Loading scripts after page load?

I work with an advertising company, where we tag certain pages to track activity. A client of mine wants to fire off a javascript tag to track activity AFTER the page has finished loading entirely (to prevent the page content from loading slowly due to slow tag load times).

An example tag that should load AFTER the page has fully loaded is:

<script>document.write('<s'+'cript language="JavaScript" src="http://jact.atdmt.com/jaction/JavaScriptTest"></s'+'cript>')</script>

I was looking at some stackoverflow threads and I came across the below implementation which I think will work:

window.onload = function(){
  <script language="JavaScript" src="http://jact.atdmt.com/jaction/JavaScriptTest"></script>
};

I tested this on my own webpage and I did get the tag to fire off, but I'm wondering if there are any alternate or more robust methods, ideally using jquery of some kind.

Below is a sample implementation that the client tried, but it seems to break their page:

<script>
jQuery(window).load(function () {$('<script language="JavaScript" src="http://jact.atdmt.com/jaction/JavaScriptTest"></script>').insertAfter('#div_name');});
</script>

I haven't done JQuery in a while and was hoping I could get some input from other members here. Is there any other way I can call the above script after page load using JQuery?

Thanks,

So, there's no way that this works:

window.onload = function(){
  <script language="JavaScript" src="http://jact.atdmt.com/jaction/JavaScriptTest"></script>
};

You can't freely drop HTML into the middle of javascript.

If you have jQuery, you can just use:

$.getScript("http://jact.atdmt.com/jaction/JavaScriptTest")

whenever you want. If you want to make sure the document has finished loading, you can do this:

$(document).ready(function() {
    $.getScript("http://jact.atdmt.com/jaction/JavaScriptTest");
});

In plain javascript, you can load a script dynamically at any time you want to like this:

var tag = document.createElement("script");
tag.src = "http://jact.atdmt.com/jaction/JavaScriptTest";
document.getElementsByTagName("head")[0].appendChild(tag);

The second approach is right to execute JavaScript code after the page has finished loading - but you don't actually execute JavaScript code there, you inserted plain HTML.
The first thing works, but loads the JavaScript immediately and clears the page (so your tag will be there - but nothing else).
(Plus: language="javascript" has been deprecated for years, use type="text/javascript" instead!)

To get that working, you have to use the DOM manipulating methods included in JavaScript. Basically you'll need something like this:

var scriptElement=document.createElement('script');
scriptElement.type = 'text/javascript';
scriptElement.src = filename;
document.head.appendChild(scriptElement);

For a Progressive Web App I wrote a script to easily load javascript files async on demand. Scripts are only loaded once. So you can call loadScript as often as you want for the same file. It wouldn't be loaded twice. This script requires JQuery to work.

For example:

loadScript("js/myscript.js").then(function(){
    // Do whatever you want to do after script load
});

or when used in an async function:

await loadScript("js/myscript.js");
// Do whatever you want to do after script load

In your case you may execute this after document ready:

$(document).ready(async function() {
    await loadScript("js/myscript.js");
    // Do whatever you want to do after script is ready
});

Function for loadScript:

function loadScript(src) {
  return new Promise(function (resolve, reject) {
    if ($("script[src='" + src + "']").length === 0) {
        var script = document.createElement('script');
        script.onload = function () {
            resolve();
        };
        script.onerror = function () {
            reject();
        };
        script.src = src;
        document.body.appendChild(script);
    } else {
        resolve();
    }
});
}

Benefit of this way:

• It uses browser cache
• You can load the script file when a user performs an action which needs the script instead loading it always.

Focusing on one of the accepted answer's jQuery solutions, $.getScript() is an .ajax() request in disguise. It allows to execute other function on success by adding a second parameter:

$.getScript(url, function() {console.log('loaded script!')})

Or on the request's handlers themselves, i.e. success (.done() - script was loaded) or failure (.fail()):

$.getScript(
    "https://code.jquery.com/color/jquery.color.js",
    () => console.log('loaded script!')
  ).done((script,textStatus ) => {
    console.log( textStatus );
    $(".block").animate({backgroundColor: "green"}, 1000);
  }).fail(( jqxhr, settings, exception ) => {
    console.log(exception + ': ' + jqxhr.status);
  }
);

.block {background-color: blue;width: 50vw;height: 50vh;margin: 1rem;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="block"></div>