old_contents: 
  
  function Square(number) {
    this.number = number;
  }
  
  function Board(string) {
    this.input = string;
    this.board = [];
    var smallArray = [new Square(0), new Square(0), new Square(0), new Square(0)];
    for (i=0; i<4; i++) {
      this.board.push(smallArray);
    }
  }
  
  Board.prototype.setUp = function() {
    var initialValues = this.input.split("");
    // find the index of values are NOT 0.
    for (i=0; i<initialValues.length; i++) {
        // find index of the value IF it's not zero
        var numToTest = parseInt(initialValues[i]);
      if (numToTest !== 0) {
        // convert that index to the index of this.board
        var innerArrayIndex = i%4;
        var outerArrayIndex = parseInt(i/4);
      // change the number property of the square at that index
      //   to the value from the input string.
        this.board[outerArrayIndex][innerArrayIndex].number = parseInt(initialValues[i]);
      }
    }
  }
  
  
  Board.prototype.toString = function(){
    console.log(_.flatten(this.board));
  }
  
  
  
new_contents: 
  
  function Square(number) {
    this.number = number;
  }
  
  function Board(string) {
    this.input = string;
    this.board = [[new Square(0), new Square(0), new Square(0), new Square(0)],
                  [new Square(0), new Square(0), new Square(0), new Square(0)],
                  [new Square(0), new Square(0), new Square(0), new Square(0)],
                  [new Square(0), new Square(0), new Square(0), new Square(0)]];
  }
  
  Board.prototype.setUp = function() {
    var initialValues = this.input.split("");
    for (i=0; i<initialValues.length; i++) {
        var numToTest = parseInt(initialValues[i]);
      if (numToTest !== 0) {
        var innerArrayIndex = i%4;
        var outerArrayIndex = parseInt(i/4);
        this.board[outerArrayIndex][innerArrayIndex].number = parseInt(initialValues[i]);
      }
    }
  }
  
  
  Board.prototype.toString = function(){
    var flattenedBoard = _.flatten(this.board);
    // console.log(flattenedBoard);
    for (i=0; i<flattenedBoard.length; i++){
      flattenedBoard[i] = flattenedBoard[i].number.toString();
      if (i%4 === 3){
        flattenedBoard[i] += "\n";
      }
    }
    console.log(flattenedBoard.toString());
  }
  
  
  
subject: debug accidentally cloning arrays in Board initialization; toString is working for testing