Now most browsers support getBoundingClientRect method, which has become the best practice. Using an old answer is very slow, not accurate and has several bugs.
The solution selected as correct is almost never precise.
This solution was tested on Internet Explorer 7 (and later), iOS 5 (and later) Safari, Android 2.0 (Eclair) and later, BlackBerry, Opera Mobile, and Internet Explorer Mobile 9.
function isElementInViewport (el) {
// Special bonus for those using jQuery
if (typeof jQuery === "function" && el instanceof jQuery) {
el = el[0];
}
var rect = el.getBoundingClientRect();
return (
rect.top >= 0 &&
rect.left >= 0 &&
rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /* or $(window).height() */
rect.right <= (window.innerWidth || document.documentElement.clientWidth) /* or $(window).width() */
);
}
How to use:
You can be sure that the function given above returns correct answer at the moment of time when it is called, but what about tracking element's visibility as an event?
Place the following code at the bottom of your <body> tag:
function onVisibilityChange(el, callback) {
var old_visible;
return function () {
var visible = isElementInViewport(el);
if (visible != old_visible) {
old_visible = visible;
if (typeof callback == 'function') {
callback();
}
}
}
}
var handler = onVisibilityChange(el, function() {
/* Your code go here */
});
// jQuery
$(window).on('DOMContentLoaded load resize scroll', handler);
/* // Non-jQuery
if (window.addEventListener) {
addEventListener('DOMContentLoaded', handler, false);
addEventListener('load', handler, false);
addEventListener('scroll', handler, false);
addEventListener('resize', handler, false);
} else if (window.attachEvent) {
attachEvent('onDOMContentLoaded', handler); // Internet Explorer 9+ :(
attachEvent('onload', handler);
attachEvent('onscroll', handler);
attachEvent('onresize', handler);
}
*/
If you do any DOM modifications, they can change your element's visibility of course.
Guidelines and common pitfalls:
Maybe you need to track page zoom / mobile device pinch? jQuery should handle zoom/pinch cross browser, otherwise first or second link should help you.
If you modify DOM, it can affect the element's visibility. You should take control over that and call handler() manually. Unfortunately, we don't have any cross browser onrepaint event. On the other hand that allows us to make optimizations and perform re-check only on DOM modifications that can change an element's visibility.
Never Ever use it inside jQuery $(document).ready() only, because there is no warranty CSS has been applied in this moment. Your code can work locally with your CSS on a hard drive, but once put on a remote server it will fail.
After DOMContentLoaded is fired, styles are applied, but the images are not loaded yet. So, we should add window.onload event listener.
We can't catch zoom/pinch event yet.
The last resort could be the following code:
/* TODO: this looks like a very bad code */
setInterval(handler, 600);
You can use the awesome feature pageVisibiliy of the HTML5 API if you care if the tab with your web page is active and visible.
TODO: this method does not handle two situations:
For syntax errors, you need to enable error display in the php.ini. By default these are turned off because you don't want a "customer" seeing the error messages. Check this page in the PHP documentation for information on the 2 directives: error_reporting and display_errors. display_errors is probably the one you want to change. If you can't modify the php.ini, you can also add the following lines to an .htaccess file:
php_flag display_errors on
php_value error_reporting 2039
You may want to consider using the value of E_ALL (as mentioned by Gumbo) for your version of PHP for error_reporting to get all of the errors. more info
3 other items: (1) You can check the error log file as it will have all of the errors (unless logging has been disabled). (2) Adding the following 2 lines will help you debug errors that are not syntax errors:
error_reporting(-1);
ini_set('display_errors', 'On');
(3) Another option is to use an editor that checks for errors when you type, such as PhpEd. PhpEd also comes with a debugger which can provide more detailed information. (The PhpEd debugger is very similar to xdebug and integrates directly into the editor so you use 1 program to do everything.)
Cartman's link is also very good: http://www.ibm.com/developerworks/library/os-debug/
Best Answer
In Javascript, use document.documentMode
See http://msdn.microsoft.com/en-us/library/cc196988%28VS.85%29.aspx for details.