Dynamically load a JavaScript file
How can you reliably and dynamically load a JavaScript file? This will can be used to implement a module or component that when 'initialized' the component will dynamically load all needed JavaScript library scripts on demand.
The client that uses the component isn't required to load all the library script files (and manually insert <script> tags into their web page) that implement this component - just the 'main' component script file.
How do mainstream JavaScript libraries accomplish this (Prototype, jQuery, etc)? Do these tools merge multiple JavaScript files into a single redistributable 'build' version of a script file? Or do they do any dynamic loading of ancillary 'library' scripts?
An addition to this question: is there a way to handle the event after a dynamically included JavaScript file is loaded? Prototype has document.observe for document-wide events. Example:
document.observe("dom:loaded", function() {
// initially hide all containers for tab content
$$('div.tabcontent').invoke('hide');
});
What are the available events for a script element?
You may write dynamic script tags (using Prototype):
new Element("script", {src: "myBigCodeLibrary.js", type: "text/javascript"});
The problem here is that we do not know when the external script file is fully loaded.
We often want our dependant code on the very next line and like to write something like:
if (iNeedSomeMore) {
Script.load("myBigCodeLibrary.js"); // includes code for myFancyMethod();
myFancyMethod(); // cool, no need for callbacks!
}
There is a smart way to inject script dependencies without the need of callbacks. You simply have to pull the script via a synchronous AJAX request and eval the script on global level.
If you use Prototype the Script.load method looks like this:
var Script = {
_loadedScripts: [],
include: function(script) {
// include script only once
if (this._loadedScripts.include(script)) {
return false;
}
// request file synchronous
var code = new Ajax.Request(script, {
asynchronous: false,
method: "GET",
evalJS: false,
evalJSON: false
}).transport.responseText;
// eval code on global level
if (Prototype.Browser.IE) {
window.execScript(code);
} else if (Prototype.Browser.WebKit) {
$$("head").first().insert(Object.extend(
new Element("script", {
type: "text/javascript"
}), {
text: code
}
));
} else {
window.eval(code);
}
// remember included script
this._loadedScripts.push(script);
}
};
There is no import / include / require in javascript, but there are two main ways to achieve what you want:
1 - You can load it with an AJAX call then use eval.
This is the most straightforward way but it's limited to your domain because of the Javascript safety settings, and using eval is opening the door to bugs and hacks.
2 - Add a script tag with the script URL in the HTML.
Definitely the best way to go. You can load the script even from a foreign server, and it's clean as you use the browser parser to evaluate the code. You can put the tag in the head of the web page, or at the bottom of the body.
Both of these solutions are discussed and illustrated here.
Now, there is a big issue you must know about. Doing that implies that you remotely load the code. Modern web browsers will load the file and keep executing your current script because they load everything asynchronously to improve performances.
It means that if you use these tricks directly, you won't be able to use your newly loaded code the next line after you asked it to be loaded, because it will be still loading.
EG : my_lovely_script.js contains MySuperObject
var js = document.createElement("script");
js.type = "text/javascript";
js.src = jsFilePath;
document.body.appendChild(js);
var s = new MySuperObject();
Error : MySuperObject is undefined
Then you reload the page hitting F5. And it works! Confusing...
So what to do about it ?
Well, you can use the hack the author suggests in the link I gave you. In summary, for people in a hurry, he uses en event to run a callback function when the script is loaded. So you can put all the code using the remote library in the callback function. EG :
function loadScript(url, callback)
{
// adding the script tag to the head as suggested before
var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = url;
// then bind the event to the callback function
// there are several events for cross browser compatibility
script.onreadystatechange = callback;
script.onload = callback;
// fire the loading
head.appendChild(script);
}
Then you write the code you want to use AFTER the script is loaded in a lambda function :
var myPrettyCode = function() {
// here, do what ever you want
};
Then you run all that :
loadScript("my_lovely_script.js", myPrettyCode);
Ok, I got it. But it's a pain to write all this stuff.
Well, in that case, you can use as always the fantastic free jQuery framework, which let you do the very same thing in one line :
$.getScript("my_lovely_script.js", function() {
alert("Script loaded and executed.");
// here you can use anything you defined in the loaded script
});
I used a much less complicated version recently with jQuery:
<script src="scripts/jquery.js"></script>
<script>
var js = ["scripts/jquery.dimensions.js", "scripts/shadedborder.js", "scripts/jqmodal.js", "scripts/main.js"];
var $head = $("head");
for (var i = 0; i < js.length; i++) {
$head.append("<script src="" + js[i] + ""></scr" + "ipt>");
}
</script>
It worked great in every browser I tested it in: IE6/7, Firefox, Safari, Opera.
Update: jQuery-less version:
<script>
var js = ["scripts/jquery.dimensions.js", "scripts/shadedborder.js", "scripts/jqmodal.js", "scripts/main.js"];
for (var i = 0, l = js.length; i < l; i++) {
document.getElementsByTagName("head")[0].innerHTML += ("<script src="" + js[i] + ""></scr" + "ipt>");
}
</script>
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