I had a situation where I had created a configurable variable value that expected a c# string.Format replacement structure.  Then I was required to move the processing to the client.  The replacement values wouldn't be available until the page had been served.  So I created this function to port the functionality to javascript.

uses regular expressions (in javascript) and an variable length argument list.  Now you can use your c# habits on the client... 

function stringFormat(sFormat){
  var sRet="";
  var args = stringFormat.arguments;
  var sAry = sFormat.split(/{[0-9]}+/g);
  for(i=1;i<args.length;i++){
   sRet+= sAry[i-1] + args[i];
  }
  return sRet;
 }

The usage of this function is as follows:

var sArgument1 = "red";

var sArgument2 = "black";

var sResultString = stringFormat("I have a {0} car and a {1} pair of shoes.", sArgument1, sArgument2);

//the result will be

"I have a red car and a black pair of shoes."