Game of Life - Iterate through 2D array and check contents of elements

I’m working on the Game of Life project. Mostly done but I am having difficult in implementing the game logic in componentDidUpdate.

I have a 2D array that serves as the game board. I want to iterate through the 2D array and check the status of each array element (individual board square) and see if it is “alive” in the Game of Life sense.

The algorithm for determining if an individual square is “alive” or not is as follows:

   // Compare current square in array to each of the following neighboring squares:
   [x][y - 1];
   [x + 1][y - 1];
   [x - 1][y];
   [x + 1][y];
   [x - 1][y + 1];
   [x][y + 1];
   [x + 1][y + 1];

I need to compare the current 2D array element to each of those neighboring squares. Then I figure I would create a tally variable and keep track of how many of it’s neighbors are alive or dead.

  • Any live cell with fewer than two live neighbors dies, as if by under population.
  • Any live cell with two or three live neighbors lives on to the next generation.
  • Any live cell with more than three live neighbors dies, as if by overpopulation.
  • Any dead cell with exactly three live neighbors becomes a live cell, as if by reproduction.
  • I’d keep the tally in a variable and then have if/else statements that handle whether or not that individual square lives on, dies, or reproduces.

    The problem is my componentDidUpdate function is running into errors. I am not properly iterating through the 2D array. Been stuck on this for a while and I think I’m overthinking it. Any pointers would be helpful.

    The project on is Codesandox and here is the particular function that I’m having trouble with:

     // Should apply Game of Life rules and decide with squares are alive/dead every time the board is updated
      componentDidUpdate = (prevProps, prevState) => {
        //console.log('PrevState is : ' + prevProps, prevState);
        const oldBoard = this.state.board;
        const newBoard = this.state.nextBoard;
        console.log('oldBoard ' + oldBoard);
        console.log('newBoard ' + newBoard);
        // Checks that board has changed and prevents infinite loop in componentDidUpdate
          for(let x = 0; x < this.state.boardHeight; x++) {
            for(let y = 0; y < this.state.boardWidth; y++) {
              let neighborCount = 0;
              // Game of Life logic pertaining to squares being alive/dead
              neighborCount += oldBoard[x - 1][y - 1];
              neighborCount += oldBoard[x][y - 1];
              neighborCount += oldBoard[x + 1][y - 1];
              neighborCount += oldBoard[x - 1][y];
              neighborCount += oldBoard[x + 1][y];
              neighborCount += oldBoard[x - 1][y + 1];
              neighborCount += oldBoard[x][y + 1];
              neighborCount += oldBoard[x + 1][y + 1];
              console.log('neighborCount ' + neighborCount[x]);
              // If square has 2 live neighbors it stays alive
              if(neighborCount == 2) {
                newBoard[x][y] = oldBoard[x][y];
              // If square has exactly 3 neighbors a new life square is born
              else if (neighborCount == 3) {
                newBoard[x][y] = 1;
              // Is square has more than 3 live neighbors it dies
              else if(neighborCount > 3){
                newBoard[x][y] = 0;
          if(newBoard !== oldBoard) {
            // after applying rules set the nextBoard
            this.setState({ board: newBoard });

    I get an error that says these blocks neighborCount += oldBoard[x - 1][y - 1]; of code are undefined.

    I’ve also unsuccessfully made a Stackoverflow post for this question.

    Without actually seeing those errors or tracing your code I would guess that maybe you are accessing array indices that are out of bounds. Do you account for cells at the edge of the playing board not having all 8 neighbors?