How to onload two javascript files? - javascript

I have two different javascripts. I would like to start a function only after both of those js files or scripts have loaded, because they need to access each other's functions and/or variables.
I don't know which one will load first.
What is the proper way of doing it?
NOTE: one of the scripts (A) loads data asynchronously and the script (B) needs to wait till all the data is loaded and started before it can continue (ex youtube video is loaded from (A) and needs to start playing before (B) can execute)


<script> are loaded synchronously in general. The evaluation of the document stops an waits until the script is loaded and executed. It is handled that way, because the script might interfere with the document by using something like document.write(), so the parser can not be sure, that the rest of the document stays the way it appears at that moment.
So to execute you scripts after they have loaded, just place a 3rd <script> tag holding the start-code behind those two, that are loading the scripts.
<script src="scriptA.js"></script>
<script src="scriptB.js"></script>
<script>
// start something of scriptB
scriptB.start();
</script>


As scripts are loaded synchronously, just load the script that executes their functions after those two files.
Anyway, if you have scripts with dependencies, I would encourage you to use a module loader, such as RequireJS. That way, you could define which modules/files should be loaded before the execution begins.

Related

Google Analytics GA4 code in an external javascript file (.js) [duplicate]

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 sorce 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 required files into the pages from these folder.
js: to keep script related files.
css: to keep style related files.
img: to keep image/icon related files
fonts: to keep fonts related files
When to use defer and async attribute
defer attribute: First it will download the script file and then wait of html parsing. After the end of html parsing, script will execute. In other words, It will guarantee all the scripts will execute after the html parsing.
Defer attribute is useful when script is using for DOM manipulations. Means script will apply on document html.
async attribute: It will download the script file and execute without wait the end of html parsing. In other words, It will not guarantee all the scripts will execute after the html parsing.
Async attribute is useful when script is not using for DOM manipulation. Some time you need script only for server side operations or for handling cache or cookie but not for DOM manipulations. Means script is not related to the used html.
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.

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.

Difference between script tag with and without "async"? [duplicate]

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 sorce 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 required files into the pages from these folder.
js: to keep script related files.
css: to keep style related files.
img: to keep image/icon related files
fonts: to keep fonts related files
When to use defer and async attribute
defer attribute: First it will download the script file and then wait of html parsing. After the end of html parsing, script will execute. In other words, It will guarantee all the scripts will execute after the html parsing.
Defer attribute is useful when script is using for DOM manipulations. Means script will apply on document html.
async attribute: It will download the script file and execute without wait the end of html parsing. In other words, It will not guarantee all the scripts will execute after the html parsing.
Async attribute is useful when script is not using for DOM manipulation. Some time you need script only for server side operations or for handling cache or cookie but not for DOM manipulations. Means script is not related to the used html.
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.

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.

HTML DOM appendChild script async [duplicate]

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.

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.

AJAX ready event once HTML page and scripts are loaded

I'm creating a web application that has multiple pages of content that I'm loading dynamically with AJAX and the HTML5 History API. When a user attempts to change the page, the new content is loaded with a $.get and injected into the body, like so:
$.get("somepage.html", function (data)
{
$("body").html(data);
});
Most of these pages require additional scripts to be loaded. This wouldn't be an issue except for the fact that $(document).ready fires before these scripts are loaded. Somepage.html looks something like this.
<script src='http://getjquerysomewhere/'></script>
<script src='my_script_that_depends_on_jQuery'></script>
This issue is complicated by the fact that these pages must have the ability to be loaded on their own. I'm therefore unsure how I can eliminate the $(document).ready functions without affecting this behavior as well.
How should I approach this problem?

What you are trying to do is certainly possible, but it's not going to be very maintainable in the long-run.
One of the biggest issues you'll run into is properly injecting the code from the ajax loaded html into the current page. You can't just ignore it and let it all run because then you'll be including libraries multiple times (resulting in plugins getting overwritten/removed), and the code for the page you are loading may happen too soon due to the dom already being ready.
This pretty much leaves you with two options: dependency injection or front-loading.
Dependency injection will probably be the easiest of the two for you to implement because it requires the least amount of changes to your current code-base. All you would have to do is ensure that all pages requested with ajax only include the content of the <body> (which can be done with server-side code), and ensure that all page-specific code is included before the closing </body> of each page. Then you would just have to use the dependency-injection methods to run your code with the proper dependencies.
You could also have it only include <div id="#content">...</div> for your partials, which ever makes more sense for your use-case.
Front-loading would be a little more difficult because you'll have this one giant file that has all of your code for all of the pages, unless you use a build process (if you've never used a build-process before, you really should try it, even if you don't think you need it.) With front-loading, you'll either have to use event delegation, or have init methods for each page that you selectively execute as you load each page. This could become a maintainability nightmare without good build processes.

You can call functions from the original scripts on the page which you have loaded. For Instance you could do this in your main:
<script>
function ExternalScriptLoaded(){}
</script>
Then on your external page:
<script>
try{ ExternalScriptLoaded(); }catch(err){alert('This page was not loaded with ajax because i can't find the function');}
</script>
The alert will trigger if the script can't find the function on your main page.
i.e. call the function after you know the script has finished runnng.
Hope this helped.

putting jquery and javascript into external file

A lot of my pages have amll bits of jquery.
Im thinking of putting them into one external file with one $(document).ready(function() {
and everything in there.
is this a good/bad idea?
will each page be slower overall if there is more code to execute even if its not relevant to the page? i imagine each line of code in the external script gets executed when the dom is ready..? or is my understanding wrong?

will each page be slower overall if there is more code to execute even if its not relvamnt to the page?
The external script file may have some overheads for loading, but if you use the script on any number of pages more than one, external is a good idea; it'll be cached and be instant.
i imagine each line of code in the external script gets executed when the dom is ready..? or is my understanding wrong?
Yes. If you wrap your code in a function as an argument to $(document).ready() it gets executed on DOMContentLoaded.

If you put all your bits of JS into one page - without calling them as functions- then, yes, they will get executed everytime you include them. It would be better to put the common functions into an external scripts and keep it there. This will increase your code reuse as well as speed up page load because your JS will be cached.

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