I am trying to write a HTML page that asks users a series of questions. The answers to these questions are evaluated by my JavaScript code and used to determine which additional JavaScript file the user needs to access. My code then adds the additional JavaScript file to the head tag of my HTML page. I don't want to merge the code into a single JavaScript file because these additional files are large enough to be a nightmare if they're together, and I don't want to add them all to the head when the page first loads because I will have too many variable conflicts. I'm reluctant to redirect to a new webpage for each dictionary because this will make a lot of redundant coding. I'm not using any libraries.
I begin with the following HTML code:
<head>
<link rel="stylesheet" type="text/css" href="main.css">
<script src="firstSheet.js" type="text/JavaScript"></script>
</head>
//Lots of HTML.
<div id="mainUserMenu">
</div>
And I have the following JavaScript function:
function thirdLevelQuestions(secondLevelAnswer) {
//Code here to calculate the variables. This part works.
activeDictionary = firstKey + secondKey + '.js';
//Changing the HTML header to load the correct dictionary.
document.head.innerHTML = '<link rel="stylesheet" type="text/css" href="main.css"><script src="' + activeDictionary + '" type="text/JavaScript"></script><script src="firstSheet.js" type="text/JavaScript"></script>';
//for loop to generate the next level of buttons.
for (var i = 0; i < availableOptions.length; i++) {
document.getElementById('mainUserMenu').innerHTML += '<button onclick="fourthLevelQuestions(' + i + ')">' + availableOptions[i] + '</button>';
}
}
This creates the buttons that I want, and when I inspect the head element I can see both JavaScript files there. When I click on any of the buttons at this level they should call a function in the second file. Instead Chrome tells me "Uncaught ReferenceError: fourthLevelQuestions is not defined" (html:1). If I paste the code back into firstSheet.js the function works, so I assume the problem is that my HTML document is not actually accessing the activeDictionary file. Is there a way to do this?
What Can be done
You are trying to load Javascript on Demand. This has been a well thought out problem lately and most of the native solutions didn't work well across bowser implementations. Check a study here with different solutions and background of the problem explained well.
For the case of large web applications the solution was to use some javascript library that helped with modularising code and loading them on demand using some script loaders. The focus is on modularizing code and not in just script loading. Check some libraries here. There are heavier ones which includes architectures like MVC with them.
If you use AJAX implementation of jQuery with the correct dataType jQuery will help you evaluate the scripts, they are famous for handling browser differences. You can as well take a look at the exclusive getScript() which is indeed a shorthand for AJAX with dataType script. Keep in mind that loading script with native AJAX does not guarantee evaluation of the javascript included, jQuery is doing the evaluation internally during the processing stage.
What is wrong
What you have done above might work in most modern browsers (not sure), but there is an essential flaw in your code. You are adding the script tags to your head using innerHTML which inserts those lines to your HTML. Even if your browser loads the script it takes a network delay time and we call it asynchronous loading, you cannot use the script right away. Then what do you do? Use the script when its ready or loaded. Surprisingly you have an event for that, just use it. Can be something like:
var head= document.getElementsByTagName('head')[0];
var script= document.createElement('script');
script.type= 'text/javascript';
script.onreadystatechange= function () {
if (this.readyState == 'complete') helper();
}
script.onload= helper;
script.src= 'helper.js';
head.appendChild(script);
Check this article for help with implementation without using external libraries
From the variable name activeDictionary If I guess that you are loading some data sets as opposed to javascript programs, you should try looking into JSON and loading and using them dynamically.
If this Question/Answer satisfies your needs, you should delete your question to avoid duplicate entries in SO.
The best way to achieve this would be with jQuery:
$(document).ready(function() {
$('#button').click(function() {
var html = "<script src='newfile.js' type='text/javascript'></script>";
var oldhtml = "<script src='firstSheet.js' type='text/javascript'></script>";
if ($(this).attr('src') == 'firstSheet.js') {
$('script[src="firstSheet.js"]').replace(html);
return;
}
$('script[src="newfile.js"]').replace(oldhtml);
});
});
I would suggest you create the elements how they should be and then append them. Also, if you are dynamically adding the firstSheet.js you shouldn't include it in your .html file.
function thirdLevelQuestions(secondLevelAnswer) {
var mainUserMenu = document.getElementById('mainUserMenu');
activeDictionary = firstKey + secondKey + '.js';
var css = document.createElement('link');
css.rel = 'stylesheet';
css.type = 'text/css';
css.href = 'main.css';
var script1 = document.createElement('script');
script1.type = 'text/javascript';
script1.src = 'firstSheet.js';
var script2 = document.createElement('script');
script2.type = 'text/javascript';
script2.src = activeDictionary;
document.head.appendChild(css);
document.head.appendChild(script1);
document.head.appendChild(script2);
for (var i = 0; i < availableOptions.length; i++) {
var btn = document.createElement('button');
btn.onclick = 'fourthLevelQuestions(' + i + ')';
var val = document.createTextNode(availableOptions[i]);
btn.appendChild(val);
mainUserMenu.appendChild(btn);
}
}
The company which developped my website just added this javascript code on the Zend Guard encrypted index.php file (I saw it with "View source") :
(function ()
{
var smrs = document.createElement("script");
smrs.type = "text/javascript";
smrs.async = true;
smrs.src = document.location.protocol + "//www.domain.com/file.js";
var s = document.getElementsByTagName("script")[0];
s.parentNode.insertBefore(smrs, s);
})();
It injects a very agressive javascript code which adds a image link to their website (with a SetInterval each 10sec), at the bottom of the page.
The problem ? A local competitor, which is currently being accused of significant fraud, have the same CMS and the same image link.
Being associated with that competitor is prejudicial for me. I would like to know if there is a way to block the "www.domain.com/file.js" loading with a .htaccess.
Thanks.
You can't (using htaccess). This javascript creates a script tag to load the external javascript. The call never passes through the server. So apache (htaccess) can't block that.
The easiest way is to search in the source code and remove the script (if you have access).
UPDATE:
I see the script is encrypted... If you can insert a script at the very beginning (before the code gets executed you can create a hook on the insertBefore method. Here is a working fiddle
var ALLOWED_DOMAINS = ['www.klaartjedevoecht.be', 'jsfiddle.net'];
function creatHook(){
function getDomain(url) {
return url.match(/:\/\/(.[^/]+)/)[1];
}
var insertBefore = Element.prototype.insertBefore;
Element.prototype.insertBefore = function(new_node,existing_node){
if(new_node.tagName.toLowerCase() === 'script' && ALLOWED_DOMAINS.indexOf(getDomain(new_node.src)) > -1){
insertBefore.call(this, new_node, existing_node);
}
}
}
creatHook();
//TESTING CODE:
var smrs = document.createElement("script");
smrs.type = "text/javascript";
smrs.async = true;
smrs.src = document.location.protocol + "//www.klaartjedevoecht.be/test.js";
//var smrs = document.createElement("img");
// smrs.src= "http://img52.imageshack.us/img52/7653/beaverl.gif";
var s = document.getElementsByTagName("div")[0];
s.parentNode.insertBefore(smrs, s);
I agree it's a bit hacking, but at least its cleaner then the timer solution. If you can't remove it, there is no clean solution.
Problem
In order to improve the page performance I need to preload scripts that I will need to run on the bottom page.
I would like to take control of when the script is parsed, compiled and executed.
I must avoid the script tag, because it is blocker for common render engines (geeko, etc).
I can't load it using defer property, because I need to control when the script is executed.
Also, async property is not a possibility.
sample:
<html><head>
//preload scripts ie: a.js without use the script
</head><body> ..... all my nice html here
//execute here a.js
</body></html>
This allows me to maximize the render performance of my page, because the browser will start to donwload the scripts content, and it will render the page at the same time in parallel. Finally, I can add the script tag, so the browser will parse, compile and execute the code.
The only way that I could do that is using a hidden image tag. (This is a simplified version of Stoyan)
i.e.
<html><head>
<img src="a.js" style=display:none;>
</head><body> ..... all my nice html here
<script src="a.js">
</body></html>
Question
I didn't find any problem using this technique, but does anyone know a better way to do this?
Is there any meta prefetch?
Additional information
I'm using requirejs, so I'm trying to preload the modules code, without executing it, because this code depends of DOM elements.
With similar technique you may preload scripts and stylesheets using img for Internet Explorer and object tag for every other browser.
var isMSIE = /*#cc_on!#*/false;
var resources = ['a.js', 'b.js', 'c.css'];
for (var i=0; i<resources.length; i++){
if (isMSIE){
new Image().src = resources[i];
} else {
var o = document.createElement('object');
o.data = resources[i];
document.body.appendChild(o);
}
}
There is a blog post describing such a technique and outlining caveats: Preload CSS/JavaScript without execution.
But why don't you want to just use dynamically added scripts just like suggested in other answer, this will probably lead to a cleaner solution with more control.
You can use the prefetch attribute of a link tag to preload any resource, javascript included. As of this writing (Aug 10, 2016) it isn't supported in Safari, but is pretty much everywhere else:
<link rel="prefetch" href="(url)">
More info on support here:
http://caniuse.com/#search=prefetch
Note that IE 9,10 aren't listed in the caniuse matrix because Microsoft has discontinued support for them.
More info here and more options for preloading, like prerender and more
For each script you'd like to download without executing, make an object containing a name and a url, and put those objects into an array.
Looping through the array, use jQuery.ajax with dataType: "text" to download your scripts as text. In the done handler of the ajax call, store the text content of the file (which is passed in first argument) in the appropriate object, increment a counter, and call an "alldone" function when that counter is equal to the number of files you are downloading in this manner.
In the "alldone" function (or later) do the following: Loop through your array again, and for each entry, use document.createElement("script"), document.createTextNode(...), and (...scriptNode...).appendChild(...) to dynamically generate scripts having the intended source inline, rather than via "src" attribute. Finally, do document.head.appendChild(...scriptNode...), which is the point when that script is executed.
I have used this technique in a project where I needed to use frames, where several frames and/or the frameset need identical JavaScript files, in order to make sure each of those files is requested only once from the server.
Code (tested and working) follows
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
<html>
<head>
<script id="scriptData">
var scriptData = [
{ name: "foo" , url: "path/to/foo" },
{ name: "bar" , url: "path/to/bar" }
];
</script>
<script id="scriptLoader">
var LOADER = {
loadedCount: 0,
toBeLoadedCount: 0,
load_jQuery: function (){
var jqNode = document.createElement("script");
jqNode.setAttribute("src", "/path/to/jquery");
jqNode.setAttribute("onload", "LOADER.loadScripts();");
jqNode.setAttribute("id", "jquery");
document.head.appendChild(jqNode);
},
loadScripts: function (){
var scriptDataLookup = this.scriptDataLookup = {};
var scriptNodes = this.scriptNodes = {};
var scriptNodesArr = this.scriptNodesArr = [];
for (var j=0; j<scriptData.length; j++){
var theEntry = scriptData[j];
scriptDataLookup[theEntry.name] = theEntry;
}
//console.log(JSON.stringify(scriptDataLookup, null, 4));
for (var i=0; i<scriptData.length; i++){
var entry = scriptData[i];
var name = entry.name;
var theURL = entry.url;
this.toBeLoadedCount++;
var node = document.createElement("script");
node.setAttribute("id", name);
scriptNodes[name] = node;
scriptNodesArr.push(node);
jQuery.ajax({
method : "GET",
url : theURL,
dataType : "text"
}).done(this.makeHandler(name, node)).fail(this.makeFailHandler(name, node));
}
},
makeFailHandler: function(name, node){
var THIS = this;
return function(xhr, errorName, errorMessage){
console.log(name, "FAIL");
console.log(xhr);
console.log(errorName);
console.log(errorMessage);
debugger;
}
},
makeHandler: function(name, node){
var THIS = this;
return function (fileContents, status, xhr){
THIS.loadedCount++;
//console.log("loaded", name, "content length", fileContents.length, "status", status);
//console.log("loaded:", THIS.loadedCount, "/", THIS.toBeLoadedCount);
THIS.scriptDataLookup[name].fileContents = fileContents;
if (THIS.loadedCount >= THIS.toBeLoadedCount){
THIS.allScriptsLoaded();
}
}
},
allScriptsLoaded: function(){
for (var i=0; i<this.scriptNodesArr.length; i++){
var scriptNode = this.scriptNodesArr[i];
var name = scriptNode.id;
var data = this.scriptDataLookup[name];
var fileContents = data.fileContents;
var textNode = document.createTextNode(fileContents);
scriptNode.appendChild(textNode);
document.head.appendChild(scriptNode); // execution is here
//console.log(scriptNode);
}
// call code to make the frames here
}
};
</script>
</head>
<frameset rows="200pixels,*" onload="LOADER.load_jQuery();">
<frame src="about:blank"></frame>
<frame src="about:blank"></frame>
</frameset>
</html>
other question closely related to above approach
other related question
You should have a look at the following links:
http://calendar.perfplanet.com/2011/lazy-evaluation-of-commonjs-modules/
http://tomdale.net/2012/01/amd-is-not-the-answer/
And at how ember.js is using a tool called minispade and preprocessing with ruby to make the process of loading, parsing and running javascript modules fast.
Why not to try this?
var script = document.createElement('script');
script.src = 'http://path/to/your/script.js';
script.onload = function() {
// do something here
}
document.head.appendChild(script);
you can use .onload event to control when it is loaded. One caveat is that .onload() doesn't work in IE and you can use this:
script.onreadystatechange = function() {
if (/^loaded|complete$/i.test(this.readyState)) {
// loaded
};
}
Additionally adding scripts via DOM is non-blocking and i believe you can perfectly achieve your goals with this approach.
I've answered the same question there:
https://stackoverflow.com/a/46121439/1951947
just use the <link> tag to preload your script and then you can use it with the <script> tag
eg: <link href="/js/script-to-preload.js" rel="preload" as="script">
I'm trying to add Reddit buttons to my site, but they are not asynchronous, and Reddit tends to lag, slowing down page loads. When I look at what the script returns, I get something like this:
(function () {
var write_string = ...
document.write(write_string);
})()
I try to inject it into my page after a page load. I've tried both these methods in javascript after page load to no avail:
placeholder.innerHTML = '<script type="text/javascript" src="http://www.reddit.com/buttonlite.js?i=5"></script>'
var js = document.createElement('script');
js.type = 'text/javascript';
js.src = 'http://www.reddit.com/buttonlite.js?i=5';
placeholder.appendChild(js);
where placeholder is a DOM element <div class="reddit-button"></div>. Any ideas on how I could go about this?
You can "override" the document.write method:
window.onload = function() {
var oScript = document.createElement("script");
document.write = function(text) {
document.getElementById("placeholder").innerHTML += text;
};
oScript.src = "http://www.reddit.com/buttonlite.js?i=0";
document.body.appendChild(oScript);
};
This way the external code can call document.write as much as it wants to and you push the HTML to the proper place in your document.
Live test case - Tested OK under Chrome, Firefox and IE9 so guess it should be enough.
The following are the first lines of code in a <script> tag just above the closing body tag in my document (it specifies that a locally-served copy of jQuery is run in the event that Google's CDN fails):
if(!window.jQuery){
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = '/js/jquery.js';
var scriptHook = document.getElementsByTagName('script')[0];
scriptHook.parentNode.insertBefore(script, scriptHook);
}
jQuery(document).ready(function($){
// page behaviors
});
It does execute successfully, in the sense that if my computer is not connected to the Internet (this is a locally-served page), the local copy of jQuery is inserted. However, the document.ready() section below does not execute. I'm guessing this is because it is invoked before the fallback copy of jQuery takes effect. What's the proper practice for somehow "delaying" its execution so that either copy of jQuery will work properly?
Consider using an existing script loader such as yepnope. There's an example of exactly what you're trying to do on the home page.
You need to be sure that the script you are appending to the dom has finished loading before calling jQuery. You can do this with the technique described here:
if(!window.jQuery){
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = '/js/jquery.js';
script.onreadystatechange= function () {
if (this.readyState == 'complete') jQueryLoaded();
}
script.onload = jQueryLoaded;
var scriptHook = document.getElementsByTagName('script')[0];
scriptHook.parentNode.insertBefore(script, scriptHook);
}
function jQueryLoaded() { };
You can also fetch the jQuery contents as an Ajax request, create a script tag with those as the body of the script and append it. That would also work.
Try that
<script>window.jQuery || document.write('<script src="js/libs/jquery-1.6.2.min.js"><\/script>')</script>
<script>
jQuery(document).ready(function($){
// page behaviors
});
</script>
This way the script tag will be loaded synchronously.
The question "of how do I cope with my CDN failing and load a file hosted on my server" seems to come up a few times lately.
Question I'd ask is whether adding yet more js is the way to achieve the resilience and what level of resilience do the js approaches really add e.g. if the CDN is down they'll be a quick failure but how well do these approaches if the CDN is slow to respond how well do these solutions cope?
An alternative way to approach this is treat it as an infrastructure problem...
Run a CDN based on a domain/sub-domain you own. Have automated monitoring on it's availability, when it fails switch the DNS over to a backup server (anycast may provide an alternative solution too)
A php solution would be something like this:
$google_jquery = 'https://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js';
$fp = #fsockopen($google_jquery, 'r');
if (!$fp)
{
echo '<script type="text/javascript" src="js/jquery.js"></script>';
}
else
{
echo '<script src="'.$google_jquery.'"></script>' }
}