Entry
Can I check with client side JavaScript whether a http server is up?
Can I check with client side JavaScript whether a http server is up?
Can I check with client side JavaScript whether a http server is up?
Mar 19th, 2000 10:32
Martin Honnen,
Client side Javascript has no means to ping an internet host and it
can't directly check whether a http server is responding or whether a
url exists. It has however (starting with js 1.1) the Image object with
two event handlers
onload
and
onerror
which you can use to check whether an image from a server is loadable
and react to it. So if you are are able to place content on a http
server you can place a (small) test image on it which you try to load
to find out whether the server is currently up and responding to web
requests. If the onload handler of the image fires the server is up and
reachable and you can load other content from it, if the onerror
handler fires you do something else (load content from another server
for instance).
The following is an example which assumes that two servers
javascript1.faqts.com
and
javascript2.faqts.com
exist and tries to load a test image from them. As these servers don't
exists the default action namely accessing
javascript.faqts.com
is always executed but the example serves to demonstrate the use of
Image onerror/onload for server checking.
<HTML>
<HEAD>
<SCRIPT>
function errorHandler (evt) {
if (this.src.indexOf('javascript1') != -1)
// error with javascript1, check next host
this.src = 'http://javascript2.faqts.com/testimg.gif';
else if (this.src.indexOf('javascript2') != -1)
// error with javascript2, load default host
location.href = 'http://javascript.faqts.com/';
}
function loadHandler (evt) {
if (this.src.indexOf('javascript1') != -1)
location.href = 'http://javascript1.faqts.com/';
else if (this.src.indexOf('javascript2') != -1)
location.href = 'http://javascript2.faqts.com';
}
var img = new Image();
img.onload = loadHandler;
img.onerror = errorHandler;
img.src = 'http://javascript1.faqts.com/testimg.gif';
</SCRIPT>
</HEAD>
<BODY>
</BODY>
</HTML>
