my version of Lea Verou's game of life life.js file

life.js

(function() {

  var _ = self.Life = function(seed) {
    this.seed = seed;
    this.height = seed.length;
    this.width = seed[0].length;

    this.prevBoard = [];
    this.board = cloneArray(seed);
  };

  _.prototype = {
    next: function() {
      this.prevBoard = cloneArray(this.board);

      for(var y = 0; y < this.height; y++) {
        for(var x = 0; x < this.width; x++) {
          var neighbors = this.aliveNeighbors(this.prevBoard, x, y);
          // console.log("y is: " + y, "x is: " + x, ": " + neighbors);
          var alive = !!this.board[y][x];

          if(alive) {
            if(neighbors < 2 || neighbors > 3) {
              this.board[y][x] = 0;
            }
          }
          else {
            if(neighbors === 3) {
              this.board[y][x] = 1;
            }
          }
        }
      }
    },

    aliveNeighbors: function(array, x, y) {
      var prevRow = array[y - 1] || [];
      var nextRow = array[y + 1] || [];

      var neighbors = [
        prevRow[x - 1], prevRow[x], prevRow[x + 1],
        array[y][x - 1], array[y][x + 1],
        nextRow[x - 1], nextRow[x], nextRow[x + 1]
      ].reduce(function(prev, cur) {
          return prev + +!!cur;
      }, 0);
    },

    toString: function() {
      return this.board.map(function(row) { return row.join(' ') }).join('\n');
    }
  };
  // Helpers
  // Warning: only clones 2D arrays
  function cloneArray(array) {
    return array.slice().map(function(row) { return row.slice(); });
  }
})();

var game = new Life([
  [0, 0, 0, 0, 0],
  [0, 0, 1, 0, 0],
  [0, 0, 1, 0, 0],
  [0, 0, 1, 0, 0],
  [0, 0, 0, 0, 0]
]);

console.log(game + '');

game.next();

console.log(game + '');

game.next();

console.log(game + '');

You forgot to return the result of the function aliveNeighbors. Use return [ instead of var neighbors = [.