How can i add new script and load it on scroll? - javascript

I am trying to add new script and want to it only on scroll. i tried this but didn't worked. any idea why?
$(window).scroll(function() {
var heightT = $('body').offset().top,
outerH = $('body').outerHeight(),
windowH = $(window).height(),
wS = $(this).scrollTop();
if (wS > (heightT+outerH-windowH-200)){
<script src="myscript.js"></script>
}

You can add a script dynamically.
let scriptAdded = false;
$(window).scroll(function() {
var heightT = $('body').offset().top,
outerH = $('body').outerHeight(),
windowH = $(window).height(),
wS = $(this).scrollTop();
if (wS > (heightT+outerH-windowH-200) && !scriptAdded){
var script = document.createElement('script');
script.setAttribute('src','myscript.js');
document.head.appendChild(script);
// So that not add many time
scriptAdded = true;
}
}

If you're using jQuery then you can use $.getScript.
$.getScript("your/script.js", function(){
console.log("Script was loaded");
});

Here we want to make sure that a script is loaded correctly.
You can try and make use of listeners when try and load a script dynamically, and to be sure that the script is loaded correctly.
You can listen to Mutation Events but it is not longer recommended because of performance and browser compatibility issues
https://developer.mozilla.org/en-US/docs/Web/Guide/Events/Mutation_events
A second alternative is attach events to the appended childNode.
In fact, the appendChild method returns the childNode and it can listen to 3 events that are fired by window.
Here a potential vanilla javascript implementation:
const id = "scriptID";
const src = "your script src";
const scriptElement = document.getElementById(id);
if (!scriptElement) {
const script = document.createElement("script");
if (script) {
script.src = src;
script.id = id;
const loadError = () => throw new Error("can't load the script")
const loadSuccess = () => console.log("loaded");
const attachedScript = document.body.appendChild(script);
const onloadEvent = attachedScript.addEventListener("load", loadSuccess);
const onerrorEvent = attachedScript.addEventListener("error", loadError);
const onavortEvent = attachedScript.addEventListener("abort", loadError);
} else {
const error = "can't load the script";
throw new Error(error);
}
}
However, the above solution listens to the load event that is fired by window when page is fully loaded (including assets), that is in contrast with DocumentContentLoaded event that is fired when DOM is loaded but it does not wait for assets to be loaded:
https://developer.mozilla.org/en-US/docs/Web/API/Document/DOMContentLoaded_event
What is wrong with the above is that the load event should be used on first load and not when DOM is mutated.
https://developer.mozilla.org/en-US/docs/Web/API/Window/load_event
A more elegant solution would be make use instead of MutationObserver, that is meant to replace the MutationEvents feature:
https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
const id = "scriptID";
const src = "https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.7.9/angular.js";
const scriptElement = document.getElementById(id);
if (!scriptElement) {
const script = document.createElement("script");
if (script) {
script.src = src;
script.id = id;
const observer = new MutationObserver(() => {
console.log("change being done");
observer.disconnect();
});
const config = { attributes: false, childList: true, subtree: true };
observer.observe( document.body, config);
document.body.appendChild(script)
} else {
const error = "can't load the script";
throw new Error(error);
}
}
MutationObserver looks for DOM mutation but it does not trigger when a script is actually loaded, so using the load event from window as well would make the solution stronger.
Now, why all of this is needed for just append a script in the DOM?
Some of the reasons are:
one thing is appending a script in the DOM, but what counts is when a script is loaded
we should be very careful when listening to DOM manipulation events, because it can easily bring to a performance degradation
there is no need to listen to error when appending a child as the cases of fail are rare

Related

How to load an external script in an iframe that will reference the iframe's document not the parent document

I am using an iframe in order to isolate its content's CSS. Which is working great. But, I need the iframe to load an external script, specifically the script I am loading is: https://web.squarecdn.com/v1/square.js
You can see that this external script calls document often. I need it to reference the document of my iframe not the document of the parent HTML.
I have tried this and it does not accomplish this:
const insertScriptToIframe = (doc, target, src, callback) => {
var s = doc.createElement("script");
s.type = "text/javascript";
if(callback) {
if (s.readyState){ //IE
s.onreadystatechange = function(){
if (s.readyState == "loaded" ||
s.readyState == "complete"){
s.onreadystatechange = null;
callback();
}
};
} else { //Others
s.onload = function(){
callback();
};
}
}
s.src = src;
target.appendChild(s);
}
// add web payments script
const createWebPaymentsNode = () => {
const iFrame = document.createElement('iframe');
const context = iFrame.contentDocument;
const frameHead = context.getElementsByTagName('head').item(0);
insertScriptToIframe(context, frameHead, 'https://web.squarecdn.com/v1/square.js');
}
It adds the external script to the iframe. But the external script is referencing the parent document. I know it is doing this because the external script, itself, loads other scripts and iframes that are being placed in the parent HTML, instead of the iframe's.
I need to be able to do this in vanilla js. Thank you

How to load script dynamically without polluting the page namespace?

I need to dynamically load a JS file which exports a variable to the window object (e.g. React or jQuery) and get the exported value without changing the page window object. How can I do it using JavaScript?
It should work like this:
(function () {
var React = someMagic();
assertNotEmpty(React);
assertEmpty(window.React);
})();
What is my end goal: make a script for embedding to other websites. The script performs some actions with the page where it is installed, requires some dependencies, but the dependencies must not interfere with the page dependencies.
Using AMD or Require.js is not suitable because it changes the page scripts behaviour (they stop exporting variables to window) which can break the page.
Using a solution like this:
<script src="jquery.js"></script>
<script>
(function () {
var jQuery = jQuery.noConflict();
})();
</script>
Is also not suitable because it requires changing HTML while I can use only JavaScript.
Joining the script with the dependencies and wrapping it all with (function(){ ... })() is not suitable too.
Use Iframe to separate the namespaces. When a script is loaded inside an Iframe, it doesn't change the page namespace. A variable from an Iframe namespace can be exported to the page namespace when required.
// Imagine this is a content of the embedded script
const onMyJqueryLoad = jQuery => {
console.log('Page jQuery:', window.jQuery.fn.jquery, window.jQuery.ui);
console.log('My jQuery:', jQuery.fn.jquery, jQuery.ui);
};
const loadScript = (src, callback = () => {}, document = window.document) => {
const script = document.createElement('script');
script.type = 'text/javascript';
script.async = true;
script.onload = script.onerror = () => {
script.parentNode.removeChild(script);
callback();
};
script.src = src;
document.body.appendChild(script);
}
// Create an Iframe and do the dirty job inside it without affecting the page
let iframe = document.createElement('iframe');
iframe.style.position = 'absolute';
iframe.style.top = '-1000px';
iframe.style.width = 0;
iframe.style.height = 0;
iframe.onload = () => {
loadScript(
'https://cdn.jsdelivr.net/npm/jquery#1.12.4/dist/jquery.min.js',
() => loadScript(
'https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.8.24/jquery-ui.min.js',
() => {
const jQuery = iframe.contentWindow.jQuery;
iframe.parentNode.removeChild(iframe);
iframe = null;
onMyJqueryLoad(jQuery);
},
iframe.contentWindow.document
),
iframe.contentWindow.document
);
iframe.onload = null;
};
document.body.appendChild(iframe);
<!-- The page -->
<script src="https://cdn.jsdelivr.net/npm/jquery#1.12.4/dist/jquery.min.js"></script>
Unfortunately the code doesn't work inside a Stack Overflow snippet because of the snippet sandboxing. You can view a demo on CodePen.

Reload javascript resource programmatically in Chrome

I'm trying to reload javascript resources programmatically in Chrome and if I run that from console, then it works fine, but if I put that into the code to reload after an event was fired, then the resource doesn't change. What is the problem?
I use this reloader:
var docHeadObj = document.getElementsByTagName("head")[0];
var dynamicScript = document.createElement("script");
dynamicScript.type = "text/javascript";
dynamicScript.src = 'js/resource.js';
docHeadObj.appendChild(dynamicScript);
And here is the event handler:
obj.onclick(function(){
var docHeadObj = document.getElementsByTagName("head")[0];
var dynamicScript = document.createElement("script");
dynamicScript.type = "text/javascript";
dynamicScript.src = 'js/resource.js';
docHeadObj.appendChild(dynamicScript);
});
After both cases the new and proper <script> element is appended to the <head> and on network tab the resource is downloaded, but in case of the second one the resource is never change.
EDIT:
I'm closer to the problem. If I have this code in the resource which will be reloaded alert("aaa");
And I open the application then after performing a click event I get an alert with 3 'a' letter. Then I decrease the number of the letters to 2, reload the resource, perform a click event, then I again see 3 'a' letter. Then I decrease the number of letters to 1, save and click, then I see 2 'a' letter. So it seems to be the Chrome (and also FF) stores the last modified resource except if I reload that from console.
You should wait before the script is loaded before you use it. It can be achieved with using onload event for the dynamic script
function loadScript(url, callback) {
var docHeadObj = document.getElementsByTagName("head")[0];
var dynamicScript = document.createElement("script");
dynamicScript.type = "text/javascript";
dynamicScript.src = url;
// bind the event to the callback function
dynamicScript.onreadystatechange = callback; //for ie<9
dynamicScript.onload = callback;
// Fire the loading
docHeadObj.appendChild(dynamicScript);
}
var testJQuery = function () {
console.log($(this));
};
var obj = document.getElementById("test");
obj.onclick = function () {
loadScript("//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js", testJQuery);
};
Working example on fiddle
If you just delete the last instance of the script tag before reloading it, that should force it to reload.
function load(link, id) {
var exists = document.getElementById(id)
if(exists){
exists.parentNode.removeChild(exists);
}
var script = document.createElement('script');
script.src = link;
script.id = id;
document.body.appendChild(script);
}
obj.onclick = function(){
load('js/resource.js','dynamicScript')
}

JavaScript to detect when external javascripts are loading

Is there a way (event listener or otherwise) to detect when a particular external javascript is about to load / is loading / has finished loading?
In otherwords, does the browser fire an event when it's about to load, is loading, and/or has finished loading a particular external script?
For my purposes it's not enough to simply check to see if a known object exists or anything like that. Instead, I need something that will detect a JS file is loading/loaded regardless of the contents of the JS file.
The following example works in Chrome. It attaches an handler on the onload event of the head tag and then adds an external javascript file. When the file is loaded, the event is captured and an alert appears.
http://jsfiddle.net/francisfortier/uv9Fh/
window.onload = function() {
var head = document.getElementsByTagName("head")[0];
var script = document.createElement("script");
script.setAttribute("type", "text/javascript");
script.setAttribute("src", "http://code.jquery.com/jquery-1.7.1.min.js");
head.addEventListener("load", function(event) {
if (event.target.nodeName === "SCRIPT")
{
alert("Script loaded: " + event.target.getAttribute("src"));
}
}, true);
head.appendChild(script);
}
Since all browsers blocks the "UI thread" when processing <script> tags, you can rely that pre-existing tags are loaded.
If you are loading a script dynamically, you can listen to the load event of the <script> tag.
function loadScript(src, callback) {
var script = document.createElement("script");
script.setAttribute("src", src);
script.addEventListener("load", callback);
document.getElementsByTagName("script")[0].insertBefore(script);
};
loadScript("http://code.jquery.com/jquery-1.7.1.min.js", function(){
alert("loading is done");
});
<script onload> will fire when a script is finished loading.
You will not be able to do something like:
<script src="/foo.js"></script>
<script src="/bar.js"></script>
<script>
function alertonload(src){
console.log(src+' is loaded');
}
scripts = document.getElementsByTagName('script');
for(var i=0; i<scripts.length; i++){
scripts[i].onload = function(){ alertonload(scripts[i].src); };
}
</script>
This is pure conjecture and speculation; I have not tried it and there's probably better ways to write it, but this will not do what you're looking to do. EDIT: The scripts are loaded as the browser sees them, not after the fact. They will be loaded before this occurs.
I wrote a script for those who wants to call a function after all external files (dynamically added) are loaded. It goes like this:
var file = [];
var loaded = [];
var head = document.getElementsByTagName('head')[0];
var fileOnLoad =
// Pass the arrays to your function
(function(file, loaded){ return function(){
loaded.push(true);
// Print the number of files loaded
document.getElementById("demo").innerHTML +=
"<br>"+loaded.length+" files loaded";
if(file.length == loaded.length){
alert("All files are loaded!");
}
}})(file, loaded);
////////////////////////////////////////////////
//// ////
//// Add the external files dynamically ////
//// ////
////////////////////////////////////////////////
file[0] = document.createElement('script');
file[0].src =
"https://maps.googleapis.com/maps/api/js?v=3.exp";
file[0].onload = fileOnLoad;
head.appendChild(file[0]);
file[1] = document.createElement('script');
file[1].src =
"http://code.jquery.com/jquery-latest.js";
file[1].onload = fileOnLoad;
head.appendChild(file[1]);
file[2] = document.createElement('script');
file[2].src =
"https://maxcdn.bootstrapcdn.com/bootstrap/3.3.1/js/bootstrap.min.js";
file[2].onload = fileOnLoad;
head.appendChild(file[2]);
file[3] = document.createElement('link');
file[3].rel = "stylesheet";
file[3].href =
"https://maxcdn.bootstrapcdn.com/bootstrap/3.3.1/css/bootstrap.min.css";
file[3].onload = fileOnLoad;
head.appendChild(file[3]);
file[4] = document.createElement('link');
file[4].rel = "stylesheet";
file[4].href =
"https://maxcdn.bootstrapcdn.com/bootstrap/3.3.1/css/bootstrap-theme.min.css";
file[4].onload = fileOnLoad;
head.appendChild(file[4]);
<div id="demo">0 file loaded</div>
Hope it helps!
If you can use jQuery, try $.getScript(). Docs here.

load scripts asynchronously

I am using several plugins, custom widgets and some other libraries from JQuery. as a result I have several .js and .css files. I need to create a loader for my site because it takes some time to load. it will be nice if I can display the loader before importing all the:
<script type="text/javascript" src="js/jquery-1.6.2.min.js"></script>
<script type="text/javascript" src="js/myFunctions.js"></script>
<link type="text/css" href="css/main.css" rel="stylesheet" />
...
....
etc
I have found several tutorials that enable me to import a JavaScript library asynchronously. for example I can do something like:
(function () {
var s = document.createElement('script');
s.type = 'text/javascript';
s.async = true;
s.src = 'js/jquery-ui-1.8.16.custom.min.js';
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
})();
for some reason when I do the same thing for all my files the pages does not work. I have been trying for so long to try to find where the problem is but I just cannot find it. First I thought that it was probably because some javascript functions depended on the others. but I loaded them in the right order using the time out function when one completed I proceeded with the next and the page still behaves weird. for example I am not able to click on links etc... animations still work though..
Anyways
Here is what I have been thinking... I believe browsers have a cache that's why it takes a long time to load the page for the first time and the next time it is quick. so what I am thinking of doing is replacing my index.html page with a page that loads all this files asynchronously. when ajax is done loading all those files redirect to the page that I plan on using. when using that page it should not take long to load since the files should alredy be included on the cache of the browser. on my index page (page where .js and .css file get loaded asynchronously) I don't care of getting errors. I will just be displaying a loader and redirecting the page when done...
Is this idea a good alternative? or should I keep trying on implementing the asynchronously methods?
EDIT
the way I load everything async is like:
importScripts();
function importScripts()
{
//import: jquery-ui-1.8.16.custom.min.js
getContent("js/jquery-1.6.2.min.js",function (code) {
var s = document.createElement('script');
s.type = 'text/javascript';
//s.async = true;
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext1,1);
});
//import: jquery-ui-1.8.16.custom.min.js
function insertNext1()
{
getContent("js/jquery-ui-1.8.16.custom.min.js",function (code) {
var s = document.createElement('script');
s.type = 'text/javascript';
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext2,1);
});
}
//import: jquery-ui-1.8.16.custom.css
function insertNext2()
{
getContent("css/custom-theme/jquery-ui-1.8.16.custom.css",function (code) {
var s = document.createElement('link');
s.type = 'text/css';
s.rel ="stylesheet";
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext3,1);
});
}
//import: main.css
function insertNext3()
{
getContent("css/main.css",function (code) {
var s = document.createElement('link');
s.type = 'text/css';
s.rel ="stylesheet";
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext4,1);
});
}
//import: jquery.imgpreload.min.js
function insertNext4()
{
getContent("js/farinspace/jquery.imgpreload.min.js",function (code) {
var s = document.createElement('script');
s.type = 'text/javascript';
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext5,1);
});
}
//import: marquee.js
function insertNext5()
{
getContent("js/marquee.js",function (code) {
var s = document.createElement('script');
s.type = 'text/javascript';
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext6,1);
});
}
//import: marquee.css
function insertNext6()
{
getContent("css/marquee.css",function (code) {
var s = document.createElement('link');
s.type = 'text/css';
s.rel ="stylesheet";
s.innerHTML=code;
var x = document.getElementsByTagName('script')[0];
x.parentNode.insertBefore(s, x);
setTimeout(insertNext,1);
});
}
function insertNext()
{
setTimeout(pageReadyMan,10);
}
}
// get the content of url and pass that content to specified function
function getContent( url, callBackFunction )
{
// attempt to create the XMLHttpRequest and make the request
try
{
var asyncRequest; // variable to hold XMLHttpRequest object
asyncRequest = new XMLHttpRequest(); // create request object
// register event handler
asyncRequest.onreadystatechange = function(){
stateChange(asyncRequest, callBackFunction);
}
asyncRequest.open( 'GET', url, true ); // prepare the request
asyncRequest.send( null ); // send the request
} // end try
catch ( exception )
{
alert( 'Request failed.' );
} // end catch
} // end function getContent
// call function whith content when ready
function stateChange(asyncRequest, callBackFunction)
{
if ( asyncRequest.readyState == 4 && asyncRequest.status == 200 )
{
callBackFunction(asyncRequest.responseText);
} // end if
} // end function stateChange
and the weird part is that all the style's work plus all the javascript functions. the page is frozen for some reason though...
A couple solutions for async loading:
//this function will work cross-browser for loading scripts asynchronously
function loadScript(src, callback)
{
var s,
r,
t;
r = false;
s = document.createElement('script');
s.type = 'text/javascript';
s.src = src;
s.onload = s.onreadystatechange = function() {
//console.log( this.readyState ); //uncomment this line to see which ready states are called.
if ( !r && (!this.readyState || this.readyState == 'complete') )
{
r = true;
callback();
}
};
t = document.getElementsByTagName('script')[0];
t.parentNode.insertBefore(s, t);
}
If you've already got jQuery on the page, just use:
$.getScript(url, successCallback)*
Additionally, it's possible that your scripts are being loaded/executed before the document is done loading, meaning that you'd need to wait for document.ready before events can be bound to the elements.
It's not possible to tell specifically what your issue is without seeing the code.
The simplest solution is to keep all of your scripts inline at the bottom of the page, that way they don't block the loading of HTML content while they execute. It also avoids the issue of having to asynchronously load each required script.
If you have a particularly fancy interaction that isn't always used that requires a larger script of some sort, it could be useful to avoid loading that particular script until it's needed (lazy loading).
* scripts loaded with $.getScript will likely not be cached
For anyone who can use modern features such as the Promise object, the loadScript function has become significantly simpler:
function loadScript(src) {
return new Promise(function (resolve, reject) {
var s;
s = document.createElement('script');
s.src = src;
s.onload = resolve;
s.onerror = reject;
document.head.appendChild(s);
});
}
Be aware that this version no longer accepts a callback argument as the returned promise will handle callback. What previously would have been loadScript(src, callback) would now be loadScript(src).then(callback).
This has the added bonus of being able to detect and handle failures, for example one could call...
loadScript(cdnSource)
.catch(loadScript.bind(null, localSource))
.then(successCallback, failureCallback);
...and it would handle CDN outages gracefully.
HTML5's new 'async' attribute is supposed to do the trick. 'defer' is also supported in most browsers if you care about IE.
async - The HTML
<script async src="siteScript.js" onload="myInit()"></script>
defer - The HTML
<script defer src="siteScript.js" onload="myInit()"></script>
While analyzing the new adsense ad unit code I noticed the attribute and a search lead me here: http://davidwalsh.name/html5-async
I loaded the scripts asynchronously (html 5 has that feature) when all the scripts where done loading I redirected the page to index2.html where index2.html uses the same libraries. Because browsers have a cache once the page redirects to index2.html, index2.html loads in less than a second because it has all it needs to load the page. In my index.html page I also load the images that I plan on using so that the browser place those images on the cache. so my index.html looks like:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Project Management</title>
<!-- the purpose of this page is to load all the scripts on the browsers cache so that pages can load fast from now on -->
<script type="text/javascript">
function stylesheet(url) {
var s = document.createElement('link');
s.type = 'text/css';
s.async = true;
s.src = url;
var x = document.getElementsByTagName('head')[0];
x.appendChild(s);
}
function script(url) {
var s = document.createElement('script');
s.type = 'text/javascript';
s.async = true;
s.src = url;
var x = document.getElementsByTagName('head')[0];
x.appendChild(s);
}
//load scritps to the catche of browser
(function () {
stylesheet('css/custom-theme/jquery-ui-1.8.16.custom.css');
stylesheet('css/main.css');
stylesheet('css/marquee.css');
stylesheet('css/mainTable.css');
script('js/jquery-ui-1.8.16.custom.min.js');
script('js/jquery-1.6.2.min.js');
script('js/myFunctions.js');
script('js/farinspace/jquery.imgpreload.min.js');
script('js/marquee.js');
})();
</script>
<script type="text/javascript">
// once the page is loaded go to index2.html
window.onload = function () {
document.location = "index2.html";
}
</script>
</head>
<body>
<div id="cover" style="position:fixed; left:0px; top:0px; width:100%; height:100%; background-color:Black; z-index:100;">Loading</div>
<img src="images/home/background.png" />
<img src="images/home/3.png"/>
<img src="images/home/6.jpg"/>
<img src="images/home/4.png"/>
<img src="images/home/5.png"/>
<img src="images/home/8.jpg"/>
<img src="images/home/9.jpg"/>
<img src="images/logo.png"/>
<img src="images/logo.png"/>
<img src="images/theme/contentBorder.png"/>
</body>
</html>
another nice thing about this is that I may place a loader in the page and when the page is done loading the loader will go away and in a matte of milliseconds the new page will be running.
Example from google
<script type="text/javascript">
(function() {
var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true;
po.src = 'https://apis.google.com/js/plusone.js?onload=onLoadCallback';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s);
})();
</script>
Several notes:
s.async = true is not very correct for HTML5 doctype, correct is s.async = 'async' (actually using true is correct, thanks to amn who pointed it out in the comment just below)
Using timeouts to control the order is not very good and safe, and you also make the loading time much larger, to equal the sum of all timeouts!
Since there is a recent reason to load files asynchronously, but in order, I'd recommend a bit more functional-driven way over your example (remove console.log for production use :) ):
(function() {
var prot = ("https:"===document.location.protocol?"https://":"http://");
var scripts = [
"path/to/first.js",
"path/to/second.js",
"path/to/third.js"
];
function completed() { console.log('completed'); } // FIXME: remove logs
function checkStateAndCall(path, callback) {
var _success = false;
return function() {
if (!_success && (!this.readyState || (this.readyState == 'complete'))) {
_success = true;
console.log(path, 'is ready'); // FIXME: remove logs
callback();
}
};
}
function asyncLoadScripts(files) {
function loadNext() { // chain element
if (!files.length) completed();
var path = files.shift();
var scriptElm = document.createElement('script');
scriptElm.type = 'text/javascript';
scriptElm.async = true;
scriptElm.src = prot+path;
scriptElm.onload = scriptElm.onreadystatechange = \
checkStateAndCall(path, loadNext); // load next file in chain when
// this one will be ready
var headElm = document.head || document.getElementsByTagName('head')[0];
headElm.appendChild(scriptElm);
}
loadNext(); // start a chain
}
asyncLoadScripts(scripts);
})();
Thanks to HTML5, you can now declare the scripts that you want to load asynchronously by adding "async" in the tag:
<script async>...</script>
Note: The async attribute is only for external scripts (and should only be used if the src attribute is present).
Note: There are several ways an external script can be executed:
If async is present: The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
If async is not present and defer is present: The script is executed when the page has finished parsing
If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page
See this: http://www.w3schools.com/tags/att_script_async.asp
I wrote a little post to help out with this, you can read more here https://timber.io/snippets/asynchronously-load-a-script-in-the-browser-with-javascript/, but I've attached the helper class below. It will automatically wait for a script to load and return a specified window attribute once it does.
export default class ScriptLoader {
constructor (options) {
const { src, global, protocol = document.location.protocol } = options
this.src = src
this.global = global
this.protocol = protocol
this.isLoaded = false
}
loadScript () {
return new Promise((resolve, reject) => {
// Create script element and set attributes
const script = document.createElement('script')
script.type = 'text/javascript'
script.async = true
script.src = `${this.protocol}//${this.src}`
// Append the script to the DOM
const el = document.getElementsByTagName('script')[0]
el.parentNode.insertBefore(script, el)
// Resolve the promise once the script is loaded
script.addEventListener('load', () => {
this.isLoaded = true
resolve(script)
})
// Catch any errors while loading the script
script.addEventListener('error', () => {
reject(new Error(`${this.src} failed to load.`))
})
})
}
load () {
return new Promise(async (resolve, reject) => {
if (!this.isLoaded) {
try {
await this.loadScript()
resolve(window[this.global])
} catch (e) {
reject(e)
}
} else {
resolve(window[this.global])
}
})
}
}
Usage is like this:
const loader = new Loader({
src: 'cdn.segment.com/analytics.js',
global: 'Segment',
})
// scriptToLoad will now be a reference to `window.Segment`
const scriptToLoad = await loader.load()
I would complete zzzzBov's answer with a check for the presence of callback and allow passing of arguments:
function loadScript(src, callback, args) {
var s, r, t;
r = false;
s = document.createElement('script');
s.type = 'text/javascript';
s.src = src;
if (typeof(callback) === 'function') {
s.onload = s.onreadystatechange = function() {
if (!r && (!this.readyState || this.readyState === 'complete')) {
r = true;
callback.apply(args);
}
};
};
t = document.getElementsByTagName('script')[0];
t.parent.insertBefore(s, t);
}
Here is a great contemporary solution to the asynchronous script loading though it only address the js script with async false.
There is a great article written in www.html5rocks.com - Deep dive into the murky waters of script loading .
After considering many possible solutions, the author concluded that adding js scripts to the end of body element is the best possible way to avoid blocking page rendering by js scripts.
In the mean time, the author added another good alternate solution for those people who are desperate to load and execute scripts asynchronously.
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' || pendingScripts[0].readyState == 'complete' ) ) {
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>');
}
}
for HTML5, you can use the 'prefetch'
<link rel="prefetch" href="/style.css" as="style" />
have a look at 'preload' for js.
<link rel="preload" href="used-later.js" as="script">
One reason why your scripts could be loading so slowly is if you were running all of your scripts while loading the page, like this:
callMyFunctions();
instead of:
$(window).load(function() {
callMyFunctions();
});
This second bit of script waits until the browser has completely loaded all of your Javascript code before it starts executing any of your scripts, making it appear to the user that the page has loaded faster.
If you're looking to enhance the user's experience by decreasing the loading time, I wouldn't go for the "loading screen" option. In my opinion that would be much more annoying than just having the page load more slowly.
I would suggest you take a look at Modernizr. Its a small light weight library that you can asynchronously load your javascript with features that allow you to check if the file is loaded and execute the script in the other you specify.
Here is an example of loading jquery:
Modernizr.load([
{
load: '//ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.js',
complete: function () {
if ( !window.jQuery ) {
Modernizr.load('js/libs/jquery-1.6.1.min.js');
}
}
},
{
// This will wait for the fallback to load and
// execute if it needs to.
load: 'needs-jQuery.js'
}
]);
You might find this wiki article interesting : http://ajaxpatterns.org/On-Demand_Javascript
It explains how and when to use such technique.
Well, x.parentNode returns the HEAD element, so you are inserting the script just before the head tag. Maybe that's the problem.
Try x.parentNode.appendChild() instead.
Check out this https://github.com/stephen-lazarionok/async-resource-loader. It has an example that shows how to load JS, CSS and multiple files with one shot.
Have you considered using Fetch Injection? I rolled an open source library called fetch-inject to handle cases like these. Here's what your loader might look like using the lib:
fetcInject([
'js/jquery-1.6.2.min.js',
'js/marquee.js',
'css/marquee.css',
'css/custom-theme/jquery-ui-1.8.16.custom.css',
'css/main.css'
]).then(() => {
'js/jquery-ui-1.8.16.custom.min.js',
'js/farinspace/jquery.imgpreload.min.js'
})
For backwards compatibility leverage feature detection and fall-back to XHR Injection or Script DOM Elements, or simply inline the tags into the page using document.write.
Here is my custom solution to eliminate render-blocking JavaScript:
// put all your JS files here, in correct order
const libs = {
"jquery": "https://code.jquery.com/jquery-2.1.4.min.js",
"bxSlider": "https://cdnjs.cloudflare.com/ajax/libs/bxslider/4.2.5/jquery.bxslider.min.js",
"angular": "https://ajax.googleapis.com/ajax/libs/angularjs/1.5.0-beta.2/angular.min.js",
"ngAnimate": "https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.0-beta.2/angular-animate.min.js"
}
const loadedLibs = {}
let counter = 0
const loadAsync = function(lib) {
var http = new XMLHttpRequest()
http.open("GET", libs[lib], true)
http.onload = () => {
loadedLibs[lib] = http.responseText
if (++counter == Object.keys(libs).length) startScripts()
}
http.send()
}
const startScripts = function() {
for (var lib in libs) eval(loadedLibs[lib])
console.log("allLoaded")
}
for (var lib in libs) loadAsync(lib)
In short, it loads all your scripts asynchronously, and then executes them consequently.
Github repo: https://github.com/mudroljub/js-async-loader
Here a little ES6 function if somebody wants to use it in React for example
import {uniqueId} from 'lodash' // optional
/**
* #param {String} file The path of the file you want to load.
* #param {Function} callback (optional) The function to call when the script loads.
* #param {String} id (optional) The unique id of the file you want to load.
*/
export const loadAsyncScript = (file, callback, id) => {
const d = document
if (!id) { id = uniqueId('async_script') } // optional
if (!d.getElementById(id)) {
const tag = 'script'
let newScript = d.createElement(tag)
let firstScript = d.getElementsByTagName(tag)[0]
newScript.id = id
newScript.async = true
newScript.src = file
if (callback) {
// IE support
newScript.onreadystatechange = () => {
if (newScript.readyState === 'loaded' || newScript.readyState === 'complete') {
newScript.onreadystatechange = null
callback(file)
}
}
// Other (non-IE) browsers support
newScript.onload = () => {
callback(file)
}
}
firstScript.parentNode.insertBefore(newScript, firstScript)
} else {
console.error(`The script with id ${id} is already loaded`)
}
}
A concise answer, the explanations in the code
function loadScript(src) {
let script = document.createElement('script');
script.type = 'text/javascript';
script.src = src;
//
// script.async = false; // uncomment this line and scripts will be executed in the document order, like 'defer' option
//
// script.defer = true; // uncomment this line when scripts need whole DOM and/or relative order execution is important
//
// the script starts loading as it's append to the document and dynamic script behave as “async” by default
// other scripts don’t wait for 'async' scripts, and 'async' scripts don’t wait for them
// scripts that loads first – runs first (“load-first” order)
document.body.append(script);
}
loadScript('js/example01.js');
loadScript('js/example02.js');
/*
the 'defer' attribute tells the browser not to wait for the script
the 'async' attribute make script to load in the background and run when ready
the 'async' and 'defer' attribute are only for external scripts
'defer' is used for scripts that need the whole DOM and/or their relative execution order is important
'async' is used for independent scripts, like counters or ads, when their relative execution order does not matter
More: https://javascript.info/script-async-defer
*/
You can use LABJS or RequreJS
Script loaders like LABJS, RequireJS will improve the speed and quality of your code.
const dynamicScriptLoading = async (src) =>
{
const response = await fetch(src);
const dataResponse = await response.text();
eval.apply(null, [dataResponse]);
}
I would suggest looking into minifying the files first and see if that gives you a big enough speed boost. If your host is slow, could try putting that static content on a CDN.

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