String#mapConcat: splits a string by character; iterates through each character; runs a callback on each character; and concat the callback return into a new string

string.map_concat.coffee

String.prototype.mapConcat = (args...) ->
  separator = args.slice(0,1) if $.type(args[0]) == 'string' or $.type(args[0]) == 'regexp'
  separator or= ''
  chars = @.split(separator)
  rtn = []
  for char, i in chars
    rtn.push(args[0].call(@, char, i) or char)

  rtn.join('')

string.map_concat.js

/*
  "This is my string".mapConcat(function(char, index){
    if(char == 'i') return '#'
  }) => "Th#s #s my str#ng"
  
  @param String: the character itself
  @param Integer: indexOf the character in the string
  @return String: mutated string after concatination
*/

String.prototype.mapConcat = function() {
  var args, char, chars, i, rtn, separator, _len;
  args = 1 <= arguments.length ? Array.prototype.slice.call(arguments, 0) : [];
  if (typeof args[0] === 'string' || (typeof (args[0]) === 'object' && args[0] instanceof RegExp)) {
      separator = args.slice(0, 1);
  }
  separator || (separator = '');
  chars = this.split(separator);
  rtn = [];
  for (i = 0, _len = chars.length; i < _len; i++) {
      char = chars[i];
      rtn.push(args[0].call(this, char, i) || char);
  }
  return rtn.join('');
};