How to Check If an External (Cross-Domain) CSS File Is Loaded Using JavaScript

HOW TO check if an external (cross-domain) CSS file is loaded using Javascript

This is actually what I did.

To ensure a specific CSS file is loaded, I added a style in the end of the CSS file. For example:

#ensure-cssload-8473649 {
  display: none
}

Now I have a JavaScript function which will fire the callback specified when the above style is loaded on the page:

var onCssLoad = function (options, callback) {
    var body = $("body");
    var div = document.createElement(constants.TAG_DIV);
    for (var key in options) {
        if (options.hasOwnProperty(key)) {
            if (key.toLowerCase() === "css") {
                continue;
            }
            div[key] = options[key];
        }
    }

    var css = options.css;
    if (css) {
        body.appendChild(div);
        var handle = -1;
        handle = window.setInterval(function () {
            var match = true;
            for (var key in css) {
                if (css.hasOwnProperty(key)) {
                    match = match && utils.getStyle(div, key) === css[key];
                }
            }

            if (match === true) {
                window.clearTimeout(handle);
                body.removeChild(div);
                callback();
            }
        }, 100);
    }
}

And this is how I used the function above:

onCssLoad({
    "id": "ensure-cssload-8473649",
    css: {
        display: "none"
    }
}, function () {
    // code when you want to execute 
    // after your CSS file is loaded
});

Here the 1st parameter takes the options where id is to check against the test style and css property to verify against what loaded from the CSS.

How to check if all CSS files have been downloaded to insert HTML using JavaScript?

You could use YepNope.js, YepNope allows you to build asynchronous conditional tests to see whether resources have loaded. Once your tests have passed you can tell it to inject new CSS or JS files.

Example below has been taken from the YepNope.js site.

yepnope.injectCss( stylesheetSource [, callback ] [, elemAttributes ] [, timeout ]);

// Example
yepnope.injectCss("print.css", function () {
  console.log("css injected!");
}, {
  media: "print"
}, 5000);

You can even make YepNope load the initial CSS files first and then once they have completed loading YepNope can trigger a callback to do additional tasks, such as loading more JS or CSS files.

Cross domain iframe content load detection

The easiest way (if you can get code added to the external sites) is to have them add an invisible iframe pointing to a special html file on your domain. This could then use parent.parent.foo() to notify the original window about the load event.

Listening for the "load" event will only tell you if the window loaded, not what was loaded or if the document is ready for interaction.

How to load up CSS files using Javascript?

Here's the "old school" way of doing it, which hopefully works across all browsers. In theory, you would use setAttribute unfortunately IE6 doesn't support it consistently.

var cssId = 'myCss';  // you could encode the css path itself to generate id..
if (!document.getElementById(cssId))
{
    var head  = document.getElementsByTagName('head')[0];
    var link  = document.createElement('link');
    link.id   = cssId;
    link.rel  = 'stylesheet';
    link.type = 'text/css';
    link.href = 'http://website.example/css/stylesheet.css';
    link.media = 'all';
    head.appendChild(link);
}

This example checks if the CSS was already added so it adds it only once.

Put that code into a JavaScript file, have the end-user simply include the JavaScript, and make sure the CSS path is absolute so it is loaded from your servers.

VanillaJS

Here is an example that uses plain JavaScript to inject a CSS link into the head element based on the filename portion of the URL:

<script type="text/javascript">
var file = location.pathname.split( "/" ).pop();

var link = document.createElement( "link" );
link.href = file.substr( 0, file.lastIndexOf( "." ) ) + ".css";
link.type = "text/css";
link.rel = "stylesheet";
link.media = "screen,print";

document.getElementsByTagName( "head" )[0].appendChild( link );
</script>

Insert the code just before the closing head tag and the CSS will be loaded before the page is rendered. Using an external JavaScript (.js) file will cause a Flash of unstyled content (FOUC) to appear.

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