add parameter for ordered script execution

README.md

@@ -42,18 +42,51 @@ You can execute code after the Script has loaded via a callback:
 </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](http://caniuse.com/#search=async) (No IE8/IE9 support).:
+
+```js
+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: 
 
 1. Some older browsers do not support `async` ([though defer has broader support so it's typically not a problem](https://www.igvita.com/2014/05/20/script-injected-async-scripts-considered-harmful/))
-2. There's no way to use these attributes to *conditionally* load a script, depending on feature or environmental conditions. 
+2. There's no way to use these attributes to *conditionally* load a script, depending on feature or environmental conditions.
+3. 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
 
-`loadJS` does nothing to manage execution order of requested scripts, so we do not advise using it to load multiple javascript files that depend on one another. It simply fetches a script and executes it as soon as possible. You can certainly use `loadJS` to load multiple scripts as long as those scripts are designed to execute independently of any other scripts being loaded by `loadJS`. 
+* Ordered execution does not work in IE9-, so we do not advise using it to load multiple javascript files that depend on one another. It simply fetches a script and executes it as soon as possible. You can certainly use `loadJS` to load multiple scripts as long as those scripts are designed to execute independently of any other scripts being loaded by `loadJS`.
+* 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`/`requestAnimationFrame`.
+
+```html
+<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
 

loadJS.js

@@ -1,12 +1,21 @@
 /*! loadJS: load a JS file asynchronously. [c]2014 @scottjehl, Filament Group, Inc. (Based on http://goo.gl/REQGQ by Paul Irish). Licensed MIT */
 (function( w ){
-	var loadJS = function( src, cb ){
+	var loadJS = function( src, cb, ordered ){
 		"use strict";
+		var tmp;
 		var ref = w.document.getElementsByTagName( "script" )[ 0 ];
 		var script = w.document.createElement( "script" );
+
+		if (typeof(cb) === 'boolean') {
+			tmp = ordered;
+			ordered = cb;
+			cb = tmp;
+		}
+
 		script.src = src;
-		script.async = true;
+		script.async = !ordered;
 		ref.parentNode.insertBefore( script, ref );
+
 		if (cb && typeof(cb) === "function") {
 			script.onload = cb;
 		}