A pattern for loading CSS asynchronously [c]2020 @scottjehl, @zachleat Filament Group, Inc. Licensed MIT
Referencing CSS stylesheets with link[rel=stylesheet] or @import causes browsers to delay page rendering while a stylesheet loads. When loading stylesheets that are not critical to the initial rendering of a page, this blocking behavior is undesirable. The pattern below allows us to fetch and apply CSS asynchronously. If necessary, this repo also offers a separate (and optional) JavaScript function for loading stylesheets dynamically.
As a primary pattern, we recommend loading asynchronous CSS like this from HTML:
<link rel="stylesheet" href="/path/to/my.css" media="print" onload="this.media='all'; this.onload=null;">
This article explains why this approach is best: https://www.filamentgroup.com/lab/load-css-simpler/
That is probably all you need! But if you want to load a CSS file from a JavaScript function, read on...
First, include the loadCSS library into your project. This can be done by directly including the loadCSS.js file which exposes a global loadCSS function.
If you prefer, you can also install loadCSS via npm: npm i fg-loadcss and import the loadCSS function into the necessary files.
Once you have loadCSS available installed in your project, use it as so:
loadCSS( "path/to/mystylesheet.css" );The code above will insert a new CSS stylesheet link after the last stylesheet or script that it finds in the page, and the function will return a reference to that link element, should you want to reference it later in your script. Multiple calls to loadCSS will reference CSS files in the order they are called, but keep in mind that they may finish loading in a different order than they were called.
The loadCSS function has 3 optional arguments.
before: By default, loadCSS attempts to inject the stylesheet link after all CSS and JS in the page. However, if you desire a more specific location in your document, such as before a particular stylesheet link, you can use thebeforeargument to specify a particular element to use as an insertion point. Your stylesheet will be inserted before the element you specify. For example, here's how that can be done by simply applying anidattribute to yourscript.
<head>
...
<script id="loadcss">
// load a CSS file just before the script element containing this code
loadCSS( "path/to/mystylesheet.css", document.getElementById("loadcss") );
</script>
...
</head>media: You can optionally pass a string to the media argument to set themedia=""of the stylesheet - the default value isall.attributes: You can also optionally pass an Object of attribute name/attribute value pairs to set on the stylesheet. This can be used to specify Subresource Integrity attributes:
loadCSS(
"https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css",
null,
null,
{
"crossorigin": "anonymous",
"integrity": "sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm"
}
);Onload event support for link elements is spotty in some browsers, so if you need to add an onload callback, include onloadCSS function on your page and use the onloadCSS function:
var stylesheet = loadCSS( "path/to/mystylesheet.css" );
onloadCSS( stylesheet, function() {
console.log( "Stylesheet has loaded." );
});The loadCSS patterns attempt to load a css file asynchronously in any JavaScript-capable browser. However, some older browsers such as Internet Explorer 8 and older will block rendering while the stylesheet is loading. This merely means that the stylesheet will load as if you referenced it with an ordinary link element.
As of version 3.0, we no longer support or include a polyfill for a rel=preload markup pattern. This is because we have since determined that the markup pattern described at the top of this readme is simpler and better for performance, while the former preload pattern could sometimes conflict with resource priorities in ways that aren't helpful for loading CSS in a non-blocking way.
To update, you can change your preload markup to this HTML pattern and delete the JS from your build.
Since this change breaks the API from prior versions, we made it a major version bump. That way, if you are still needing to use the now-deprecated preload pattern, you can keep your code pointing at prior versions that are still on NPM, such as version 2.1.0 https://github.com/filamentgroup/loadCSS/releases/tag/v2.1.0
Both are very much appreciated - especially bug fixes. As for contributions, the goals of this project are to keep things very simple and utilitarian, so if we don't accept a feature addition, it's not necessarily because it's a bad idea. It just may not meet the goals of the project. Thanks!