When embedding JavaScript in an HTML document, where is the proper place to put the <script> tags and included JavaScript? I seem to recall that you are not supposed to place these in the <head> section, but placing at the beginning of the <body> section is bad, too, since the JavaScript will have to be parsed before the page is rendered completely (or something like that). This seems to leave the end of the <body> section as a logical place for <script> tags.
So, where is the right place to put the <script> tags?
(This question references this question, in which it was suggested that JavaScript function calls should be moved from <a> tags to <script> tags. I'm specifically using jQuery, but more general answers are also appropriate.)
Here's what happens when a browser loads a website with a <script> tag on it:
Fetch the HTML page (e.g. index.html)
Begin parsing the HTML
The parser encounters a <script> tag referencing an external script file.
The browser requests the script file. Meanwhile, the parser blocks and stops parsing the other HTML on your page.
After some time the script is downloaded and subsequently executed.
The parser continues parsing the rest of the HTML document.
Step #4 causes a bad user experience. Your website basically stops loading until you've downloaded all scripts. If there's one thing that users hate it's waiting for a website to load.
Why does this even happen?
Any script can insert its own HTML via document.write() or other DOM manipulations. This implies that the parser has to wait until the script has been downloaded and executed before it can safely parse the rest of the document. After all, the script could have inserted its own HTML in the document.
However, most JavaScript developers no longer manipulate the DOM while the document is loading. Instead, they wait until the document has been loaded before modifying it. For example:
<!-- index.html -->
<html>
<head>
<title>My Page</title>
<script src="my-script.js"></script>
</head>
<body>
<div id="user-greeting">Welcome back, user</div>
</body>
</html>
JavaScript:
// my-script.js
document.addEventListener("DOMContentLoaded", function() {
// this function runs when the DOM is ready, i.e. when the document has been parsed
document.getElementById("user-greeting").textContent = "Welcome back, Bart";
});
Because your browser does not know my-script.js isn't going to modify the document until it has been downloaded and executed, the parser stops parsing.
Antiquated recommendation
The old approach to solving this problem was to put <script> tags at the bottom of your <body>, because this ensures the parser isn't blocked until the very end.
This approach has its own problem: the browser cannot start downloading the scripts until the entire document is parsed. For larger websites with large scripts and stylesheets, being able to download the script as soon as possible is very important for performance. If your website doesn't load within 2 seconds, people will go to another website.
In an optimal solution, the browser would start downloading your scripts as soon as possible, while at the same time parsing the rest of your document.
The modern approach
Today, browsers support the async and defer attributes on scripts. These attributes tell the browser it's safe to continue parsing while the scripts are being downloaded.
async
<script src="path/to/script1.js" async></script>
<script src="path/to/script2.js" async></script>
Scripts with the async attribute are executed asynchronously. This means the script is executed as soon as it's downloaded, without blocking the browser in the meantime.
This implies that it's possible that script 2 is downloaded and executed before script 1.
According to http://caniuse.com/#feat=script-async, 97.78% of all browsers support this.
defer
<script src="path/to/script1.js" defer></script>
<script src="path/to/script2.js" defer></script>
Scripts with the defer attribute are executed in order (i.e. first script 1, then script 2). This also does not block the browser.
Unlike async scripts, defer scripts are only executed after the entire document has been loaded.
(To learn more and see some really helpful visual representations of the differences between async, defer and normal scripts check the first two links at the references section of this answer)
Conclusion
The current state-of-the-art is to put scripts in the <head> tag and use the async or defer attributes. This allows your scripts to be downloaded ASAP without blocking your browser.
The good thing is that your website should still load correctly on the 2% of browsers that do not support these attributes while speeding up the other 98%.
References
async vs defer attributes
Efficiently load JavaScript with defer and async
Remove Render-Blocking JavaScript
Async, Defer, Modules: A Visual Cheatsheet
Just before the closing body tag, as stated on Put Scripts at the Bottom:
Put Scripts at the Bottom
The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.
Non-blocking script tags can be placed just about anywhere:
<script src="script.js" async></script>
<script src="script.js" defer></script>
<script src="script.js" async defer></script>
async script will be executed asynchronously as soon as it is available
defer script is executed when the document has finished parsing
async defer script falls back to the defer behavior if async is not supported
Such scripts will be executed asynchronously/after document ready, which means you cannot do this:
<script src="jquery.js" async></script>
<script>jQuery(something);</script>
<!--
* might throw "jQuery is not defined" error
* defer will not work either
-->
Or this:
<script src="document.write(something).js" async></script>
<!--
* might issue "cannot write into document from an asynchronous script" warning
* defer will not work either
-->
Or this:
<script src="jquery.js" async></script>
<script src="jQuery(something).js" async></script>
<!--
* might throw "jQuery is not defined" error (no guarantee which script runs first)
* defer will work in sane browsers
-->
Or this:
<script src="document.getElementById(header).js" async></script>
<div id="header"></div>
<!--
* might not locate #header (script could fire before parser looks at the next line)
* defer will work in sane browsers
-->
Having said that, asynchronous scripts offer these advantages:
Parallel download of resources:
Browser can download stylesheets, images and other scripts in parallel without waiting for a script to download and execute.
Source order independence:
You can place the scripts inside head or body without worrying about blocking (useful if you are using a CMS). Execution order still matters though.
It is possible to circumvent the execution order issues by using external scripts that support callbacks. Many third party JavaScript APIs now support non-blocking execution. Here is an example of loading the Google Maps API asynchronously.
The standard advice, promoted by the Yahoo! Exceptional Performance team, is to put the <script> tags at the end of the document's <body> element so they don't block rendering of the page.
But there are some newer approaches that offer better performance, as described in this other answer of mine about the load time of the Google Analytics JavaScript file:
There are some great slides by Steve Souders (client-side performance expert) about:
Different techniques to load external JavaScript files in parallel
their effect on loading time and page rendering
what kind of "in progress" indicators the browser displays (e.g. 'loading' in the status bar, hourglass mouse cursor).
The modern approach is using ES6 'module' type scripts.
<script type="module" src="..."></script>
By default, modules are loaded asynchronously and deferred. i.e. you can place them anywhere and they will load in parallel and execute when the page finishes loading.
Further reading:
The differences between a script and a module
The execution of a module being deferred compared to a script(Modules are deferred by default)
Browser Support for ES6 Modules
If you are using jQuery then put the JavaScript code wherever you find it best and use $(document).ready() to ensure that things are loaded properly before executing any functions.
On a side note: I like all my script tags in the <head> section as that seems to be the cleanest place.
<script src="myjs.js"></script>
</body>
The script tag should always be used before the body close or at the bottom in HTML file.
The Page will load with HTML and CSS and later JavaScript will load.
Check this if required:
http://stevesouders.com/hpws/rule-js-bottom.php
The best place to put <script> tag is before closing </body> tag, so the downloading and executing it doesn't block the browser to parse the HTML in document,
Also loading the JavaScript files externally has its own advantages like it will be cached by browsers and can speed up page load times, it separates the HTML and JavaScript code and help to manage the code base better.
But modern browsers also support some other optimal ways, like async and defer to load external JavaScript files.
Async and Defer
Normally HTML page execution starts line by line. When an external JavaScript <script> element is encountered, HTML parsing is stopped until a JavaScript is download and ready for execution. This normal page execution can be changed using the defer and async attribute.
Defer
When a defer attribute is used, JavaScript is downloaded parallelly with HTML parsing, but it will be execute only after full HTML parsing is done.
<script src="/local-js-path/myScript.js" defer></script>
Async
When the async attribute is used, JavaScript is downloaded as soon as the script is encountered and after the download, it will be executed asynchronously (parallelly) along with HTML parsing.
<script src="/local-js-path/myScript.js" async></script>
When to use which attributes
If your script is independent of other scripts and is modular, use async.
If you are loading script1 and script2 with async, both will run
parallelly along with HTML parsing, as soon as they are downloaded
and available.
If your script depends on another script then use defer for both:
When script1 and script2 are loaded in that order with defer, then script1 is guaranteed to execute first,
Then script2 will execute after script1 is fully executed.
Must do this if script2 depends on script1.
If your script is small enough and is depended by another script
of type async then use your script with no attributes and place it above all the async scripts.
Reference: External JavaScript JS File – Advantages, Disadvantages, Syntax, Attributes
It turns out it can be everywhere.
You can defer the execution with something like jQuery so it doesn't matter where it's placed (except for a small performance hit during parsing).
The most conservative (and widely accepted) answer is "at the bottom just before the ending tag", because then the entire DOM will have been loaded before anything can start executing.
There are dissenters, for various reasons, starting with the available practice to intentionally begin execution with a page onload event.
It depends. If you are loading a script that's necessary to style your page / using actions in your page (like click of a button) then you better place it at the top. If your styling is 100% CSS and you have all fallback options for the button actions then you can place it at the bottom.
Or the best thing (if that's not a concern) is you can make a modal loading box, place your JavaScript code at the bottom of your page and make it disappear when the last line of your script gets loaded. This way you can avoid users using actions in your page before the scripts are loaded. And also avoid the improper styling.
Including scripts at the end is mainly used where the content/ styles of the web page is to be shown first.
Including the scripts in the head loads the scripts early and can be used before the loading of the whole web page.
If the scripts are entered at last the validation will happen only after the loading of the entire styles and design which is not appreciated for fast responsive websites.
You can add JavaScript code in an HTML document by employing the dedicated HTML tag <script> that wraps around JavaScript code.
The <script> tag can be placed in the <head> section of your HTML, in the <body> section, or after the </body> close tag, depending on when you want the JavaScript to load.
Generally, JavaScript code can go inside of the document <head> section in order to keep them contained and out of the main content of your HTML document.
However, if your script needs to run at a certain point within a page’s layout — like when using document.write to generate content — you should put it at the point where it should be called, usually within the <body> section.
Depending on the script and its usage the best possible (in terms of page load and rendering time) may be to not use a conventional <script>-tag per se, but to dynamically trigger the loading of the script asynchronously.
There are some different techniques, but the most straightforward is to use document.createElement("script") when the window.onload event is triggered. Then the script is loaded first when the page itself has rendered, thus not impacting the time the user has to wait for the page to appear.
This naturally requires that the script itself is not needed for the rendering of the page.
For more information, see the post Coupling async scripts by Steve Souders (creator of YSlow, but now at Google).
Script blocks DOM load until it's loaded and executed.
If you place scripts at the end of <body>, all of the DOM has a chance to load and render (the page will "display" faster). <script> will have access to all of those DOM elements.
On the other hand, placing it after the <body> start or above will execute the script (where there still aren't any DOM elements).
You are including jQuery which means you can place it wherever you wish and use .ready().
You can place most of <script> references at the end of <body>.
But if there are active components on your page which are using external scripts, then their dependency (.js files) should come before that (ideally in the head tag).
The best place to write your JavaScript code is at the end of the document after or right before the </body> tag to load the document first and then execute the JavaScript code.
<script> ... your code here ... </script>
</body>
And if you write in jQuery, the following can be in the head document and it will execute after the document loads:
<script>
$(document).ready(function(){
// Your code here...
});
</script>
If you still care a lot about support and performance in Internet Explorer before version 10, it's best to always make your script tags the last tags of your HTML body. That way, you're certain that the rest of the DOM has been loaded and you won't block and rendering.
If you don't care too much any more about in Internet Explorer before version 10, you might want to put your scripts in the head of your document and use defer to ensure they only run after your DOM has been loaded (<script type="text/javascript" src="path/to/script1.js" defer></script>). If you still want your code to work in Internet Explorer before version 10, don't forget to wrap your code in a window.onload even, though!
I think it depends on the webpage execution.
If the page that you want to display can not displayed properly without loading JavaScript first then you should include the JavaScript file first.
But if you can display/render a webpage without initially download JavaScript file, then you should put JavaScript code at the bottom of the page. Because it will emulate a speedy page load, and from a user's point of view, it would seems like that the page is loading faster.
Always, we have to put scripts before the closing body tag expect some specific scenario.
For Example :
`<html> <body> <script> document.getElementById("demo").innerHTML = "Hello JavaScript!"; </script> </body> </html>`
Prefer to put it before the </body> closing tag.
Why?
As per the official doc: https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web/JavaScript_basics#a_hello_world!_example
Note: The reason the instructions (above) place the element
near the bottom of the HTML file is that the browser reads code in the
order it appears in the file.
If the JavaScript loads first and it is supposed to affect the HTML
that hasn't loaded yet, there could be problems. Placing JavaScript
near the bottom of an HTML page is one way to accommodate this
dependency. To learn more about alternative approaches, see Script
loading strategies.
In an attempt to create polymer elements that use requirejs modules I ran into a blocking issue. I understand that polymer is not designed to work with requirejs, but for the time being It is my only option.
Searching for answers I found two solutions:
Don't use requirejs and make your modules compatible with HTML imports.
Put Polymer() call inside the requirejs callback as described here
Since I have to use require, at least for the time being, I went with the solution no.2. However, it turns out the solution causes asynchronous delays of element registration and incorrect data binding prior to Polymer upgrading the element.
Digging deeper into this issue, I started hacking undocumented Polymer internals with an intention to stop Polymer entirely until requirejs does its thing. Here is what I came up with:
Polymer.require = function(tag, deps, func) {
var stopper = {}
Polymer.queue.wait(stopper);
require(deps, function() {
delete stopper.__queue;
Polymer.queue.check();
Polymer(tag, func.apply(this, arguments));
});
};
I know this is terribly wrong. Is there a better solution?
I found that if I embed the call to require within the Polymer script I avoid this issue.
<link rel="import" href="../polymer/polymer.html"/>
<script src="../requirejs/require.js"></script>
<script src="../something/something.js"></script>
<polymer-element name="some-component">
<template>...</template>
<script>
(function() {
Polymer('some-component', {
someMethod: function () {
require(['something'], function (Something) {
var something = new Something();
...
}
}
)();
</script>
</polymer-element>
So there's this solution from Scott Miles but I find it a bit simplistic and inflexible as it relies on:
<script> tags to be executed in order, therefore ruling out:
async script tags
xhr based script loading
polymer getting loaded from a <script> tag, therefore:
layout.html and associated css won't be loaded
any future call to polymer.html won't be deduped
If you want more control over your bootstrapping logic you will need to enforce some amount of synchronisation between your components (which is what both requirejs and polymer are competing to do) before those are fully loaded.
The previous example is a more declarative (read polymer) way of doing things but falls short of fine grained tuning. I've started working on a repository to show how you can fully customise your load ordering, by using a more imperative approach where requirejs is given priority to orchestrate the rest of the bootstrapping.
At the time of writing, this extra control comes at the price of worse performance as the various scripts can't be loaded in parallel but I'm still working on optimising this.
Is it possible to access google closure library functions from google app scripts via HtmlService? The html files in the google scripts seems to be filtering out anything related to closure library.
project: I am exploring DOM manipulation utilities from Google Closure library from within the google app scripts using HtmlService. I intend to run this as a stand alone web app.
The closure functions work when directly loaded into the browser from its local client environment - but they dont work when injected from GAS app via the HtmlService utility.
Here is the code I am using in the GAS.
html file
<html>
<head>
<script src="http://closure-library.googlecode.com/svn/trunk/closure/goog/base.js"></script>
<script>
goog.require('goog.dom');
function c_sayHi() {
var newHeader = goog.dom.createDom('h1', {'style': 'background-color:#EEE'},'Hello world!');
goog.dom.appendChild(document.body, newHeader);
}
</script>
</head>
<script>
function c_updateButton(date, button) {
button.value = "clicked at " + date;
}
</script>
<body onload="c_sayHi()">
<input type='button' value='Never Clicked'
onclick='google.script.run.withSuccessHandler(c_updateButton).withUserObject(this).s_getCurrentDate()'>
<input type='button' value='Never Clicked'
onclick='google.script.run.withSuccessHandler(c_updateButton).withUserObject(this).s_getCurrentDate()'>
</body>
</html>
Google Script file
function s_getCurrentDate() {
return new Date().toString();
}
function doGet(e) {
return HtmlService.createTemplateFromFile('hello').evaluate();
}
I have prefixed c_ to client side functions and s_ for server side fns. When running this as a web app,
Function c_sayHi has no effect - I am not sure if it is even invoked.
Functions s_getCurrentDate and c_updateButton work fine as described in google's documentation https://developers.google.com/apps-script/html_service.
Is there a way to get closure library working from the web apps as attempted above?
Couple of things here -
All .gs files is JavaScript that runs on the server side. So the DOM is not really relevant there.
You can run client side JavaScript by returning code in HtmlService. This is what I believe you want to do. However, jQuery is the best supported library on this approach. Closure might end up working but the team does not specifically test against that library.
The problem is that Closure's dependency structure is executing before the window load event, otherwise it will not work. So any require and provide statements are taken care of way before window load. When you inject them through the HTML Service, you are forcing their execution at a different stage then required, which causes everything to fail.
If you would be using a COMPILED Closure Library source, you will not have any problems with running Closure. Learn how to use the Compiler and Builder to make Closure Work properly. Also, you can use lazy loading to simulate your HTML Service.
With that, you can make javascript load dynamically onclick, onload or whatever the hell you want. This is called lazy-loading and it is used as a standard practice for all large web applications. Monitor the Network tab of Firebug when browsing through Gmail or Facebook.
Arun Nagarajan is right, jQuery is the easier solution but if you are doing something proper that requires breadth, scale and speed, jQuery is a toy for kids.
I have been looking at a HTML 5 boilerplate template (from http://html5boilerplate.com/) and noticed the use of "?v=1" in URLs when referring to CSS and JavaScript files.
What does appending "?v=1" to CSS and JavaScript URLs in link and script tags do?
Not all JavaScript URLs have the "?v=1" (example from the sample below: js/modernizr-1.5.min.js). Is there a reason why this is the case?
Sample from their index.html:
<!-- CSS : implied media="all" -->
<link rel="stylesheet" href="css/style.css?v=1">
<!-- For the less-enabled mobile browsers like Opera Mini -->
<link rel="stylesheet" media="handheld" href="css/handheld.css?v=1">
<!-- All JavaScript at the bottom, except for Modernizr which enables HTML5 elements & feature detects -->
<script src="js/modernizr-1.5.min.js"></script>
<!------ Some lines removed ------>
<script src="js/plugins.js?v=1"></script>
<script src="js/script.js?v=1"></script>
<!--[if lt IE 7 ]>
<script src="js/dd_belatedpng.js?v=1"></script>
<![endif]-->
<!-- yui profiler and profileviewer - remove for production -->
<script src="js/profiling/yahoo-profiling.min.js?v=1"></script>
<script src="js/profiling/config.js?v=1"></script>
<!-- end profiling code -->
These are usually to make sure that the browser gets a new version when the site gets updated with a new version, e.g. as part of our build process we'd have something like this:
/Resources/Combined.css?v=x.x.x.buildnumber
Since this changes with every new code push, the client's forced to grab a new version, just because of the querystring. Look at this page (at the time of this answer) for example:
<link ... href="http://sstatic.net/stackoverflow/all.css?v=c298c7f8233d">
I think instead of a revision number the SO team went with a file hash, which is an even better approach, even with a new release, the browsers only forced to grab a new version when the file actually changes.
Both of these approaches allow you to set the cache header to something ridiculously long, say 20 years...yet when it changes, you don't have to worry about that cache header, the browser sees a different querystring and treats it as a different, new file.
This makes sure you are getting the latest version from of the css or js file from the server.
And later you can append "?v=2" if you have a newer version and "?v=3", "?v=4" and so on.
Note that you can use any querystring, 'v' is not a must for example:
"?blah=1" will work as well.
And
"?xyz=1002" will work.
And this is a common technique because browsers are now caching js and css files better and longer.
The hash solution is nice but not really human readable when you want to know what version of file is sitting in your local web folder. The solution is to date/time stamp your version so you can easily compare it against your server file.
For example, if your .js or .css file is dated 2011-02-08 15:55:30 (last modification) then the version should equal to .js?v=20110208155530
Should be easy to read properties of any file in any language. In ASP.Net it's really easy...
".js?v=" + File.GetLastWriteTime(HttpContext.Current.Request.PhysicalApplicationPath + filename).ToString("yyMMddHHHmmss");
Of coz get it nicely refactored into properties/functions first and off you go. No more excuses.
Good luck, Art.
In order to answer you questions;
"?v=1" this is written only beacuse to download a fresh copy of the css and js files instead of using from the cache of the browser.
If you mention this query string parameter at the end of your stylesheet or the js file then it forces the browser to download a new file, Due to which the recent changes in the .css and .js files are made effetive in your browser.
If you dont use this versioning then you may need to clear the cache of refresh the page in order to view the recent changes in those files.
Here is an article that explains this thing How and Why to make versioning of CSS and JS files
Javascript files are often cached by the browser for a lot longer than you might expect.
This can often result in unexpected behaviour when you release a new version of your JS file.
Therefore, it is common practice to add a QueryString parameter to the URL for the javascript file. That way, the browser caches the Javascript file with v=1. When you release a new version of your javascript file you change the url's to v=2 and the browser will be forced to download a new copy.
During development / testing of new releases, the cache can be a problem because the browser, the server and even sometimes the 3G telco (if you do mobile deployment) will cache the static content (e.g. JS, CSS, HTML, img). You can overcome this by appending version number, random number or timestamp to the URL e.g: JSP: <script src="js/excel.js?time=<%=new java.util.Date()%>"></script>
In case you're running pure HTML (instead of server pages JSP, ASP, PHP) the server won't help you. In browser, links are loaded before the JS runs, therefore you have to remove the links and load them with JS.
// front end cache bust
var cacheBust = ['js/StrUtil.js', 'js/protos.common.js', 'js/conf.js', 'bootstrap_ECP/js/init.js'];
for (i=0; i < cacheBust.length; i++){
var el = document.createElement('script');
el.src = cacheBust[i]+"?v=" + Math.random();
document.getElementsByTagName('head')[0].appendChild(el);
}
As you can read before, the ?v=1 ensures that your browser gets the version 1 of the file. When you have a new version, you just have to append a different version number and the browser will forget about the old version and loads the new one.
There is a gulp plugin which takes care of version your files during the build phase, so you don't have to do it manually. It's handy and you can easily integrate it in you build process. Here's the link: gulp-annotate
As mentioned by others, this is used for front end cache busting. To implement this, I have personally find grunt-cache-bust npm package useful.