We do a lot of string manipulation on the Firefox Addons site. A lot of it has to do with localization so one thing that comes up is being able to format strings. Here’s a little snippet to give yourself python like string formatting:
/* Python(ish) string formatting:
* >>> format('{0}', ['zzz'])
* "zzz"
* >>> format('{x}', {x: 1})
* "1"
*/
function format(s, args) {
var re = /\{([^}]+)\}/g;
return s.replace(re, function(_, match){ return args[match]; });
}