Dynamically load a JavaScript file

Question

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?

Answer 1

Something like this...

<script>
     $(document).ready(function() {
          $('body').append('<script src="https://maps.googleapis.com/maps/api/js?key=KEY&libraries=places&callback=getCurrentPickupLocation" async defer><\/script>');
     });
</script>

Answer 2

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>

Answer 3

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.

E.G : 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. E.G :

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
});

Answer 4

If you want a SYNC script loading, you need to add script text directly to HTML HEAD tag. Adding it as will trigger an ASYNC load. To load script text from external file synchronously, use XHR. Below a quick sample (it is using parts of other answers in this and other posts):

/*sample requires an additional method for array prototype:*/

if (Array.prototype.contains === undefined) {
Array.prototype.contains = function (obj) {
    var i = this.length;
    while (i--) { if (this[i] === obj) return true; }
    return false;
};
};

/*define object that will wrap our logic*/
var ScriptLoader = {
LoadedFiles: [],

LoadFile: function (url) {
    var self = this;
    if (this.LoadedFiles.contains(url)) return;

    var xhr = new XMLHttpRequest();
    xhr.onload = function () {
        if (xhr.readyState === 4) {
            if (xhr.status === 200) {
                self.LoadedFiles.push(url);
                self.AddScript(xhr.responseText);
            } else {
                if (console) console.error(xhr.statusText);
            }
        }
    };
    xhr.open("GET", url, false);/*last parameter defines if call is async or not*/
    xhr.send(null);
},

AddScript: function (code) {
    var oNew = document.createElement("script");
    oNew.type = "text/javascript";
    oNew.textContent = code;
    document.getElementsByTagName("head")[0].appendChild(oNew);
}
};

/*Load script file. ScriptLoader will check if you try to load a file that has already been loaded (this check might be better, but I'm lazy).*/

ScriptLoader.LoadFile("Scripts/jquery-2.0.1.min.js");
ScriptLoader.LoadFile("Scripts/jquery-2.0.1.min.js");
/*this will be executed right after upper lines. It requires jquery to execute. It requires a HTML input with id "tb1"*/
$(function () { alert($('#tb1').val()); });

Answer 5

There are scripts that are designed specifically for this purpose.

yepnope.js is built into Modernizr, and lab.js is a more optimized (but less user friendly version.

I wouldn't reccomend doing this through a big library like jquery or prototype - because one of the major benefits of a script loader is the ability to load scripts early - you shouldn't have to wait until jquery & all your dom elements load before running a check to see if you want to dynamically load a script.

Answer 6

I know my answer is bit late for this question, but, here is a great article in www.html5rocks.com - Deep dive into the murky waters of script loading .

In that article it is concluded that in regards of browser support, the best way to dynamically load JavaScript file without blocking content rendering is the following way:

Considering you've four scripts named script1.js, script2.js, script3.js, script4.js then you can do it with applying async = false:

[
  'script1.js',
  'script2.js',
  'script3.js',
  'script4.js'
].forEach(function(src) {
  var script = document.createElement('script');
  script.src = src;
  script.async = false;
  document.head.appendChild(script);
});

Now, Spec says: Download together, execute in order as soon as all download.

Firefox < 3.6, Opera says: I have no idea what this “async” thing is, but it just so happens I execute scripts added via JS in the order they’re added.

Safari 5.0 says: I understand “async”, but don’t understand setting it to “false” with JS. I’ll execute your scripts as soon as they land, in whatever order.

IE < 10 says: No idea about “async”, but there is a workaround using “onreadystatechange”.

Everything else says: I’m your friend, we’re going to do this by the book.

Now, the full code with IE < 10 workaround:

var scripts = [
  'script1.js',
  'script2.js',
  'script3.js',
  'script4.js'
];
var src;
var script;
var pendingScripts = [];
var firstScript = document.scripts[0];

// Watch scripts load in IE
function stateChange() {
  // Execute as many scripts in order as we can
  var pendingScript;
  while (pendingScripts[0] && pendingScripts[0].readyState == 'loaded') {
    pendingScript = pendingScripts.shift();
    // avoid future loading events from this script (eg, if src changes)
    pendingScript.onreadystatechange = null;
    // can't just appendChild, old IE bug if element isn't closed
    firstScript.parentNode.insertBefore(pendingScript, firstScript);
  }
}

// loop through our script urls
while (src = scripts.shift()) {
  if ('async' in firstScript) { // modern browsers
    script = document.createElement('script');
    script.async = false;
    script.src = src;
    document.head.appendChild(script);
  }
  else if (firstScript.readyState) { // IE<10
    // create a script and add it to our todo pile
    script = document.createElement('script');
    pendingScripts.push(script);
    // listen for state changes
    script.onreadystatechange = stateChange;
    // must set src AFTER adding onreadystatechange listener
    // else we’ll miss the loaded event for cached scripts
    script.src = src;
  }
  else { // fall back to defer
    document.write('<script src="' + src + '" defer></'+'script>');
  }
}

A few tricks and minification later, it’s 362 bytes

!function(e,t,r){function n(){for(;d[0]&&"loaded"==d[0][f];)c=d.shift(),c[o]=!i.parentNode.insertBefore(c,i)}for(var s,a,c,d=[],i=e.scripts[0],o="onreadystatechange",f="readyState";s=r.shift();)a=e.createElement(t),"async"in i?(a.async=!1,e.head.appendChild(a)):i[f]?(d.push(a),a[o]=n):e.write("<"+t+' src="'+s+'" defer></'+t+">"),a.src=s}(document,"script",[
  "//other-domain.com/1.js",
  "2.js"
])