I created a vanillaJS script that is used by several other websites. The script-server went down for a couple of minutes last week and the result was that several of the sites using the script were not loading correctly (slow, pages not loading at all, errors, etc.) because the sites kept on "waiting" for the external script to load.
I noticed ALL these sites had the script in de head. I suggested moving the script-tag to the footer plus adding the 'async' attribute. But is this the best solution?
Sidenotes:
Can't use jQuery
Can't use a framework like Angular or React
Preferably don't use additional JS on the site itself
The script created it's own content and does not rely on anything on
the page it's served on. It simply created a div with content from a
datababase staticly served in very basic JS to avoid cross-site
errors.
Thanks in advance.
async will make the script to load asynchronously and will be be executed while the page is reading.
defer will make the script to execute once the page is loaded although it's highly depends on the browser at to my tests IE9, IE8 supports this. You can make a quick check with this defer in fiddler
But there is one more alternative to have the script tag at the bottom as
<script>
window.onload = function() {
var element = document.createElement("script");
element.src = "vanillaJS .js";
document.body.appendChild(element);
};
</script>
But i guess these will not solve your problem where the script is not accessible cause of server fault. I suggest to have the local copy of the script in the website folder and reference it.
I want to write a npm package to localize an html url.
1. using the html url download the html page
2. parse the html file, extract all the js, css and img files used in the html and local these resources.
3. If these js, css and img files using some external resources, localize these resources. For example, extract background image in the css.
The first and second requirements are easy to meet. But I have no idea about the last one.
I can parse the all the css files and localize the resources used in it. But how can I parse the js files?
For example:
If the js adds a 'script src = XXX' tag into the html dom, how can I extract the src?
I think I would try to use a headless browser to catch every network calls instead of trying to parse the code.
I didn't used it personally but PhantomJS seems to fit the bill.
It can be used to load a webpage then execute any script / css that would normally happen on the request and execute stuff once the page is loaded.
The network monitoring features are probably what you'll want to use.
I have a Java Web Application, and I'm wondering if the javascript files are downloaded with the HTML-body, or if the html body is loaded first, then the browser request all the JavaScript files.
The reason for this question is that I want to know if importing files with jQuery.getScript() would result in poorer performance. I want to import all files using that JQuery function to avoid duplication of JavaScript-imports.
The body of the html document is retrieved first. After it's been downloaded, the browser checks what resources need to be retrieved and gets those.
You can actually see this happen if you open Chrome Dev Console, go to network tab (make sure caching is disabled and logs preserved) and just refresh a page.
That first green bar is the page loading and the second chunk are the scripts, a stylesheet, and some image resources
The HTML document is downloaded first, and only when the browser has finished downloading the HTML document can it find out which scripts to fetch
That said, heavy scripts that don't influence the appearance of the HTML body directly should be loaded at the end of the body and not in the head, so that they do not block the rendering unless necessary
I'm wondering if the javascript are downloaded with the html body during a request
If it's part of that body then yes. If it's in a separate resource then no.
For example, suppose your HTML file has this:
<script type="text/javascript">
$(function () {
// some code here
});
</script>
That code, being part of the HTML file, is included in the HTML resource. The web server doesn't differentiate between what kind of code is in the file, it just serves the response regardless of what's there.
On the other hand, if you have this:
<script type="text/javascript" src="someFile.js"></script>
In that case the code isn't in the same file. The HTML is just referencing a separate resource (someFile.js) which contains the code. In that case the browser would make a separate request for that resource. Resulting in two requests total.
The HTML document is downloaded first, or at least it starts to download first. While it is parsed, any script includes that the browser finds are downloaded. That means that some scripts may finish loading before the document is completely loaded.
While the document is being downloaded, the browser parses it and displays as much as it can. When the parsing comes to a script include, the parsing stops and the browser will suspend it until the script has been loaded and executed, then the parsing continues. That means that
If you put a call to getScript instead of a script include, the behaviour will change. The method makes an asynchronous request, so the browser will continue parsing the rest of the page while the script loads.
This has some important effects:
The parsing of the page will be completed earlier.
Scripts will no longer run in a specific order, they run in the order that the loading completes.
If one script is depending on another, you have to check yourself that the first script has actually loaded before using it in the other script.
You can use a combination of script includes and getScript calls to get the best effect. You can use regular scripts includes for scripts that other scripts depend on, and getScript for scripts that are not affected by the effects of that method.
I have a couple of questions about the attributes async & defer for the <script> tag which to my understanding only work in HTML5 browsers.
One of my sites has two external JavaScript files that currently sit just above the </body> tag; the first is jquery sourced from google and the second is a local external script.
With respects to site load speed
Is there any advantage in adding async to the two scripts I have at the bottom of the page?
Would there be any advantage in adding the async option to the two scripts and putting them at the top of the page in the <head>?
Would this mean they download as the page loads?
I assume this would cause delays for HTML4 browsers, but would it speed up page load for HTML5 browsers?
Using <script defer src=...
Would loading the two scripts inside <head> with the attribute defer the same affect as having the scripts before </body>?
Once again I assume this would slow up HTML4 browsers.
Using <script async src=...
If I have two scripts with async enabled
Would they download at the same time?
Or one at a time with the rest of the page?
Does the order of scripts then become a problem? For example one script depends on the other so if one downloads faster, the second one might not execute correctly etc.
Finally am I best to leave things as they are until HTML5 is more commonly used?
This image explains normal script tag, async and defer
Async scripts are executed as soon as the script is loaded, so it
doesn't guarantee the order of execution (a script you included at
the end may execute before the first script file )
Defer scripts guarantees the order of execution in which they appear
in the page.
Ref this link : http://www.growingwiththeweb.com/2014/02/async-vs-defer-attributes.html
Keep your scripts right before </body>. Async can be used with scripts located there in a few circumstances (see discussion below). Defer won't make much of a difference for scripts located there because the DOM parsing work has pretty much already been done anyway.
Here's an article that explains the difference between async and defer: http://peter.sh/experiments/asynchronous-and-deferred-javascript-execution-explained/.
Your HTML will display quicker in older browsers if you keep the scripts at the end of the body right before </body>. So, to preserve the load speed in older browsers, you don't want to put them anywhere else.
If your second script depends upon the first script (e.g. your second script uses the jQuery loaded in the first script), then you can't make them async without additional code to control execution order, but you can make them defer because defer scripts will still be executed in order, just not until after the document has been parsed. If you have that code and you don't need the scripts to run right away, you can make them async or defer.
You could put the scripts in the <head> tag and set them to defer and the loading of the scripts will be deferred until the DOM has been parsed and that will get fast page display in new browsers that support defer, but it won't help you at all in older browsers and it isn't really any faster than just putting the scripts right before </body> which works in all browsers. So, you can see why it's just best to put them right before </body>.
Async is more useful when you really don't care when the script loads and nothing else that is user dependent depends upon that script loading. The most often cited example for using async is an analytics script like Google Analytics that you don't want anything to wait for and it's not urgent to run soon and it stands alone so nothing else depends upon it.
Usually the jQuery library is not a good candidate for async because other scripts depend upon it and you want to install event handlers so your page can start responding to user events and you may need to run some jQuery-based initialization code to establish the initial state of the page. It can be used async, but other scripts will have to be coded to not execute until jQuery is loaded.
HTML5: async, defer
In HTML5, you can tell browser when to run your JavaScript code. There are 3 possibilities:
<script src="myscript.js"></script>
<script async src="myscript.js"></script>
<script defer src="myscript.js"></script>
Without async or defer, browser will run your script immediately, before rendering the elements that's below your script tag.
With async (asynchronous), browser will continue to load the HTML page and render it while the browser load and execute the script at the same time.
With defer, browser will run your script when the page finished parsing. (not necessary finishing downloading all image files. This is good.)
Both async and defer scripts begin to download immediately without pausing the parser and both support an optional onload handler to address the common need to perform initialization which depends on the script.
The difference between async and defer centers around when the script is executed. Each async script executes at the first opportunity after it is finished downloading and before the window’s load event. This means it’s possible (and likely) that async scripts are not executed in the order in which they occur in the page. Whereas the defer scripts, on the other hand, are guaranteed to be executed in the order they occur in the page. That execution starts after parsing is completely finished, but before the document’s DOMContentLoaded event.
Source & further details: here.
Faced same kind of problem and now clearly understood how both will works.Hope this reference link will be helpful...
Async
When you add the async attribute to your script tag, the following will happen.
<script src="myfile1.js" async></script>
<script src="myfile2.js" async></script>
Make parallel requests to fetch the files.
Continue parsing the document as if it was never interrupted.
Execute the individual scripts the moment the files are downloaded.
Defer
Defer is very similar to async with one major differerence. Here’s what happens when a browser encounters a script with the defer attribute.
<script src="myfile1.js" defer></script>
<script src="myfile2.js" defer></script>
Make parallel requests to fetch the individual files.
Continue parsing the document as if it was never interrupted.
Finish parsing the document even if the script files have downloaded.
Execute each script in the order they were encountered in the document.
Reference :Difference between Async and Defer
async and defer will download the file during HTML parsing. Both will not interrupt the parser.
The script with async attribute will be executed once it is downloaded. While the script with defer attribute will be executed after completing the DOM parsing.
The scripts loaded with async doesn't guarantee any order. While the scripts loaded with defer attribute maintains the order in which they appear on the DOM.
Use <script async> when the script does not rely on anything.
when the script depends use <script defer>.
Best solution would be add the <script> at the bottom of the body. There will be no issue with blocking or rendering.
Good practice is to keep all the files in your source folder to load source files fast. You need to download all the script, style, icon, and image-related files and put these files into your project folder.
Create these folders in your project to keep different source files and then load the required files into the pages from this folder.
JS: to keep script-related files.
CSS: to keep style-related files.
images: to keep image/icon-related files
fonts: to keep font-related files
When to use defer and async attributes on the <script> tag
defer attribute: First the defer attribute will download the script file and then wait for HTML parsing. After the end of the HTML parsing, the script will execute. In other words, it will guarantee all the scripts will execute after the HTML parsing.
The defer attribute is useful when the script is used for DOM manipulations.
async attribute: The async attribute will download the script file and execute without waiting for the end of HTML parsing. In other words, it does not guarantee that all the scripts will execute after the HTML parsing.
The async attribute is useful when the script is not used for DOM manipulation. Sometimes you need a script only for server-side operations or for handling cache or cookies, but not for DOM manipulations.
Useful link when to use defer and async:
https://stackoverflow.com/a/68929270/7186739
I think Jake Archibald presented us some insights back in 2013 that might add even more positiveness to the topic:
https://www.html5rocks.com/en/tutorials/speed/script-loading/
The holy grail is having a set of scripts download immediately without blocking rendering and execute as soon as possible in the order they were added. Unfortunately HTML hates you and won’t let you do that.
(...)
The answer is actually in the HTML5 spec, although it’s hidden away at the bottom of the script-loading section.
"The async IDL attribute controls whether the element will execute asynchronously or not. If the element's "force-async" flag is set, then, on getting, the async IDL attribute must return true, and on setting, the "force-async" flag must first be unset…".
(...)
Scripts that are dynamically created and added to the document are async by default, they don’t block rendering and execute as soon as they download, meaning they could come out in the wrong order. However, we can explicitly mark them as not async:
[
'//other-domain.com/1.js',
'2.js'
].forEach(function(src) {
var script = document.createElement('script');
script.src = src;
script.async = false;
document.head.appendChild(script);
});
This gives our scripts a mix of behaviour that can’t be achieved with plain HTML. By being explicitly not async, scripts are added to an execution queue, the same queue they’re added to in our first plain-HTML example. However, by being dynamically created, they’re executed outside of document parsing, so rendering isn’t blocked while they’re downloaded (don’t confuse not-async script loading with sync XHR, which is never a good thing).
The script above should be included inline in the head of pages, queueing script downloads as soon as possible without disrupting progressive rendering, and executes as soon as possible in the order you specified. “2.js” is free to download before “1.js”, but it won’t be executed until “1.js” has either successfully downloaded and executed, or fails to do either. Hurrah! async-download but ordered-execution!
Still, this might not be the fastest way to load scripts:
(...) With the example above the browser has to parse and execute script to discover which scripts to download. This hides your scripts from preload scanners. Browsers use these scanners to discover resources on pages you’re likely to visit next, or discover page resources while the parser is blocked by another resource.
We can add discoverability back in by putting this in the head of the document:
<link rel="subresource" href="//other-domain.com/1.js">
<link rel="subresource" href="2.js">
This tells the browser the page needs 1.js and 2.js. link[rel=subresource] is similar to link[rel=prefetch], but with different semantics. Unfortunately it’s currently only supported in Chrome, and you have to declare which scripts to load twice, once via link elements, and again in your script.
Correction: I originally stated these were picked up by the preload scanner, they're not, they're picked up by the regular parser. However, preload scanner could pick these up, it just doesn't yet, whereas scripts included by executable code can never be preloaded. Thanks to Yoav Weiss who corrected me in the comments.
Rendering engine goes several steps till it paints anything on the screen.
it looks like this:
Converting HTML bytes to characters depending on encoding we set to the document;
Tokens are created according to characters. Tokens mean analyze characters and specify opening tangs and nested tags;
From tokens separated nodes are created. they are objects and according to information delivered from tokenization process, engine creates objects which includes all necessary information about each node;
after that DOM is created. DOM is tree data structure and represents whole hierarchy and information about relationship and specification of tags;
The same process goes to CSS. for CSS rendering engine creates different/separated data structure for CSS but it's called CSSOM (CSS Object Model)
Browser works only with Object models so it needs to know all information about DOM and CSSDOM.
The next step is combining somehow DOM and CSSOM. because without CSSOM browser do not know how to style each element during rendering process.
All information above means that, anything you provide in your html (javascript, css ) browser will pause DOM construction process. If you are familiar with event loop, there is simple rule how event loop executes tasks:
Execute macro tasks;
execute micro tasks;
Rendering;
So when you provide Javascript file, browser do not know what JS code is going to do and stops all DOM construction process and Javascript interptreter starts parsing and executing Javascript code.
Even you provide Javascript in the end of body tag, Browser will proceed all above steps to HTML and CSS but except rendering. it will find out Script tag and will stop until JS is done.
But HTML provided two additional options for script tag: async and defer.
Async - means execute code when it is downloaded and do not block DOM construction during downloading process.
Defer - means execute code after it's downloaded and browser finished DOM construction and rendering process.
It seems the behavior of defer and async is browser dependent, at least on the execution phase. NOTE, defer only applies to external scripts. I'm assuming async follows same pattern.
In IE 11 and below, the order seems to be like this:
async (could partially execute while page loading)
none (could execute while page loading)
defer (executes after page loaded, all defer in order of placement in file)
In Edge, Webkit, etc, the async attribute seems to be either ignored or placed at the end:
data-pagespeed-no-defer (executes before any other scripts, while page is loading)
none (could execute while page is loading)
defer (waits until DOM loaded, all defer in order of placement in file)
async (seems to wait until DOM loaded)
In newer browsers, the data-pagespeed-no-defer attribute runs before any other external scripts. This is for scripts that don't depend on the DOM.
NOTE: Use defer when you need an explicit order of execution of your external scripts. This tells the browser to execute all deferred scripts in order of placement in the file.
ASIDE: The size of the external javascripts did matter when loading...but had no effect on the order of execution.
If you're worried about the performance of your scripts, you may want to consider minification or simply loading them dynamically with an XMLHttpRequest.
Default - By default, as soon as the browser sees a script tag it downloads the file and then executes the script file. The script files are executed in the order of their occurrence.
async - The browser will download the script file and continue parsing HTML parallelly until the file is downloaded. The file is executed as soon as it is downloaded.
defer - The browser will download the script and do HTML parsing at the same time. After parsing is done, the script files are executed in the order of their occurrence.
Note:
In defer, the js files are executed in the order of their occurrence in the HTML file while in the case of the async attribute the script files are executed in the order of download time.
Both the async and defer attributes are used to load external JavaScript files asynchronously, which means that the HTML parsing and rendering process is not blocked while the external file is being downloaded and executed.
When you download a web page, 2 major things happen in your browser:
HTML Parsing
Loading of the scripts
2.1 Fetching scripts from the network
2.2 Executing the scripts line by line
However, there are some differences between the two attributes:
1.async attribute:
The async attribute allows the browser to download the script file asynchronously while continuing to parse the HTML document. Once the script file is downloaded, it will be executed immediately, regardless of whether or not the HTML document has finished parsing. This means that the script may execute before the rest of the page has loaded.
Example:
<script async src="script.js"></script>
2.defer attribute:
The defer attribute also downloads the script file asynchronously, but it will not be executed until the HTML document has finished parsing. This means that the script will be executed in the order in which it appears in the HTML document, and after the page has finished loading.
Example:
<script defer src="script.js"></script>
In general, the async attribute is used when the script file does not depend on other scripts or on the HTML document being fully loaded, while the defer attribute is used when the script file depends on other scripts or on the HTML document being fully loaded.
Summary:
Async is suitable if your script doesn’t contains DOM manipulation and other scripts doesn’t depend upon on this.
Eg: bootstrap cdn,jquery
Defer is suitable if your script contains DOM manipulation and other scripts depend upon on this.
Eg: <script src=”createfirst.js”> //let this will create element <script src=”showfirst.js”> //after createfirst create element it will show that.
Thus make it:
Eg: <script defer src=”createfirst.js”> //let this will create element <script defer src=”showfirst.js”> //after createfirst create element it will
This will execute scripts in order.
But if i made:
Eg: <script async src=”createfirst.js”> //let this will create element <script defer src=”showfirst.js”> //after createfirst create element it will
Then, this code might result unexpected results.
Coz: if html parser access createfirst script.It won’t stop DOM creation and starts downloading code from src .Once src got resolved/code got downloaded, it will execute immediately parallel with DOM.
What if showfirst.js execute first than createfirst.js.This might be possible if createfirst takes long time (Assume after DOM parsing finished).Then, showfirst will execute immediately.
I wonder if anyone has found a way to send at mid rendering the head tag so CSS and Javascript are loaded before the page render has finished? Our page takes about 523ms to be rendered and resources aren't loaded until the page is received. I've done a lot of PHP and it is possible to flush the buffer before the end of the script. I've tried to add a Response.flush() at the end of the Masterpage page_load, but the page layout is horribly broken afterward. I've seen a lot of people using an update panel to send the content using AJAX afterward but I don't quite know what impact it would have on the SEO.
If I don't find a solution I guess I'd have to go the reverse proxy route and find a way to invalidate the proxy cache when the pages content change.
Do not place the Flush on code behind but on your html page as:
</head>
<%Response.Flush();%>
<body >
This can make something like fleekering effect on the page, so you can try to move the flush even a little lower to the page.
Also on Yahoo tips page at Flush the Buffer Early
http://developer.yahoo.com/performance/rules.html
Cache on Static
Additionally you can add client cache on static content like the css and javascript. In this page have all the ways for all iis versions.
http://www.iis.net/ConfigReference/system.webServer/staticContent/clientCache
Follow up
One more think that I suggest you to do after I see your pages is to place all css and javascript in one file each. And also use minified to minimize them.
I use this minified http://www.asp.net/ajaxlibrary/Download.ashx with very good results and real time minified.
Consider using a content-delivery-network (CDN) to host your images, CSS and JS files. Browsers have either an eight or four connection limit per domain - so once you use those up the browser has to wait for the resources to be freed up.
By hosting some files on the CDN you get another set of connections to use concurrently, allowing everything to load faster.
Also consider enabling GZIP on your server if you haven't already. This compresses files on the fly, resulting in smaller transfers.
You could use jQuery to execute your js as soon as it is loaded.
$.fn.ready(function(){
//Your code here
})
Or you could just take the standalone ready function -> $(document).ready equivalent without jQuery
You could do a fade-in or show once the document has been loaded. Just set body display:none;