Detecting Browser Using JavaScript(Urgent)

Hi I have just been given a problem at work, how do you detect
what browser your running your java script in. (Using java script
not java).
Can any body help me.

Here is the type of JavaScript that you will want to use:

function Is() {
var agent = navigator.userAgent.toLowerCase();
this.major = parseInt(navigator.appVersion);
this.minor = parseFloat(navigator.appVersion);
this.ns = ((agent.indexOf('mozilla')!=-1) &&
((agent.indexOf('spoofer')==-1) && (agent.indexOf('compatible') == -1)));
this.ns2 = (this.ns && (this.major == 2));
this.ns3 = (this.ns && (this.major == 3));
this.ns4b = (this.ns && (this.minor < 4.04));
this.ns4 = (this.ns && (this.major >= 4));
this.ie = (agent.indexOf("msie") != -1);
this.ie3 = (this.ie && (this.major == 2));
this.ie4 = (this.ie && (this.major >= 4));
this.op3 = (agent.indexOf("opera") != -1);
}
var is = new Is();
if (is.ie4){
do IE stuff
}
if (is.ns4) {
do netscape stuff
}
Obj = eval(doc + '["btn"]' + sty);
Obj.width = available_width;
}

Hope this helps!
-Ray

Thanks Ray.