Solution for a codewars task

diamond.js

var
  CHAR    = '*',
  NEWLINE = '\n',
  repeat =
    function repeat(c, n) {
      if(n === 0) { return ''; }
      return c + repeat(c, n-1);
    },

  lines =
    function lines(acc, n) {
      if(n < 1) { return acc; }

      var
        core       = repeat(CHAR, n),
        whitespace = repeat(' ', acc.length+1),
        line       = [whitespace+core];

      return lines(acc.concat(line), n-2);
    };

  diamond =
    function(n) {
      if(n <= 0 || n % 2 === 0) { return null; }

      var
        core   = repeat(CHAR, n),
        top    = lines([], n-2).reverse().join(NEWLINE),
        bottom = lines([], n-2).join(NEWLINE);

      return [top,core,bottom].join(NEWLINE);
    };