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:warning: This project is archived and the repository is no longer maintained.

loadJS

A simple function for asynchronously loading JavaScript files

[c]2014 @scottjehl, Filament Group, Inc.
Based on Surefire Dom Element Insertion by Paul Irish
Licensed MIT

Usage

Place the loadJS function inline in the head of your page (it can also be included in an external JavaScript file if preferable).

Then call it by passing it a JavaScript URL:

<head>
...
<script>
  // include loadJS here...
  function loadJS( src ){ ... }
  // load a file with loadJS
  loadJS( "path/to/script.js" );
</script>
...
</head>

You can execute code after the Script has loaded via a callback:

<head>
...
<script>
  // include loadJS here...
  function loadJS( src ){ ... }
  // load a file with loadJS
  loadJS( "path/to/script.js", function() {
    // file has loaded
  });
</script>
...
</head>

You can ensure ordered execution of multiple asynchronous by passing true as the second or third parameter. Only supported in browsers that support the async attribute (No IE8/IE9 support).:

loadJS( "path/to/library.js", true );
loadJS( "path/to/plugins.js", true );
loadJS( "path/to/last.js", function() {
	//all scripts loaded
}, true );

Why not just use `<script src="..." async defer>`?

The async and defer attributes enjoy broad browser support. They're great options when all you need to do is load a script for all users, ideally in a non-blocking manner. The limitations with these attributes are:

Some older browsers do not support async (though defer has broader support so it's typically not a problem)
There's no way to use these attributes to conditionally load a script, depending on feature or environmental conditions.
There is (still) no declarative way to load scripts async, but in order.

Number 2 above is the main reason we use loadJS. Say you want to check if querySelector is supported before requesting your entire DOM framework and UI scripting - you'll need to use a script loader to do that. But again, if you just want to load a script unconditionally, these attributes are recommended.

Limitations

If placed below external blocking scrips or stylesheets the download starts only after these files are downloaded and parsed. A good workaround for async script loading of crucial scripts is to inline loadJS before any other stylesheets and scripts and use it either immediately or within a setTimeout.
Ordered execution does not work in IE9-.

<script>
	// include loadJS here...
	function loadJS( src ){ ... }
	
	setTimeout(function(){
		loadJS( "path/to/library.js", true );
		
		if ( !hasFeature ) {
			loadJS( "path/to/polyfill.js", true );
		}
		
		loadJS( "path/to/app.js", true );
	});
</script>

...
 
<link rel="stylesheet" href="path/to/styles.css" />

Contributions and bug fixes