Develop Reference

JS function to preload images before page load

I need a javascript code which loads an image into browser cache. What is the usage? read this:
When the user logs into my site, she/he gets redirected to a page which is "Redirecting you to control panel" and a progress is displayed there too. Now, this "redirector" page has a background, since user experience this page and sees it only 3 seconds, many times, background image is missed and there remains no chance for it to be loaded, since from the page load till the page redirection there is only 3 seconds gap. Here is en example of my ajax login:
$.ajax({
// do ajax stuff
success : function(msg)
{
if(msg==true)
{
// I NEED A FUNCTION HERE TO LOAD THEM IMAGE INTO CACHE BEFORE THIS PAGE
// TO LOAD THE REDIRECTOR PAGE. USING THIS, I CAN ENSURE THE EXISTENCE OF THE
// BG IMAGE WHEN THE USER SEES NEXT PAGE. THIS BG IMAGE IS INDEED NEXT PAGE'S BG
window.locatio.href = 'process/redirection/to/user-panel';
}
}
});

This function will work:
function preloadImage(url)
{
var img = new Image();
img.src = "/test/example.jpg";
}
Also, here is a question that discusses something similar, pre-loading images on a splash screen, but the implementation is far more complex.
On the subject, if you don't have to use JavaScript, another solution using CSS and XHTML that could probably work on the redirect page can be found here. Otherwise, the code at the top should work. Hope this helps, good luck.

What to do if $(window).load(function(){}); is too early

I need to trigger a piece of code after every single bits are done downloading. The script works if injected after everything is loaded, but how do I trigger that automaticly?
My script is:
var divId = "jwplayer-0_wrapper";
if ($('#' + divId).length == 1) {
myReg = /https?:\/\/www\.youtube\.com\/watch\?v=[^"]+/;
var plainText = $('#' + divId).parent().children('script').text();
var url = plainText.match(myReg);
if (url !== null) {
window.location = url;
};
};
It is used to skip certain site that decide to use the JW player witch I find horribly buggy. So it looks for a div with the indication of the JW player and if there's one, it finds the link to the original youtube video and directly goes there.
Its triggered By Google Chrome Add-on named Javascript Injector and I apply the script on every page I visit. The plug in work perfectly well on sites like www.ayoye.co and www.veuxturire.com. But on other sites, that uses the same pathern, it seems that the script is triggerd too early. For example there www.mondedestars.com and www.lesautos.ca triggers it too early.
If I use the "inject now" fonction of the Add on after the page is really done loading, then it redirects me to the youtube page as expected. I am lost on the why it works some where and not were else.
I'm not trying to understand every single website here, I'd prefer make it dynamicly triggered after the page has done loading everything from its php, ajax, script, flash, html and CSS.
I've tryed to look to the JWplayer API, but since its terribly unclear to me, over the fact that its partialy in flash, it woudl be simpler if there was a way to trigger it after, or maybe just triggering it after i hover over the body, since every sites has a body. It cant be specific to one page.

Use something like this
var timer;
function injectYouTube() {
// DO YOUR STUFF HERE
// ONCE DONE CALL clearInterval(timer);
clearInterval(timer);
}
timer = setInterval(injectYouTube, 2000);
I am not saying this will be called after everything is loaded but instead you can make sure your code is executed when you want it to.

The JWPlayer API are not that difficult. You can retrive the informations you need even not knowing the container id.
This is an example:
var player = jwplayer(0); // get the first jwplayer element of the page
var video = player.getPlaylistItem(); // get the player video
var url = video.file // retrieve the video url
I think the setTimeout or setInterval are unreliable.
Setting up a listener on jwplayer onReady event would be better.

The pessimistic answer to this is that you can't wait until a page has finished all AJAX operations etc. because web pages can continue loading new content indefinitely if they wish.
What you might consider is running your code every time a new HTML element is added to the page. This way, you can be certain to catch JWPlayer the moment it is inserted into the page.
document.addEventListener("DOMNodeInserted", yourRemovalFunction);

.load() a web page into a site?

I'm making a site where the different pages are brought in by .load(), the problem is that index.php is initially empty and my other efforts have simply loaded the page into itself or left the page empty (pages can still be emptied and loaded from #contentspace though). My current code loads nothing, i'm sure i'm doing several things wrong, I just need to know where to start. do i need to use php for this?
//load page into contentspace by default?
var defaultpage = "blog.php";
$(document).ready(function() {
$("#contentspace").load(defaultpage);
)};

You have a syntax error with your closing of the $(document).ready )}; for one. As you can see in my demo here: http://jsfiddle.net/n1ck/NNpqq/9/ it is requesting the blog.php file.
//load page into contentspace by default?
var defaultpage = "blog.php";
$(document).ready(function() {
$("#contentspace").load(defaultpage);
});​

Detect first page load with jQuery?

I need to detect the first time a page loads in jQuery so that I can perform some actions only when the page loads the first time a user navigates to that page. Similar to server side code page.ispostbasck. I have tested $(document).ready and it fires every time the page loads so this will not provide what I need. I have also tried the jQuery Load function - it also fires every page load. So by page load an example is that I have an HTML input tag on the page of type button and it does not fire a postback (like an asp.net button) but it does reload the page and fires $(document).ready
Thanks

You will have to use cookie to store first load information:
if (! $.cookie("cookieName")){
// do your stuff
// set cookie now
$.cookie("cookieName", "firstSet", {"expires" : 7})
}
Note: Above example uses jQuery Cookie plugin.

An event doesn't exist that fires only when the page is loaded for the first time.
You should use jQuery's .ready() event, and then persist the fact that you've handled a first time page load using your method of choice (i.e. cookie, session variable, local storage, etc.).
Note: This method will never be fool proof unless you can store this information at the user level in a DB. Otherwise, as soon as the user clears their cookies, or whatever method you choose, the "first time loaded" code will fire again.

I just ran into this problem and this is how I handled it. Keep track of the first time the page loads by using a variable initialLoad:
var initialLoad = true;
$(document).ready(function() {
...
...
...
initialLoad = false;
});
Then in other functions, you can do this:
if (initialLoad) {
//Do work that is done when the page was first refreshed/loaded.
} else {
//Do work when it's not the initial load.
}
This works well for me. If the user is already on the page and some jQuery functions run, I now know if that user just loaded the page or if they were already on the page.

The easy solution is to use jQuery ‘Once’ plugin
$(element).once('class-name', function() {
// your javascript code
});

Javascript delayed loading blanks page

On a website I'm working on, I need to load a tracking script 10 seconds after the page loads. I found a snippet to do so, but I've hit a snag. After waiting 10 seconds, the page goes white. The URL doesn't seem to change, but the page is no longer visible and the throbber starts spinning.
Here's what I'm using to load the script:
function $import(src){
var scriptElem = document.createElement('script');
scriptElem.setAttribute('src',src);
scriptElem.setAttribute('type','text/javascript');
document.getElementsByTagName('head')[0].appendChild(scriptElem);
}
// import with a random query parameter to avoid caching
function $importNoCache(src){
var ms = new Date().getTime().toString();
var seed = "?" + ms;
$import(src + seed);
}
//
// Tracker options go here...
//
setTimeout(function(){
$importNoCache("http://tracking.code/url");
}, 10 * 1000);
Is there a better way to do this?
EDIT: I stepped through the code in Firebug, and the scripts works like it should. With Firebug's debugger off, it blanks the page as I described above.

This would happen if the script calls document.write.
Can you show us the script that you're loading?

The code looks fine, so the problem is probably in the tracking code. If it contains a document.write() call, it will work fine when included normally, but wipe out the page when included after the page has finished loading.
Edit:
Yep, the tracking script does d=document, then calls d.write() later on... you won't be able to include this script after the page has finished loading.