boggleboard.js

var BoggleBoard = function(letters, width, height) {
  if (letters.length != width * height) throw 'incorrect board length';

  this.width = width;
  this.height = height;

  this.letters = letters;
}

BoggleBoard.prototype.at = function(x, y) {
  return this.letters.charAt(y * this.width + x);
}