer: function (times, // number of times to try
                     delay, // delay per try
                     delayMore, // extra delay per try (additional to delay)
                     test, // called each try, timer stops if this returns true
                     failure, // called on failure
                     result // used internally, shouldn't be passed
            ) {
        var me = this;
        if (times == -1 || times > 0) {
            setTimeout(function () {
                result = (test()) ? 1 : 0;
                me.timer((result) ? 0 : (times > 0) ? --times : times, delay + ((delayMore) ? delayMore : 0), delayMore, test, failure, result);
            }, (result || delay < 0) ? 0.1 : delay);
        } else if (typeof failure == 'function') {
            setTimeout(failure, 1);
        }
    },

    addEvent: function (el, eventName, eventFunc) {
        if (typeof el != 'object') {
            return false;
        }

        if (el.addEventListener) {
            el.addEventListener(eventName, eventFunc, false);
            return true;
        }

        if (el.attachEvent) {
            el.attachEvent("on" + eventName, eventFunc);
            return true;
        }

        return false;
    },

    // add script to dom
    require: function (url, args) {
        var me = this;
        args = args || {};

        var scriptTag = document.createElement('script');
        var headTag = document.getElementsByTagName('head')[0];
        if (!headTag) {
            return false;
        }

        setTimeout(function () {
            var f = (typeof args.success == 'function') ? args.success : function () {
            };
            args.failure = (typeof args.failure == 'function') ? args.failure : function () {
            };
            var fail = function () {
                if (!scriptTag.__es) {
                    scriptTag.__es = true;
                    scriptTag.id = 'failed';
                    args.failure(scriptTag);
                }
            };
            scriptTag.onload = function () {
                scriptTag.id = 'loaded';
                f(scriptTag);
            };
            scriptTag.type = 'text/javascript';
            scriptTag.async = (typeof args.async == 'boolean') ? args.async : false;
            scriptTag.charset = 'utf-8';
            me.__es = false;
            me.addEvent(scriptTag, 'error', fail); // when supported
            // when error event is not supported fall back to timer
            me.timer(15, 1000, 0, function () {
                return (scriptTag.id == 'loaded');
            }, function () {
                if (scriptTag.id != 'loaded') {
                    fail();
                }
            });
            scriptTag.src = url;
            setTimeout(function () {
                try {
                    headTag.appendChild(scriptTag);
                } catch (e) {
                    fail();
                }
            }, 1);
        }, (typeof args.delay == 'number') ? args.delay : 1);
        return true;
    }
};

$(document).ready(function () {
    var loader = new ScriptLoader();
    loader.require('resources/templates.js', {
        async: true, success: function () {
            alert('loaded');
        }, failure: function () {
            alert('NOT loaded');
        }
    });
});

I was forced to use jQuery's $.getScript, as this function did fail for cached scripts in MSIE8-, unfortunatelly — Zathrus Writer, Commented Jun 4, 2013 at 10:35
@ZathrusWriter that's probably a better idea, thanks for letting us know! You could probably add a random int to the url in this code and it would remove the caching. — Aram Kocharyan, Commented Jun 4, 2013 at 14:23
Yes Aram, that would certainly do the trick, however it would also invalidate browser's caching, so the overhead in such case is probably not really worth it ;) — Zathrus Writer, Commented Jun 4, 2013 at 16:14
@ZathrusWriter, so how does the workings of the jQuery implementation work? — Pacerier, Commented May 7, 2014 at 9:02
@Pacerier sorry, I'm not a jQuery core developer, so I can't really answer that — Zathrus Writer, Commented May 7, 2014 at 9:11

Optimus · Accepted Answer · 2016-05-03 13:57:14Z

I know this is an old thread but I got a nice solution to you (I think). It's copied from an class of mine, that handles all AJAX stuff.

When the script cannot be loaded, it set an error handler but when the error handler is not supported, it falls back to a timer that checks for errors for 15 seconds.

function jsLoader()
{
    var o = this;

    // simple unstopable repeat timer, when t=-1 means endless, when function f() returns true it can be stopped
    o.timer = function(t, i, d, f, fend, b)
    {
        if( t == -1 || t > 0 )
        {
            setTimeout(function() {
                b=(f()) ? 1 : 0;
                o.timer((b) ? 0 : (t>0) ? --t : t, i+((d) ? d : 0), d, f, fend,b );
            }, (b || i < 0) ? 0.1 : i);
        }
        else if(typeof fend == 'function')
        {
            setTimeout(fend, 1);
        }
    };

    o.addEvent = function(el, eventName, eventFunc)
    {
        if(typeof el != 'object')
        {
            return false;
        }

        if(el.addEventListener)
        {
            el.addEventListener (eventName, eventFunc, false);
            return true;
        }

        if(el.attachEvent)
        {
            el.attachEvent("on" + eventName, eventFunc);
            return true;
        }

        return false;
    };

    // add script to dom
    o.require = function(s, delay, baSync, fCallback, fErr)
    {
        var oo = document.createElement('script'),
        oHead = document.getElementsByTagName('head')[0];
        if(!oHead)
        {
            return false;
        }

        setTimeout( function() {
            var f = (typeof fCallback == 'function') ? fCallback : function(){};
            fErr = (typeof fErr == 'function') ? fErr : function(){
                alert('require: Cannot load resource -'+s);
            },
            fe = function(){
                if(!oo.__es)
                {
                    oo.__es = true;
                    oo.id = 'failed';
                    fErr(oo);
                }
            };
            oo.onload = function() {
                oo.id = 'loaded';
                f(oo);
            };
            oo.type = 'text/javascript';
            oo.async = (typeof baSync == 'boolean') ? baSync : false;
            oo.charset = 'utf-8';
            o.__es = false;
            o.addEvent( oo, 'error', fe ); // when supported

            // when error event is not supported fall back to timer
            o.timer(15, 1000, 0, function() {
                return (oo.id == 'loaded');
            }, function(){ 
                if(oo.id != 'loaded'){
                    fe();
                }
            });
            oo.src = s;
            setTimeout(function() {
                try{
                    oHead.appendChild(oo);
                }catch(e){
                    fe();
                }
            },1); 
        }, (typeof delay == 'number') ? delay : 1);  
        return true;
    };

}

$(document).ready( function()
{
    var ol = new jsLoader();
    ol.require('myscript.js', 800, true, function(){
        alert('loaded');
    }, function() {
        alert('NOT loaded');
    });
});

@Erwinus I did not check headers, it was just a quick cross-browser check that I performed and $.getScript always ran without problems, so I stuck with it... you are welcome to try yourself, I'm using W7 and XAMPP here — Zathrus Writer, Commented Jun 5, 2013 at 13:20
Whether this works or not, it's damned painful to read - let alone debug. That's some pretty poor formatting and truly atrocious coding style. The variable naming alone is a mess. — Vala, Commented Nov 21, 2014 at 11:26
If you don't see the value of readable code (and you could make that readable without any changes to actually executed code), then I feel sorry for every single person that has to work with you and your code. — Vala, Commented Nov 22, 2014 at 23:07

Luca Matteis · Accepted Answer · 2009-02-11 21:57:14Z

10

To check if the javascript in nonexistant.js returned no error you have to add a variable inside http://fail.org/nonexistant.js like var isExecuted = true; and then check if it exists when the script tag is loaded.

However if you only want to check that the nonexistant.js returned without a 404 (meaning it exists), you can try with a isLoaded variable ...

var isExecuted = false;
var isLoaded = false;
script_tag.onload = script_tag.onreadystatechange = function() {
    if(!this.readyState ||
        this.readyState == "loaded" || this.readyState == "complete") {
        // script successfully loaded
        isLoaded = true;

        if(isExecuted) // no error
    }
}

This will cover both cases.

edited Feb 11, 2009 at 21:57

answered Feb 11, 2009 at 21:13

Luca Matteis

29.2k20 gold badges115 silver badges170 bronze badges

1

It works, but doesn't really do what I want -- a failure in that case would never fire an event. I don't want to have to poll for a variable and, if it's still false after a few seconds, fire the on-fail callback.
– David
Commented Feb 11, 2009 at 21:43
What kind of error are you trying to retrieve? Failure to load? Failure to execute the javascript in nonexstant.js? HTTP Response Error? Please be more descriptive.
– Luca Matteis
Commented Feb 11, 2009 at 21:45
Any of the above would be good. I specifically care about a 404 error, though.
– David
Commented Feb 11, 2009 at 21:50
404, meaning you want to check if the file nonexistant.js doesn't exist? But if it does you want to check if it returned no error?
– Luca Matteis
Commented Feb 11, 2009 at 21:52
Hey David, you should be able to assume that if this.readyState/... is not true, then the script failed to load. Basically an onError.
– Cody
Commented Aug 16, 2012 at 23:44

| Show 1 more comment

rogerdpack · Accepted Answer · 2016-10-27 20:25:16Z

I hope this doesn't get downvoted, because in special circumstances it is the most reliable way to solve the problem. Any time the server allows you to get a Javascript resource using CORS (http://en.wikipedia.org/wiki/Cross-origin_resource_sharing), you have a rich array of options to do so.

Using XMLHttpRequest to fetch resources will work across all modern browsers, including IE. Since you are looking to load Javascript, you have Javascript available to you in the first place. You can track the progress using the readyState (http://en.wikipedia.org/wiki/XMLHttpRequest#The_onreadystatechange_event_listener). Finally, once you receive the content of the file, you can execute it with eval ( ). Yes, I said eval -- because security-wise it is no different from loading the script normally. In fact, a similar technique is suggested by John Resig to have nicer tags (http://ejohn.org/blog/degrading-script-tags/).

This method also lets you separate the loading from the eval, and execute functions before and after the eval happens. It becomes very useful when loading scripts in parallel but evaluating them one after the other -- something browsers can do easily when you place the tags in HTML, but don't let you by adding scripts at run-time with Javascript.

CORS is also preferable to JSONP for loading scripts (http://en.wikipedia.org/wiki/XMLHttpRequest#Cross-domain_requests). However, if you are developing your own third-party widgets to be embedded in other sites, you should actually load the Javascript files from your own domain in your own iframe (again, using AJAX)

In short:

Try to see if you can load the resource using AJAX GET
Use eval after it has successfully loaded

To improve it:

Check out the cache-control headers being sent
Look into otherwise caching the content in localStorage, if you need to
Check out Resig's "degrading javascript" for cleaner code
Check out require.js

NB that this way requires the origin server to have CORS enabled, which is not always the case or controllable :| — rogerdpack, Commented Oct 27, 2016 at 20:34

Gautam Tandon · Accepted Answer · 2013-01-12 17:53:22Z

5

This trick worked for me, although I admit that this is probably not the best way to solve this problem. Instead of trying this, you should see why the javascripts aren't loading. Try keeping a local copy of the script in your server, etc. or check with the third party vendor from where you are trying to download the script.

Anyways, so here's the workaround: 1) Initialize a variable to false 2) Set it to true when the javascript loads (using the onload attribute) 3) check if the variable is true or false once the HTML body has loaded

<html>
  <head>
    <script>
      var scriptLoaded = false;

      function checkScriptLoaded() {
        if (scriptLoaded) {
          // do something here
        } else {
          // do something else here!
        }
      }
    </script>
    <script src="http://some-external-script.js" onload="scriptLoaded=true;" />
  </head>
  <body onload="checkScriptLoaded()">
    <p>My Test Page!</p>
  </body>
</html>

answered Jan 12, 2013 at 17:53

Gautam Tandon

2442 silver badges2 bronze badges

1

What if the script is still loading? How do you know if the file was not found or if you just have to wait?
– Sergey Orshanskiy
Commented Oct 14, 2013 at 3:28
This solution works in my testing, as long as the script tag is placed directly in the document source. The document's onload event doesn't fire until all resources referenced in the page source have already loaded (or failed). If you need to insert scripts into the DOM later, though, you need another approach.
– Jamey Sharp
Commented Nov 27, 2013 at 18:29
Great solution. I don't understand why it hasn't been upvoted.
– Lotfi
Commented Nov 11, 2014 at 8:35
This assumes that the script will be loaded synchronously blocking everything else, which is not always true. In fact, it is not a recommended way of loading scripts.
– Vitim.us
Commented Jun 25, 2019 at 20:02

Add a comment |

Community · Accepted Answer · 2017-05-23 12:34:19Z

Here is another JQuery-based solution without any timers:

<script type="text/javascript">
function loadScript(url, onsuccess, onerror) {
$.get(url)
    .done(function() {
        // File/url exists
        console.log("JS Loader: file exists, executing $.getScript "+url)
        $.getScript(url, function() {
            if (onsuccess) {
                console.log("JS Loader: Ok, loaded. Calling onsuccess() for " + url);
                onsuccess();
                console.log("JS Loader: done with onsuccess() for " + url);
            } else {
                console.log("JS Loader: Ok, loaded, no onsuccess() callback " + url)
            }
        });
    }).fail(function() {
            // File/url does not exist
            if (onerror) {
                console.error("JS Loader: probably 404 not found. Not calling $.getScript. Calling onerror() for " + url);
                onerror();
                console.error("JS Loader: done with onerror() for " + url);
            } else {
                console.error("JS Loader: probably 404 not found. Not calling $.getScript. No onerror() callback " + url);
            }
    });
}
</script>

Thanks to: https://stackoverflow.com/a/14691735/1243926

Sample usage (original sample from JQuery getScript documentation):

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>jQuery.getScript demo</title>
  <style>
  .block {
     background-color: blue;
     width: 150px;
     height: 70px;
     margin: 10px;
  }
  </style>
  <script src="http://code.jquery.com/jquery-1.9.1.js"></script>
</head>
<body>

<button id="go">&raquo; Run</button>
<div class="block"></div>

<script>


function loadScript(url, onsuccess, onerror) {
$.get(url)
    .done(function() {
        // File/url exists
        console.log("JS Loader: file exists, executing $.getScript "+url)
        $.getScript(url, function() {
            if (onsuccess) {
                console.log("JS Loader: Ok, loaded. Calling onsuccess() for " + url);
                onsuccess();
                console.log("JS Loader: done with onsuccess() for " + url);
            } else {
                console.log("JS Loader: Ok, loaded, no onsuccess() callback " + url)
            }
        });
    }).fail(function() {
            // File/url does not exist
            if (onerror) {
                console.error("JS Loader: probably 404 not found. Not calling $.getScript. Calling onerror() for " + url);
                onerror();
                console.error("JS Loader: done with onerror() for " + url);
            } else {
                console.error("JS Loader: probably 404 not found. Not calling $.getScript. No onerror() callback " + url);
            }
    });
}


loadScript("https://raw.github.com/jquery/jquery-color/master/jquery.color.js", function() {
  console.log("loaded jquery-color");
  $( "#go" ).click(function() {
    $( ".block" )
      .animate({
        backgroundColor: "rgb(255, 180, 180)"
      }, 1000 )
      .delay( 500 )
      .animate({
        backgroundColor: "olive"
      }, 1000 )
      .delay( 500 )
      .animate({
        backgroundColor: "#00f"
      }, 1000 );
  });
}, function() { console.error("Cannot load jquery-color"); });


</script>
</body>
</html>

Nice Sergey! So, you're using .get() to check if the file exists and .getScript() to check to see if it can be executed/loaded without errors? — jjwdesign, Commented Nov 13, 2013 at 18:24
Yes. As I recall, this approach does have limitations: you won't be able to load many scripts in this way --- if I remember correctly, because of cross-domain scripting restrictions. — Sergey Orshanskiy, Commented Nov 14, 2013 at 0:47
The problem with using jQuery (or another library) is that you need to first load that library. So how do you check if that library failed to load? — Pacerier, Commented May 7, 2014 at 9:04
I've taken this approach as well, but I recommend using cache:true for the $.getScript call (this is off by default). This way, for most requests, it automatically uses the cached version for the getScript call (saving you latency) — Laurens Rietveld, Commented Dec 10, 2014 at 16:32

Lahiru Mirihagoda · Accepted Answer · 2020-11-13 07:02:30Z

This can be done safely using promises

    function loadScript(src) {
      return new Promise(function(resolve, reject) {
        let script = document.createElement('script');
        script.src = src;
    
        script.onload = () => resolve(script);
        script.onerror = () => reject(new Error("Script load error: " + src));
    
        document.head.append(script);
      });
    }

and use like this

    let promise = loadScript("https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js");
    
    promise.then(
      script => alert(`${script.src} is loaded!`),
      error => alert(`Error: ${error.message}`)
    );

William Monahan · Accepted Answer · 2019-04-24 12:09:52Z

onerror Event

*Update August 2017: onerror is fired by Chrome and Firefox. onload is fired by Internet Explorer. Edge fires neither onerror nor onload. I wouldnt use this method but it could work in some cases. See also

*

Definition and Usage The onerror event is triggered if an error occurs while loading an external file (e.g. a document or an image).

Tip: When used on audio/video media, related events that occurs when there is some kind of disturbance to the media loading process, are:

onabort
onemptied
onstalled
onsuspend

In HTML:

element onerror="myScript">

In JavaScript, using the addEventListener() method:

object.addEventListener("error", myScript);

Note: The addEventListener() method is not supported in Internet Explorer 8 and earlier versions.

Example Execute a JavaScript if an error occurs when loading an image:

img src="image.gif" onerror="myFunction()">

user42092user42092 · Accepted Answer · 2009-02-11 21:18:45Z

1

The reason it doesn't work in Safari is because you're using attribute syntax. This will work fine though:

script_tag.addEventListener('error', function(){/*...*/}, true);

...except in IE.

If you want to check the script executed successfully, just set a variable using that script and check for it being set in the outer code.

answered Feb 11, 2009 at 21:18

user42092

That actually doesn't make any difference for Safari -- it still doesn't fire the error handler.
– David
Commented Feb 11, 2009 at 21:35
I had a problem with an onclick handler in safari and this was what fixed it, so I thought it'd be the same for onerror too. Apparently not...
– user42092
Commented Feb 11, 2009 at 21:53
1

Should this work for statically included script tags too, or just ones injected dynamically? I just tried to use it to detect if jQuery failed to load but it didn't work. I'm not sure if I'm doing it wrong or it just doesn't work in this case.
– hippietrail
Commented Dec 26, 2011 at 21:50
1

@hippietrail - an event listener will never work on a static HTML <script>. If you try to add it before then you get an error that the tag doesn't exist, and if you add it after then the script's already run and triggered its events. The only way there is to look for side effects caused by the script.
– user42092
Commented Jan 1, 2012 at 3:12
Thanks flussence. Too bad that this only works for 404, not for JS errors. Setting a variable inside the script is not always practicable.
– Jo Liss
Commented Mar 23, 2012 at 17:06

Add a comment |

Alan Wendt · Accepted Answer · 2020-09-18 17:05:53Z

This doesn't need jquery, doesn't need to load the script async, needs no timer nor to have the loaded script set a value. I've tested it in FF, Chrome, and Safari.

<script>
        function loadScript(src) {
          return new Promise(function(resolve, reject) {

            let s = window.document.createElement("SCRIPT");

            s.onload = () => resolve(s);
            s.onerror = () => reject(new Error(src));
            s.src = src;
            // don't bounce to global handler on 404.
            s.addEventListener('error', function() {});
            window.document.head.append(s);
          }); 
        }   
                        
        let successCallback = (result) => {
          console.log(scriptUrl + " loaded.");
        }   
                        
        let failureCallback = (error) => {
          console.log("load failed: " + error.message);
        }   
                        
        loadScript(scriptUrl).then(successCallback, failureCallback);
</script>

sth · Accepted Answer · 2010-01-29 03:35:46Z

0

It was proposed to set a timeout and then assume load failure after a timeout.

setTimeout(fireCustomOnerror, 4000);

The problem with that approach is that the assumption is based on chance. After your timeout expires, the request is still pending. The request for the pending script may load, even after the programmer assumed that load won't happen.

If the request could be canceled, then the program could wait for a period, then cancel the request.

edited Jan 29, 2010 at 3:35

sth

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answered Jan 28, 2010 at 23:24

Garrett

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Mi G · Accepted Answer · 2018-12-23 17:25:05Z

This is how I used a promise to detect loading errors that are emited on the window object:

<script type='module'>
window.addEventListener('error', function(error) {
  let url = error.filename
  url = url.substring(0, (url.indexOf("#") == -1) ? url.length : url.indexOf("#"));
  url = url.substring(0, (url.indexOf("?") == -1) ? url.length : url.indexOf("?"));
  url = url.substring(url.lastIndexOf("/") + 1, url.length);
  window.scriptLoadReject && window.scriptLoadReject[url] && window.scriptLoadReject[url](error);
}, true);
window.boot=function boot() {
  const t=document.createElement('script');
  t.id='index.mjs';
  t.type='module';
  new Promise((resolve, reject) => {
    window.scriptLoadReject = window.scriptLoadReject || {};
    window.scriptLoadReject[t.id] = reject;
    t.addEventListener('error', reject);
    t.addEventListener('load', resolve); // Careful load is sometimes called even if errors prevent your script from running! This promise is only meant to catch errors while loading the file.
  }).catch((value) => {
    document.body.innerHTML='Error loading ' + t.id + '! Please reload this webpage.<br/>If this error persists, please try again later.<div><br/>' + t.id + ':' + value.lineno + ':' + value.colno + '<br/>' + (value && value.message);
  });
  t.src='./index.mjs'+'?'+new Date().getTime();
  document.head.appendChild(t);
};
</script>
<script nomodule>document.body.innerHTML='This website needs ES6 Modules!<br/>Please enable ES6 Modules and then reload this webpage.';</script>
</head>

<body onload="boot()" style="margin: 0;border: 0;padding: 0;text-align: center;">
  <noscript>This website needs JavaScript!<br/>Please enable JavaScript and then reload this webpage.</noscript>

Bhargav Rao · Accepted Answer · 2019-04-05 18:00:40Z

0

Well, the only way I can think of doing everything you want is pretty ugly. First perform an AJAX call to retrieve the Javascript file contents. When this completes you can check the status code to decide if this was successful or not. Then take the responseText from the xhr object and wrap it in a try/catch, dynamically create a script tag, and for IE you can set the text property of the script tag to the JS text, in all other browsers you should be able to append a text node with the contents to script tag. If there's any code that expects a script tag to actually contain the src location of the file, this won't work, but it should be fine for most situations.

edited Apr 5, 2019 at 18:00

Bhargav Rao

51.6k29 gold badges124 silver badges141 bronze badges

answered Feb 17, 2010 at 21:00

nickjsify

397 bronze badges

This solution is way obsolete with modern browsers.
– Simon_Weaver
Commented Sep 22, 2019 at 1:43

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