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("type","text/javascript")
        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")
        document.getElementsByTagName("head")[0].appendChild(fileref)
}
 
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

 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:

document.getElementsByTagName("head")[0].appendChild(fileref)

 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"
    }
    else
        alert("file already added!")
}
 
checkloadjscssfile("myscript.js", "js") //success
checkloadjscssfile("myscript.js", "js") //redundant file, so file not added