When are JavaScripts executed?

views:

answers:

When are JavaScripts executed?

I'm currently asking myself, when javascripts in the header of a page are executed?

After all contents are loaded? During loading images? Before loading the site?

Reason is, I want to implement a barrier to prevent users accessing a site without javascript (it's for internal company use, don't blame me for the requirement) - my idea was to create a div-error-message that is hidden with javascript and a screenshot of my page greyed out, also hidden if javascript is disabled.

Problem is, such a screenshot can be quite large in terms of file-sizes. So, with a slow connection, would you have to load the whole image before javascript hides it?!

+4 A:

Why not just put the "you must have javascript" warning inside a <noscript> tag?

Paul Dixon 2009-11-30 09:44:08

Because that doesn't prevent the user from using the site anyway, which is what i need to do. It has to be (at least without editing CSS) impossible to use the site without JS...

ApoY2k 2009-11-30 09:46:21

noscripts must not necessarily follow script elements, so you can put the JS in the head and still have noscript elements in the body.

Boldewyn 2009-11-30 09:48:47

`<noscript>` tags aren't reliable. What if a corporate firewall is filtering out all JS before it even gets to the JS-enabled client? They won't see anything...

J-P 2009-11-30 11:01:15

+1 A:

Javascript will depend on what event are you using; if you're talking about a onload event, so it'll run just after all content loads.

To handle your problem, you could to create a page and, there, to write some javascript to redirect it to JavascriptEnabled.html; if you can't redirect it, you should also to add a META REFRESH tag and redirect it to JavascriptDisabled.html and to display a warning message.

Rubens Farias 2009-11-30 09:46:51

Good point, and nice idea! I will surely look into this.

ApoY2k 2009-11-30 09:56:48

+1 A:

<script> tags in the <head> of an HTML page are executed as they are loaded (unless they have the defer attribute set), that is, before the images, or even the rest of the document have been loaded. What you might want to do is contain the image you wish to hide in a <div id="something" style="display:none"> and show this div after the page and all of its content (your screenshot image included) has loaded (ie. in an onload handler -- or obviously you can use addEventListener too).

HTH!

AKX 2009-11-30 09:47:54

Up for the first answer really answering the question when it's executed ;-) So, to recap; If i would place a redirect in a JS in the header, it will be executed before the background-image loads? So that's the solution to my problem, just one redirect, if it doesn't work, the page will load with the image.

ApoY2k 2009-11-30 09:58:03

Yes. The redirect (location.href='...';) will be executed immediately.

AKX 2009-11-30 18:20:49

+2 A:

Javascript in a script tag executes whenever the browser sees it. But is stops all other execution and (probably) downloading. So, you are trying to hide an img in the body, but because the script tag is in head it will execute before the img has loaded so it won't be able to find it (document.getElementById('#noScriptImage') will fail).

Another option is to place the script tag after the img, but then there is a change that for a fraction of a second the image will be visible and the screen will flick.

Emil Ivanov 2009-11-30 09:48:56

Very good to understand, things getting clearer now.

ApoY2k 2009-11-30 09:59:11

ansaurus

tags:

views:

answers:

When are JavaScripts executed?

related questions