It is well known to everyone that using defer is an efficient way to minimize the loading time of a website.
In my project, I am using Vue (included in the header using defer) and in a circumstance, I want to use a component that is created by another person. When I try to do Vue.Component(...) in the body of the HTML, it says Vue is undefined. It seems that my script in the body is running before the external script has been loaded. Is there any way to fix this issue?
I tried to do document.onload, but the function itself is not working.
PS: Just to be clear, the external script is referring to my js file where Vue is defined and I am not talking about the third party library at all.
Instead of document.onload you need to use window.onload or document.body.onload.
But an even better way is to wait for the load event on <script> tag:
<html>
<head>
<script id="vue-script" src="vue.js" charset="utf-8" defer></script>
</head>
<body>
<script type="text/javascript">
function onVueLoaded() {
Vue.render();
}
if ('Vue' in window) {
onVueLoaded();
} else {
var script = document.getElementById('vue-script');
script.addEventListener('load', onVueLoaded);
script.addEventListener('error', () => console.warn('failed to load Vue.js'));
}
</script>
</body>
</html>
Here I also added a handler for the error event if you wanted to explicitly handle loading errors.
I have the following html code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script src="https://cdn.jsdelivr.net/blazy/1.8.2/blazy.min.js" defer></script>
<script src="https://code.jquery.com/jquery-2.1.4.min.js" integrity="sha256-8WqyJLuWKRBVhxXIL1jBDD7SDxU936oZkCnxQbWwJVw=" crossorigin="anonymous" defer></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/lightbox2/2.9.0/js/lightbox.min.js" defer></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous" defer></script>
<!-- 26 dec flexslider js -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/flexslider/2.6.3/jquery.flexslider.min.js" defer></script>
<script defer>
(function($) {
$(document).ready(function() {
//do something with b-lazy plugin, lightbox plugin and then with flexslider
});
})(jQuery);
</script>
</head>
<body>
</body>
</html>
I get an error, saying jQuery is not defined. Now even if I remove defer from my inline JS code, it says jQuery is undefined. For some reason I have to keep the jQuery plugins in the head and keep my JS code inline. My question is:
Why doesn't inline Javascript code get deferred when defer attribute is present on it?
Is there a way to imitate the defer behavior on my inline Javascript code? I can put that at the end of body tag if required.
The scripts with the defer attribute load in the order they are specified, but not before the document itself has been loaded. As defer has no effect on script tags unless they also have the src attribute, the first script that gets executed is your inline script. So at that time jQuery is not loaded yet.
You can solve this in at least two ways:
Put your inline script in a .js file and reference it with a src attribute (in addition to the defer attribute which you already had there), or
Let your inline script wait for the document and the deferred scripts to be loaded. The DOMContentLoaded event will fire when that has happened:
<script>
window.addEventListener('DOMContentLoaded', function() {
(function($) {
//do something with b-lazy plugin, lightbox plugin and then with flexslider
})(jQuery);
});
</script>
NB: Notice that in the latter case $(document).ready(function() is not included any more, as that would wait for the same event (DOMContentLoaded). You could still include it like you had in your original code, but then jQuery would just execute the callback immediately, which makes no practical difference.
You can create a Base64 URL out of the script and put it into the src!
<script src="data:text/javascript;base64,YWxlcnQoJ0hlbGxvIHdvcmxkIScpOw=="
defer>
</script>
I built a quick test to see it in action.
You should see an alert with Hello world! last if defer is working:
<script defer>
alert('Why no defer?!?');
</script>
<!-- alert('Hello world!'); -->
<script src="data:text/javascript;base64,YWxlcnQoJ0hlbGxvIHdvcmxkIScpOw=="
defer></script>
<script>
alert('Buh-bye world!');
</script>
Doing it manually is a little laborious so if you have the luxury of compiling your HTML in some way (Handlebars, Angular, etc.) then that helps a lot.
I'm currently using:
<script src="data:text/javascript;base64,{{base64 "alert('Hello world!');"}}"
defer>
</script>
You can also use type="module":
<meta charset="utf-8">
<script type="module">
let t = document.getElementById('top');
console.log(t);
</script>
<h1 id="top">Top Questions</h1>
https://developer.mozilla.org/docs/Web/HTML/Element/script#attr-type
From MDN docs:
defer
This Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded. The defer attribute should only be used on external scripts.
This is called an IIFE (Immediately Invoked Function Expression) which gets executed before DOM is available. So, in that case jQuery is undefined because it it not in the DOM.
defer loading with plain text Data URI - Chrome and FF
#noLib #vanillaJS
suggest not to use on Cross Browser PRODuction
until MS IE dies and MS Edge will adopt the Chromium open source ;)
the only way to defer script is external file or Data_URI (without using event DOMContentLoaded)
defer
spec script#attr-defer (MDN web docs): "This attribute must not be used if the src attribute is absent (i.e. for inline scripts), in this case it would have no effect.)"
Data_URI
spec Data_URI
with right type "text/javascript" there is no need to base64 at all... ;)
using plain text so you can use simple:
<script defer src="data:text/javascript,
//do something with b-lazy plugin, lightbox plugin and then with flexslider
lightbox.option({
resizeDuration: 200,
wrapAround: true
})
">
yes, it's little bit weird hack, but <script type="module"> are deferred by default, there is no other option to mix following in exact order:
module external files - deferred by default
module inline scripts - deferred by default
external files - optionally deferred
inline scripts - only with this hack - as I know (without libraries/frameworks)
Defer/async script tags are not good enough
There is a common knowledge that you should use <script src=".." async defer> (or set script.async = true before assigning src, when you do it from JS) and/or put your scripts at the very bottom of the page, so that as much as possible of the page gets loaded and rendered to the user, as fast as possible.
defer.js (note: I am the author of this script) is written in plain JavaScript, making lazy-loading other contents more fast and performant. You can defer any javascript files as well as inline script blocks efficiently.
If your page is just an HTML page enhanced with some JavaScript, then you're good with just <script async>. It takes time for browser to parse and execute those scripts, and each UI change may reflow your layout, make your load speed more slow, no one likes staring at a blank white page; users are impatient and will leave quickly.
In various cases, using async or defer does not deliver faster page speed than defer.js does.
I checked all the proposed solutions but all have their disadvantages. So I invented my own.
Put this inline script into your head tag or right after the start of body tag:
<script>var Defer = []; document.addEventListener('DOMContentLoaded', function() { while (Defer.length) Defer.shift().call(); }); </script>
This one liner will collect all the inline scripts you want to defer and run them respectively as soon as document is fully loaded. Now anytime you need to run an inline script deferred, just register it like:
<script>
alert('This alert will show immediately.');
Defer.push(function() {
alert('This alert will show only after document is loaded.');
// You can use anything which is not loaded yet, like jQuery
$(".selector").doSomeJqueryStuff();
});
// You can use it as many times as you like and in any place of your DOM.
Defer.push(function() {
// Any inline code you want to defer
});
</script>
This inline script will run only after document is loaded. That means you can run inline jQuery script having your jQuery stay at the end of your DOM.
You can use this data url as src attribute
data:application/javascript,eval(document.currentScript.textContent)
which takes this current script tag and evaluate its content as if it was inside an external file.
it also works with lazy attribute.
it uses document.currentScript which not supported by IE browsers.
<script defer src="https://cdn.jsdelivr.net/npm/vue"></script>
<script defer src="data:application/javascript,eval(document.currentScript.textContent)">
console.log('defered', typeof Vue); // function
</script>
<script>
console.log('not defered', typeof Vue); // undefined
</script>
There is a somewhat less obscure way to accomplish deferral that does not require callbacks, promises, or data urls ... although it does a little DOM manipulation in the background. The tiny library (109 bytes compressed/gziped) https://www.npmjs.com/package/deferscript let's you do this. The example below is based on the original post.
<script src="https://cdnjs.cloudflare.com/ajax/libs/flexslider/2.6.3/jquery.flexslider.min.js" defer>
</script>
<script src="./deferscript.js" defer>
(function($) {
$(document).ready(function() {
//do something with b-lazy plugin, lightbox plugin and then with flexslider
});
})(jQuery);
</script>
All you have to do is insert a src attribute with the value ./deferscript.js.
If the problem is that jQuery variable $ is not defined, maybe you can create a fake $ function that returns a ready function waiting for the DOMContentLoaded?
All my inline scripts has $(document).ready(..... and the problem is that $ is not defined as the header scripts are deferred.
So, just add a fake $ in an inline script in head:
<script type="text/javascript">
var $ = function(element) {
return {
ready: function(callback) {
// in case the document is already rendered
if (document.readyState!="loading") callback();
// modern browsers
else if (document.addEventListener)
document.addEventListener("DOMContentLoaded", callback);
// IE <= 8
else document.attachEvent("onreadystatechange", function(){
if (document.readyState=="complete") callback();
});
}
};
};
</script>
I am looking for some kind of a custom generic function that would allow me to dynamically create elements with JavaScript/jQuery. Since I am working with XHTML and the application/xhtml+xml MIME type, I am forced to work with DOM functions instead of document.write();. Take the following fallback code for jQuery:
if (!window.jQuery) {
if (!navigator.userAgent.toLowerCase().match(/msie\s[5-8]/i)) {
document.write('<script charset="UTF-8" type="application/javascript" src="/assets/scripts/jquery-2.min.js"><\/script>');
} else {
document.write('<script charset="UTF-8" type="text/javascript" src="/assets/scripts/jquery-1.min.js"><\/script>');
}
}
The browser console will display an error and the code will not be executed. So what I think I need is something like this:
function writeXHTML(tag) {
// some ugly DOM-based code to execute in order for the tag to be created. Support IE, please!
}
And then use it like this:
if (!window.jQuery) {
if (!navigator.userAgent.toLowerCase().match(/msie\s[5-8]/i)) {
writeXHTML('<script charset="UTF-8" type="application/javascript" src="/assets/scripts/jquery-2.min.js"><\/script>');
} else {
writeXHTML('<script charset="UTF-8" type="text/javascript" src="/assets/scripts/jquery-1.min.js"><\/script>');
}
}
Obviously, I will need to create the jQuery script in the <head>, before the other <script>s that depend on it. And I would also need this to be done from an external script since I don't like inlin-ing :P
I guess I could use ids to do that, but what code should the writeXHTML(); function have? Thanks in advance!
I swear I have included jquery in the page header, it is right there!
Nonetheless the following code, which I've included near the bottom of the page (and inline for now) gives me an error saying "TypeError: $ is not a function."
<script>
function displayResult(longA, latA, longB, latB, units) {
$("#distance").html(calcDist(longA, latA, longB, latB, units));
if (units=="m") {
$("#unitLabel").html("miles");
$("units").prop("selectedIndex",0);
} else {
$("#unitLabel").html("kilometers");
$("#units").prop("selectedIndex",1);
}
$("#longA").val(longA);
$("#latA").val(latA);
$("#longB").val(longB);
$("#latB").val(latB);
}
$("#calculateButton").click(function() { //This is the line it's complaining about
var longA=$("#longA").val();
var latA=$("#latA").val();
var longB=$("#longB").val();
var latB=$("#latB").val();
var units=$("#units").val();
displayResult(longA, latA, longB, latB, units);
})(jQuery);
</script>
Higher up in the page header I've got the following:
<script src="jquery.js" ></script>
<script src="calcDistSinglePage.js" ></script>
I'm not using Wordpress or anything, this is a very straightforward hand-coded HTML page.
Try wrapping your code in a closure (which is considered good practice anyways):
(function($) {
$("#calculateButton").click(function() {
// do stuff...
});
}(jQuery));
If this snippet still complains with the same error, there's bound to be a problem with the way you're loading the jQuery library.
Also, make sure that you don't overwrite the $ variable in your other code. For example, inside calcDistSinglePage.js.
The dollar-sign is a very straight-forward javascript variable and can be reassigned to whatever you want. According to the error, $ currently is something but not a function (otherwise you'd receive a ReferenceError stating that $ is undefined). So probably, somewhere in your code, you've overwritten it.
Make sure the jQuery library it's the first script you load.
Add this just before the closing </body> tag.
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/vendor/jquery-{{JQUERY_VERSION}}.min.js"><\/script>')</script>
Download the file locally and inside js/vendor/ add the file.
Replace the value {{JQUERY_VERSION}} in the script above adding your jquery version.
Here one CDN you could use.
https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js
https://code.jquery.com/jquery-2.1.3.min.js
You are probably linking it from a root folder that is not your HTML folder. Use an absolute path:
<script src="/jquery.js" ></script>
Or make sure jquery.js is in the same folder as your HTML.
Here is my code:
<!DOCTYPE html>
<html lang="en" >
<head>
<script type="text/javascript">
function showResponse(response){
var responseString = JSON.stringify(response, '', 2);
document.getElementById('response').innerHTML += responseString;
}
function onClientLoad(){
gapi.client.load('youtube','v3', onYouTubeApiLoad);
}
function onYouTubeApiLoad(){
gapi.client.setApiKey('MyActualKey');
search();
}
function search(){
var request = gapi.client.youtube.search.list({
part: 'snippet'
});
request.execute(onSearchResponse);
}
function onSearchResponse(response){
showResponse(response);
}
</script>
<title></title>
<script src="https://apis.google.com/js/client.js?onload=onClientLoad"></script>
</head>
<body>
<div id="response"></div>
</body>
</html>
This code is from Codecademy, and I thought I can use it on an html page and it would work.
I got an API key from google and I set my Youtube data api v3 setting to enabled in my google developers console, but this code gives me a blank page.
What am I doing wrong?
There are a few missing pieces, code snippets which codecademy likely took for granted but which are essential when placing it in your own server outside of their app. First of all, you need a line that actually loads the gapi library from google. You can put this in your code, just before the closing :
<script src="https://apis.google.com/js/client.js?onload=onClientLoad"></script>
In short, this will get the library from Google's servers, and when it's loaded the library will automatically call your onClientLoad method, kicking off your app.
Next, you say you have an API key; make sure you put that key into your code by replacing this:
gapi.client.setApiKey('MyKey');
with this:
gapi.client.setApiKey('{WHATEVER_YOUR_ACTUAL_KEY IS');
Finally, as the commenters mentioned, your body is empty, so when your code executes the showResponse method there's no place to put what comes back. Add this:
<div id="response"></div>