Return winning cell coordinates in JS Connect-Four

I'm working on a JS connect-four game. My 'board' is an array of arrays dimensions 7 * 6 (0 = empty, 1 = occupied by player A, -1 = occupied by player B).

var boardTwo = [[1,1,1,1,0,0],[-1,-1,-1,0,0,0],[0,0,0,0,0,0],[0,0,0,0,0,0],[0,0,0,0,0,0],[0,0,0,0,0,0],[0,0,0,0,0,0]];

This is my (long-winded) function for checking wins (has either player created an unbroken horizontal, vertical, or diagonal line length 4 in the board). How can I modify this so that It returns an array with the coordinates of the winning counters rather than simply an alert message? I appreciate the best solution may involve abandoning my original checkWin function in place of something more concise.

checkWin = function(board){
    checkVert(board);
    checkHoriz(board);
    checkDia(board);
    }
winAlert = function(checkSum){
    if(checkSum == 4){
        status = "a"
        alert("a wins"); 
    }
    else if(checkSum == -4){
        status = 'b';
        alert("b wins"); 
    }
}
checkVert = function(board){
    for(var i = 0; i < board.length; i++){
       for(var j = 0; j < board[0].length - 3; j++){
           var checkSum = board[i][j] + board[i][j+1] + board[i][j+2] + board[i][j+3];
           winAlert(checkSum);
           }
    }
}

checkHoriz = function(board){
    for(var i = 0; i < board.length-3; i++){
      for(var j = 0; j < board[0].length; j++){
        var checkSum = board[i][j] + board[i+1][j] + board[i+2][j] + board[i+3][j];
        winAlert(checkSum);
      }
    }
  }

checkDia = function(board){
    for(var i = 0; i < board.length-3; i++){
      for(var j = 0; j < board[0].length - 3; j++){
        var checkSum = board[i][j] + board[i+1][j+1] + board[i+2][j+2] + board[i+3][j+3];
    winAlert(checkSum);
        }
      }
    }
checkWin(boardTwo) //alerts "player A wins"

I want checkWin(boardTwo) to return [[0,0],[0,1],[0,2],[0,3]];

Solution

Here's an straightforward solution:

winAlert should return a number if someone won, e.g. 1 for A and -1 for B
Each check function will check this result and if it's -1 or 1 then it'll interpolate the values it checked
if a check method returns something that's not undefined i.e. the array with the answer then other checks should not be made
finally state holds who won (-1, 1 or undefined) and the returning value of checkWin will be an array of arrays if someone won

I also did an additional check on the checkDiagonal method to also check the other diagonal that exists in the matrix

function sgn(x) {
  if (x < 0) return -1;
  if (x > 0) return 1;
  return 0;
}

function interpolate(from, to) {
  var coordinates = [];
  var startX = from[0];
  var startY = from[1];
  var limit = Math.max(
    Math.abs(from[0] - to[0]),
    Math.abs(from[1] - to[1])
  ) + 1;
  for (i = 0; i < limit; i += 1) {
    coordinates.push([
      startX + i * sgn(to[0] - from[0]),
      startY + i * sgn(to[1] - from[1])
    ]);
  }
  return coordinates;
}

var state;
function checkWin(board) {
  var ans;
  ans = ans || checkVert(board);
  ans = ans || checkHoriz(board);
  ans = ans || checkDia(board);
  return ans;
}

function winAlert(checkSum) {
  if (Math.abs(checkSum) === 4) {
    state = sgn(checkSum);
    return state;
  }
}

checkVert = function(board) {
  for (var i = 0; i < board.length; i++) {
    for (var j = 0; j < board[0].length - 3; j++) {
      var checkSum = board[i][j] + board[i][j + 1] + board[i][j + 2] + board[i][j + 3];
      if (winAlert(checkSum)) {
        return interpolate([i, j], [i, j + 3]);
      }
    }
  }
}

checkHoriz = function(board) {
  for (var i = 0; i < board.length - 3; i++) {
    for (var j = 0; j < board[0].length; j++) {
      var checkSum = board[i][j] + board[i + 1][j] + board[i + 2][j] + board[i + 3][j];
      if (winAlert(checkSum)) {
        return interpolate([i, j], [i + 3, j]);
      }
    }
  }
}

checkDia = function(board) {
  for (var i = 0; i < board.length - 3; i++) {
    for (var j = 0; j < board[0].length - 3; j++) {
      var checkSum = board[i][j] + board[i + 1][j + 1] + board[i + 2][j + 2] + board[i + 3][j + 3];
      if (winAlert(checkSum)) {
        return interpolate([i, j], [i + 3, j + 3]);
      }
    }
  }
  
  // missing diagonal
  for (i = 3; i < board.length; i += 1) {
    for (var j = 0; j < board[0].length - 3; j++) {
      var checkSum = board[i][j] + board[i - 1][j + 1] + board[i - 2][j + 2] + board[i - 3][j + 3];
      if (winAlert(checkSum)) {
        return interpolate([i, j], [i - 3, j + 3]);
      }
    }
  }
}

var board = [
  [1, 1, 1, 0, 0, 0],
  [-1, -1, -1, -1, 0, 0],
  [0, 0, 0, 0, 0, 0],
  [0, 0, 0, 0, 0, 0],
  [0, 0, 0, 0, 0, 0],
  [0, 0, 0, 0, 0, 0],
  [0, 0, 0, 0, 0, 0]
];

var ans = checkWin(board) || [];
document.write(state + '<pre>' + JSON.stringify(ans) + '</pre>');