Diamond kata with a TDD approach, in Javascript. Javascript noob, any feedback is most than welcome! Particularly looking for 'idiomatic' corrections.

diamond_kata.js

// Diamond Kata with a TDD approach. First attempt

'use strict';

module.exports.Diamond = Diamond;

// Encapsulate line printing
function Letter(char, spaces) {
  this._char = char;
  this._spaces = spaces;
}
Letter.prototype.print = function print() {
  return this._char + (
    (this._spaces > 0)
      ? (Array(this._spaces + 1).join(' ') + this._char + '\n')
      : '\n'
  );
}


// Singleton list with all letters
var LETTERS = new function () {
  // Init all upper-case letters
  for (let char=65; char <= 90; char++) {
    let character = String.fromCharCode(char);
    this[character] = new Letter(character, (char - 65) + (char - 65 - 1));
  }
}();
LETTERS.__proto__.iterateLetters = function (character, callback) {
  var chars = Object.keys(this);
  var last = chars.indexOf(character);
  var pos = 0;
  var direction = 1;
  // Forwards
  while (pos >= 0) {
    callback(this[chars[pos]], pos, last);
    if (pos === last) {
      // Reached final character, return to 'A'
      direction = -1;
    }
    pos += 1 * direction;
  }
}


function Diamond (letter) {
  if (typeof(letter) !== 'string' && !(letter instanceof String)) {
    throw new Error('Letter must be an string');
  }
  this._letter = letter;
};
Diamond.prototype.process = function process() {
  var prefix = function (max, current) {
    return Array(1 + max - current).join(' ');
  }
  var result = '';
  LETTERS.iterateLetters(this._letter, function (letter, current, last) {
    result += prefix(last, current);
    result += letter.print();
  });
  return result;
}

test_diamond_kata.js

// var should = require('unit.js').should;
var should = require('should');
var diamondKata = require('../diamond_kata');


suite('Diamond', function() {

  test('should be instantable', function() {
    diamond = newDiamond();
    diamond.should.be.an.instanceof(diamondKata.Diamond);
  });

  test('takes a letter as unique required parameter', function() {
    (function newDiamondWithoutLetter() {newDiamond(null);})
      .should.throw();
  });

  test('if letter is A, returns A', function() {
    newDiamond('A').process().should.equal('A\n');
  });

  test('if letter is B, returns ABBA with proper spaces', function() {
    newDiamond('B').process().should.equal(' A\nB B\n A\n');
  });

  test('if letter is C, returns ABBCCBBA with proper spaces', function() {
    newDiamond('C').process().should.equal('  A\n B B\nC   C\n B B\n  A\n');
  });
  
});


function newDiamond (letter) {
  if (letter === undefined) {
    letter = 'A';
  }
  return new diamondKata.Diamond(letter);
}