scan.js

// assuming "Board" has a getTokenAt(row, column) function
// that returns "red", "blue", or null for each position
// and an isInBounds(row, column) function that returns
// true if row and column are valid, false if they are off the board

// returns number of players tokens that are connected in a direction
// (Board, string, int, int, int, int) -> int
function countConnectedInDirection(board, player, newTokenRow, newTokenColumn, directionX, directionY) {
  var count = 1; // number connected, starts at 1 because of the new token
  //  scan "forward" in direction
  var x = newTokenRow + directionX;
  var y = newTokenColumn + directionY;
  // while on the board and the token at (x,y) is the current players
  while (board.isInBounds(x, y) && board.getTokenAt(x, y) === player) {
    count++;
    x += directionX;
    y += directionY;
  }
  
  // scan "backward" in direction
  var x = newTokenRow - directionX;
  var y = newTokenColumn - directionY;
  // while on the board and the token at (x,y) is the current players
  while (board.isInBounds(x, y) && board.getTokenAt(x, y) === player) {
    count++;
    x -= directionX;
    y -= directionY;
  }
  
  return count;
}

// check each direction for a win
// (Board, string, int, int) -> bool
function doesNewTokenResultInWinForPlayer(board, player, newTokenRow, newTokenColumn) {
  // check column
  let columnCount = countConnectedInDirection(board, player, newTokenRow, newTokenColumn, 0, 1); // change row by 0, columnn by 1
  if (columnCount >= 4) {
    return true;
  }
  // row wins
  let rowCount = countConnectedInDirection(board, player, newTokenRow, newTokenColumn, 1, 0); // change row by 1, column by 0
  if (rowCount >= 4) {
    return true;
  }
  
  // check if diagonal that "descends left-to-right" wins
  let diagonalOne = countConnectedInDirection(board, playr, newTokenRow, newTokenColumn, 1, 1); // change row by 1, column by 1
  if (diagonalOne >= 4) {
    return true;
  }
  
  // check if diagonal that "assends left-to-right" wins
  let diagonalTwo = countConnectedInDirection(board, playr, newTokenRow, newTokenColumn, 1, -1); // change row by 1, column by 1
  if (diagonalTwo >= 4) {
    return true;
  }
  
  // if none of these conditions won, the result is false
  return false;
}