javascript replace() function replace only single characters

I have been trying to use javascript replace() to search and replace special individual characters in a string. But having some difficulties.

We can use the following way to solve the following problem.

function ReplaceAllInstanceOfString(strString,strReplaceString,strReplaceByString)

{

       var intPos= strString .indexOf('+ strReplaceString +'); 

       if ( intPos !=-1)

       {  

               strString = strString .replace(/,/g,' + strReplaceByString + '); 

        }

        return strString ;

}