Wednesday, 11 September 2013

Dynamically loading an external JavaScript or CSS file

The conventional way to loading external JavaScript (ie: .js) and CSS (ie: .css) files on a page is to stick a reference to them in the HEAD section of your page, for example:
<script type="text/javascript" src="myscript.js"></script>
<link rel="stylesheet" type="text/css" href="main.css" />
Files that are called this way are added to the page as they are encountered in the page's source, or synchronously. For the most part, this setup meets our needs just fine, though in the world of synchronous Ajax design patterns, the ability to also fire up JavaScript/ CSS on demand is becoming more and more handy. In this tutorial, lets see how it's done.

Dynamically loading external JavaScript and CSS files

To load a .js or .css file dynamically, in a nutshell, it means using DOM methods to first create a swanky new "SCRIPT" or "LINK" element, assign it the appropriate attributes, and finally, use element.appendChild() to add the element to the desired location within the document tree. It sounds a lot more fancy than it really is. Lets see how it all comes together:
function loadjscssfile(filename, filetype){
 if (filetype=="js"){ //if filename is a external JavaScript file
  var fileref=document.createElement('script')
  fileref.setAttribute("src", filename)
 else if (filetype=="css"){ //if filename is an external CSS file
  var fileref=document.createElement("link")
  fileref.setAttribute("rel", "stylesheet")
  fileref.setAttribute("type", "text/css")
  fileref.setAttribute("href", filename)
 if (typeof fileref!="undefined")

loadjscssfile("myscript.js", "js") //dynamically load and add this .js file
loadjscssfile("javascript.php", "js") //dynamically load "javascript.php" as a JavaScript file
loadjscssfile("mystyle.css", "css") ////dynamically load and add this .css file
Demo: "myscript.js" source:
var petname="Spotty"
alert("Pet Name: " + petname)
"mystyle.css" source:
#demotable td{
background-color: lightyellow;

#demotable b{
color: blue;

Since external JavaScript and CSS files can technically end with any custom file extension (ie: "javascript.php"), the function parameter "filetype" lets you tell the script what file type to expect before loading. After that, the function sets out to create the element using the appropriate DOM methods, assign it the proper attributes, and finally, add it to the end of the HEAD section. Now, where the created file gets added is worth elaborating on:

By referencing the HEAD element of the page first and then calling appendChild(), this means the newly created element is added to the very end of the HEAD tag. Furthermore, you should be aware that no existing element is harmed in the adding of the new element- that is to say, if you call loadjscssfile("myscript.js", "js") twice, you now end up with two new "SCRIPT" elements both pointing to the same JavaScript file. This is problematic only from an efficiency standpoint, as you'll be adding redundant elements to the page and using unnecessary browser memory in the process. A simple way to prevent the same file from being added more than once is to keep track of the files added by loadjscssfile(), and only load a file if it's new:
var filesadded="" //list of files already added

function checkloadjscssfile(filename, filetype){
 if (filesadded.indexOf("["+filename+"]")==-1){
  loadjscssfile(filename, filetype)
  filesadded+="["+filename+"]" //List of files added in the form "[filename1],[filename2],etc"
  alert("file already added!")

checkloadjscssfile("myscript.js", "js") //success
checkloadjscssfile("myscript.js", "js") //redundant file, so file not added
Here I'm just crudely detecting to see if a file that's set to be added already exists within a list of added files' names stored in variable filesadded before deciding whether to proceed or not.
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