Underscore mixin for making multiple string replacements in one call

_.replace.js

//this replace method can take multiple finds and one or multiple replacements at a time
//if find is an array, replace will be called on every find
//if replace is an array, the indexes must match with find to do replacements.
//if replace is a string, it will be used for all replace calls if find is an array
//if find and replace are both strings, this will simply mimmick a normal str.replace(find, replace) call
//the result of all replacements will be returned at the end
_.mixin({
  replace: function(str, find, replace){
    //if str is null, just give up now
    if (str === null){
      return str;
    }

    //explicity cast to string otherwise .replace() does not exist
    str = str.toString();

    //if find is an array we are going to be replacing a series of things
    if (_.isArray(find)){
        //go through each thing to find
        _.each(find, function(v, k){
            var replace_with = '';

            //if replacements is an array use the 'replace' at the same index as the current 'find'
            if (_.isArray(replace)){
                replace_with = replace[k];
            }
            //otherwise just replace with the string sent
            else{
                replace_with = replace;
            }

            //do the replacement
            str = str.replace(new RegExp(v, 'g'), replace_with);
        });
    }
    //nothing fancy, do a normal replacement
    else{
        str = str.replace(new RegExp(v, 'g'), replace);
    }

    //return the string with replacements applied
    return str;
  }
});