Detect whether external script has loaded

Question

Using JavaScript, is there a way to detect whether or not an external script (from a third-party vendor) has completely loaded?

The script in question is used to pull in and embed the markup for a list of jobs and, unfortunately, doesn't make use of any variables or functions. It uses document.write to output all of the content that gets embedded in my page.

Ideally, I'd like to display some kind of loading message while I'm waiting for the external script to load, and if it fails to load, display a "We're sorry, check back later..." message.

I'm using jQuery on the site, but this external script is called before I make the jQuery call.

Here's what the document.write stuff from the external script looks like:

document.write('<div class="jt_job_list">');
document.write("
    <div class=\"jt_job jt_row2\">
        <div class=\"jt_job_position\">
            <a href=\"http://example.com/job.html\">Position Title</a>
        </div>
        <div class=\"jt_job_location\">City, State</div>
        <div class=\"jt_job_company\">Job Company Name</div>
    </div>
");

Answer 1

Attach an function to the load event:

<script type="text/javascript" src="whatever.js" onload ="SomeFunction()" />

As far as your loading... problem goes, try displaying a div for loading and then just display:none-ing it in your onload function. Make sure to handle cases where your script fails to load too, though.

Answer 2

This worked for me: it does however, rely on the newer querySelector interface which most modern browsers support. But if you're using really old browsers, you can use getElement... and run a for loop.

function loadJS(file, callback, error, type) {
    var _file = file ;
    var loaded = document.querySelector('script[src="'+file+'"]') ;

    if (loaded) {
      loaded.onload = callback ;
      loaded.onreadystatechange = callback;
      return
    }

    var script = document.createElement("script");

    script.type = (typeof type ==="string" ? type : "application/javascript") ;

    script.src = file;
    script.async = false ;
    script.defer = false ;
    script.onload = callback ;

    if (error) {
       script.onerror = error ;
       }
    else {
       script.onerror = function(e) {
              console.error("Script File '" + _file + "' not found :-(");
              };
       }

    script.onreadystatechange = callback;

    document.body.appendChild(script);
}

Answer 3

Script tags block downloads, so as long as the content dependent on your script is below where your script it loaded, you should be fine. This is true even if the script is in-line in the body of your page.

This website has a great example of how this works.

This obviously does not work if you're loading the scripts asynchronously.

Scripts without async or defer attributes are fetched and executed immediately, before the browser continues to parse the page.

Source: MDN

Answer 4

You could put a script block after it on the page:

<script src="external_script.js"></script>
<script type="text/javascript">
    ExternalScriptHasLoaded();
</script>

Answer 5

Thanks for the assistance above, especially ngmiceli for the Steve Souders link!

I decided to take what's probably a "lazy" approach, and also forego the "loading" message:

$(document).ready(function(){
    if ($('.jt_job_list').length === 0){
        $('#job-board').html("<p>We're sorry, but the Job Board isn't currently available. Please try again in a few minutes.</p>");
    };
});

Pretty simple, but I'm looking to see if an element with the .jt_job_list class is in the dom. If it isn't, I display an error message.