Please Help! Alternating-Flashing-Text Script is Not working

Hi, I want to be able to have two strings that alternate. I also want to be able to control the time duration that one string is visible before it is replaced by the second string.

This does not work and is a pretty poor attempt because I keep changing my mind regarding the use of a global variable versus a return statement. (It would be nice to be able to use the function more than once on a single html page but is not necessary.)

<script type="text/javascript">

	alternateStrings(str1, str2, duration){
		var str0;  //?problem if I put this variable outside the function, it is not possible to use two instances of this function. If it is inside the function, then it is not reachable for a document.write statement in the html.
		var k = 0;
		while(true){
			if(k==0){str0 = str1;}
			else{str0 = str2;}
			setTimeout('', duration ); //???
			
			if(k==0){k=1;}
			else{k=0;}
		}
        }

...


<script type="text/javascript">

     //I want to be able to have use several instances of the function. 
     //In that case, I don't think a global variable is the way to go 
     //(i.e., use the bottom example to use multiple times).

     //method A:
     alternateStrings(abra', 'cadabra, .5); document.write(str0);

     //method B:
     document.write(alternateStrings('waaaaz', 'aaaaaaa?', .25));
</scipt>
		

Code (markup):