<script type="text/javascript">
var visitorName = "Chuck";
var myOldString = "Hello username! I hope you enjoy your stay username.";
var myNewString = myOldString.replace(/username/g, visitorName);

document.write("Old string =  " + myOldString); 
document.write("<br />New string = " + myNewString);

</script>

http://www.tizag.com/javascriptT/javascript-string-replace.php

Normal replace() function only replaces first instance.

javascript replace all instances in a string

javascript, replace

JavaScript