I have a meta http-equiv="refresh" inside the <head>.
<head>
<meta name="mymeta" http-equiv="refresh" content="2" id="myMeta">
</head>
Using Javascript, I'm trying to change the content attribute of this meta tag.
var myMeta = document.getElementById("myMeta");
myMeta.content="10";
When I display the content via document.write(myMeta.content);, I get the changed value which is 10, however, the meta tag will keep refreshing each 2 seconds.
I have tested this both in Firefox and Opera.
FULL PAGE
<!DOCTYPE html>
<html>
<head>
<meta name="mymeta" http-equiv="refresh" content="2" id="myMeta">
<script>
var myMeta=document.getElementById("myMeta");
myMeta.content="10";
document.write(myMeta.content);
</script>
</head>
<body>
</body>
</html>
This happens because the browser immediately process the <meta> tag when it is present onload.
See DEMO.
When the document is being loaded, the browser sees and processes the following:
<meta name="mymeta" http-equiv="refresh" content="2" id="myMeta"/>
Even though you try to change its content from 2 to 10, that 2 second refresh is already acknowledged and the browser waits for 2 seconds before it refreshes the page. The 10-second refresh that is injected by JavaScript actually works*, although the page has been refreshed by the time it reaches 2 seconds and nothing seems to happen. This process is then repeated again and again.
Try the opposite and see what happens.
*This only works on Safari and Chrome. Firefox and Opera does not support the modification of meta refresh through JavaScript.
The getElementsByTagName method returns a NodeList so you need to specify an index to correctly access the element:
var myMeta = document.getElementsByTagName("meta")[0];
As someone mentioned this will probably still not work as the meta tag will need to be re-appended to have the desired effect.
Since you're using JavaScript you can just use setTimeout to achieve the same behavior
setTimeout(function() {
location.reload();
},2000); // reload page after 2 seconds
Related
I know, this question looks quite the same as those asked and reasked regularly, as for example here:
How to force reloading a page when using browser back button?
I need to know if the user has...
. navigated to current page, or reloaded current page (in both cases the page has been normally loaded, I need no further specific action)
. or has arrived on current page using history back or forward button (in these cases, the page is not loaded, just anyhow taken out of browser-cache, and I need to take further actions -the simplest being just reloading the page, window.location.reload(); or equivalent).
I tried the solutions exposed here: https://stackoverflow.com/a/43043658/3872061 and here: https://stackoverflow.com/a/56851042/3872061 as well as in several other places, within Stackoverflow or not.
It works well for Firefox (105.0.1), for Edge (105.0.1343.50), but I can't get it to work with Chrome (105.0.5195.127).
Here the simplest test I could imagine.
page1.html :
<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script>
$(window).on("pageshow", function(e){
$("#type").text(window.performance.getEntriesByType("navigation")[0].type);
let historyTraversal = event.persisted
|| ( typeof window.performance != "undefined" && window.performance.getEntriesByType("navigation")[0].type === "back_forward" );
if ( historyTraversal ) { // Handle page restore.
$("#backOrNot").text('User just came back using the history "back" button');
//window.location.reload(); // this is actually the targetted action: reload page
}
else {
$("#backOrNot").text('User loaded the page');
}
});
</script>
</head>
<body>
<p id="backOrNot"></p>
<p>( window.performance.getEntriesByType("navigation")[0].type = <span id="type"></span> )</p>
<p>Just a link from where you can click the history back button</p>
</body>
</html>
page2.html :
<!doctype html>
<html>
<head>
<meta charset="utf-8" />
</head>
<body>
<p>
From here, try to:<br>
1. Navigate to page1.html<br>
2. Go back to page1.html using the history back-button
</p>
</body>
</html>
With both FF and Edge, when you come back to page1 using the history, the "back_forward" information is available.
With Chrome, you just get the status ("navigate" or "reload") which the page had on the initial load,as if you had never navigated away then returned using the back button.
What have I missed? What am I doing wrong here?
Your page will automatically reload, if you send off page headers to not allow caching of that itemsList.php page. My test code -- see answer here -- should help you understand the concept. I am still trying to find a better way to solve that problem differently. In my case, I am firing off a "page-is-now-loading-spinner" that is not resetting itself to hidden, if the back button (go back one page) is presses.
I'm trying to calculate the load time and page size of different URLs/Pages similar to the developer tools performance tab but in javascript. But the current code only calculates its current page instead of a different URL. Is there any way for me to do this because with my research I have no luck.
<html>
<head>
<meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate">
<meta http-equiv="Pragma" content="no-cache">
<meta http-equiv="Expires" content="0">
<script type="text/javascript">
var start = new Date().getTime();
function onLoad() {
var now = new Date().getTime();
var latency = now - start;
alert("page loading time: " + latency+"\n"+"Start time:"+start+"\n"+"End time:"+now);
alert("Load_size:"+document.getElementsByTagName("html")[0].outerHTML.length + "KB");
}
</script>
</head>
<body onload="onLoad()">
<!- Main page body goes from here. -->
</body>
</html>
It will not be possible to read the runtime parameters of a page outside the page your javascript is running on.
Part of the security model is to avoid being able to inspect the runtime of other pages. This is called the "sandbox". You'll need to build a plugin that breaks the sandbox to inspect the domLoad / domReady and other performance events.
Good news though, you probably have one built in! The console for modern browsers shows all those events in the timeline tab.
If you're trying to make a service that attempts to evaluate the runtime of other pages, you'll need to load those in a virtual web browser on the server and interpret the results using selenium or something similar.
You can try this to calculate the load time of a page:
<script type="text/javascript">
$(document).ready(function() {
console.log("Time until DOMready: ", Date.now()-timerStart);
});
$(window).load(function() {
console.log("Time until everything loaded: ", Date.now()-timerStart);
});
</script>
edit: this will only work on pages where this JS code will run, so if you cant insert code onto the page you wont be able to run it.
I know this question was asked many times, but I haven't found answer. So why its recommended to include scripts at the end of body tag for better rendering?
From Udacity course https://www.udacity.com/course/ud884 - rendering starts after DOM and CSSOM are ready. JS is HTML parse blocking and any script starts after CSSOM is ready.
So if we got:
<html>
<head>
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>CRP</title>
<link rel="stylesheet" href="styles.css">
</head>
<body>
<!-- content -->
<script src="script.js"></script>
</body>
</html>
CRP would be:
CSSOM ready > JS execute > DOM ready > Rendering
And if script is at head:
<html>
<head>
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>CRP</title>
<link rel="stylesheet" href="styles.css">
<script src="script.js"></script>
</head>
<body>
<!-- content -->
</body>
</html>
CRP would be the same:
CSSOM ready > JS execute > DOM ready > Rendering
This question is only about "sync" scripts (without async/defer attribute).
Scripts, historically, blocked additional resources from being downloaded more quickly. By placing them at the bottom, your style, content, and media could download more quickly giving the perception of improved performance.
Further reading: The async and defer attributes.
In my opinion, this is an outdated practice. More recently, the preference is for JavaScript to separate any code that requires the DOM to be present into a "DOMContentLoaded" event listener. This isn't necessarily all logic; lots of code can initialize without access to the complete DOM.
It's true that this causes a small moment when only the script file is being retrieved, and nothing else (for instance, images). This small window can be skipped by adding the async attribute, but even without it I recommend putting script tags in the head so that the browser knows as soon as possible to load them, rather than saving them (and any future JS-initiated requests) for last.
It is a best practice to put JavaScript tags just before the
closing tag rather than in the section of your HTML.
The reason for this is that HTML loads from top to bottom. The head
loads first, then the body, and then everything inside the body. If we
put our JavaScript links in the head section, the entire JavaScript
file will load before loading any of the HTML, which could cause a few
problems.
1.If you have code in your JavaScript that alters HTML as soon as the
JavaScript file loads, there won't actually be any HTML elements
available for it to affect yet, so it will seem as though the
JavaScript code isn't working, and you may get errors.
2.If you have a lot of JavaScript, it can visibly slow the loading of your page
because it loads all of the JavaScript before it loads any of the
HTML. When you place your JavaScript links at the bottom of your HTML
body, it gives the HTML time to load before any of the JavaScript
loads, which can prevent errors, and speed up website response time.
One more thing: While it is best to include your Javascript at the end
of your HTML , putting your Javascript in the of your
HTML doesn't ALWAYS cause errors. When using jQuery, it is common to
put all of your code inside a "document ready" function:
$("document").ready(function(){ // your code here });
This function basically says, don't run any of the code inside until
the document is ready, or fully loaded. This will prevent any errors,
but it can still slow down the loading time of your HTML, which is why
it is still best to include the script after all of the HTML.
Images placed below the script tag will wait to load until the JS script loads. By placing the script tag at the bottom you load images first, giving the appearance of a faster page load.
I think it depends on your website or app. Some web apps are based on JavaScript. Then it does not make sense to include it at the bottom of the page, but load it immediately. If JavaScript just adds some not so important features to some content based page, then better load it at the end. Loading time will almost be the same, but the user will see the important parts earlier (before the page finished loading).
It’s not about a whole site loading faster, but giving a user the impression of some website loading faster.
For example:
This is why Ajax based websites can give a much faster impression. The interface is always the same. Just some content parts will alter.
This was an extremely useful link. For any given webpage, a document object model is created from the .html. A CSS object model is also created from .css.
We also know that JS files also modify objects. When the browser encounters a tag, the creation of DOM and CSS object models are immediately halted when the script is run because it can edit everything. As a result, if the js file needed to extract information from either of the trees (DOM and CSS object model), it would not have enough information.
Therefore, script srces are generally at the end of the body where most of the trees have already been rendered.
Not sure if this helps,
But from this resource script-tag-in-web the inline script is always render blocking even if kept at the end of body tag.
Below inline script is first render blocking.Browser will not paint anything on screen till the long for loop is executed
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width,initial-scale=1" />
<link href="style.css" rel="stylesheet" />
<title>Critical Path: Script</title>
</head>
<body>
<p>Hello <span>web performance</span> students!</p>
<div><img src="awesome-photo.jpg" /></div>
<script>
let word = 0
for(let i =0; i<3045320332; i++){
word += i;
}
var span = document.getElementsByTagName('span')[0];
span.textContent = 'interactive'; // change DOM text content
span.style.display = 'inline'; // change CSSOM property
// create a new element, style it, and append it to the DOM
var loadTime = document.createElement('div');
loadTime.textContent = word + 'You loaded this page on: ' + new Date();
loadTime.style.color = 'blue';
document.body.appendChild(loadTime);
</script>
</body>
</html>
But'index.js' below is not initial render blocking, the screen will be painted , then once external 'index.js' is finished running the span tag will be updated.
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width,initial-scale=1" />
<link href="style.css" rel="stylesheet" />
<title>Critical Path: Script</title>
</head>
<body>
<p>Hello <span>web performance</span> students!</p>
<div><img src="awesome-photo.jpg" /></div>
<script type="text/javascript" src="./index.js">
</script>
</body>
</html>
index.js
let word = 0
for(let i =0; i<3045320332; i++){
word += i;
}
var span = document.getElementsByTagName('span')[0];
span.textContent = 'interactive'; // change DOM text content
span.style.display = 'inline'; // change CSSOM property
// create a new element, style it, and append it to the DOM
var loadTime = document.createElement('div');
loadTime.textContent = word + 'You loaded this page on: ' + new Date();
loadTime.style.color = 'blue';
document.body.appendChild(loadTime);
I'm trying to make a simple gallery page. The website will always reload after pressing "Next" and I want to make some prerender for the next slide (for better performance and faster load).
At the moment I'm using prefetch/prerender options from HTML5, for Chrome and FireFox:
<!DOCTYPE html>
<html>
<head>
<link rel="prefetch" href="index2.html">
<link rel="prerender" href="index2.html">
</head>
<body>
<img src="big_big_buck_bunny.jpg"/>
Next
</body>
</html>
Is there any other way to cache/prerender next page (in this example - index2.html) ? For example using JavaScript? I'm asking about it because I want to make the prerender work also on Opera 12 and IE (8/9).
Maybe use AJAX. In jquery exists .load() method (http://api.jquery.com/load/)
$('#next').click(function () {
$('#container').load('http://fiddle.jshell.net/webdevem/JfcJp/show/');
});
$('#prev').click(function () {
$('#container').load('http://fiddle.jshell.net/webdevem/JfcJp/show/ #specialContent');
});
Here You have example jsfiddle
If your going to navigate to a new page there's no way to precache html. That's what Ajax is for.
You can Ajax in the html, set the document body to the new html. But if the use reloads the page it will be at the wrong place unless you set a #! In the URL. There's not a lot of nice options with IE8.
Cant you see just preload the images, The page itself isnt gona take any time to build...is it?
I am messing around with JavaScript experimenting to get a feel for it and have already hit a problem. Here is my html code:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
<link rel="stylesheet" type="text/css" href="styles.css">
<script type="text/javascript" src="testing.js"></script>
</head>
<body onload="writeLine()">
</body>
</html>
Here is the JavaScript testing.js:
function writeLine()
{
document.write("Hello World!")
}
Here is the style sheet styles.css:
html, body {
background-color: red;
}
So a very simple example, but I may have chose an awkward example, using on-load in a body tag. So the code above loads and runs the function, but the style sheet does nothing, unless I remove the script tags in the head. I have tried putting the script tags everywhere else, but nothing works. I have researched on-line how to properly link to JavaScript files, and have no found no clear solution, can anyone point out my error?
I have used JavaScript before, but I want a clear understanding from the beginning before I use it any longer
You cannot use document.write after the document is closed (which it will be when onload fires) without destroying the existing document (including links to stylesheets).
Instead, use DOM manipulation, which is covered by chapters 8 and 9 of the W3C JavaScript Core Skills.
Your problem is with the document.write() called in a wrong moment*. This method prints given text at current place in the page as was intended to work while the page still loads. Because you are calling it when the whole page was loaded, the results are unexpected (undefined?)
Instead you should manipulate the dom tree directly:
function writeLine() {
var text = document.createTextNode("Hello World!");
document.body.appendChild(text);
}
Actually in Opera browser I see red background for few milliseconds and then it goes back to white. Try commenting out document.write() - the background is as expected. Moreover you should include <script> tag at the end of body, but this won't solve your problem.
* to be honest, there is no good moment for calling document.write(), avoid it
In your particular example it doesn't matter where the script tag is added as the document.write command executes after the content is rendered, overwriting the existing content.
If you add an alert before overwriting the content you can see your page is red before it gets overwritten with Hello World.