4 in a row

var grid = [
  ['.', '.', '.', '.', '.', '.', '.'],
  ['.', '.', '.', '.', '.', '.', '.'],
  ['.', '.', '.', '.', '.', '.', '.'],
  ['.', '.', '.', '.', '.', '.', '.'],
  ['.', '.', '.', '.', '.', '.', '.'],
  ['.', '.', '.', '.', '.', '.', '.']
];

var currentPlayer = 'X';

function checkWin() {
    //Horizontal check
    for (var i = 0; i < grid.length; i++)
        for (var j = 0; j < grid[i].length - 3; j++)
            if (grid[i][j] != '.')
                if (grid[i][j] == grid[i][j + 1] && grid[i][j + 1] == grid[i][j + 2] && grid[i][j + 2] == grid[i][j + 3])
                    return grid[i][j];

    //Vertical check
    for (var i = 0; i < grid[0].length; i++)
        for (var j = 0; j < grid.length - 3; j++)
            if (grid[j][i] != '.')
                if (grid[j][i] == grid[j + 1][i] && grid[j + 1][i] == grid[j + 2][i] && grid[j + 2][i] == grid[j + 3][i])
                    return grid[j][i];

    //Diagonal check from top left to bottom right \
    for (var i = 0; i < grid.length - 3; i++)
        for (var j = 0; j < grid[0].length - 3; j++)
            if (grid[i][j] != '.')
                if (grid[i][j] == grid[i + 1][j + 1] && grid[i + 1][j + 1] == grid[i + 2][j + 2] && grid[i + 2][j + 2] == grid[i + 3][j + 3])
                    return grid[i][j];

    //Diagonal check from top right to bottom left /
    for (var i = 0; i < grid.length - 3; i++)
        for (var j = 3; j < grid[0].length; j++)
            if (grid[i][j] != '.')
                if (grid[i][j] == grid[i + 1][j - 1] && grid[i + 1][j - 1] == grid[i + 2][j - 2] && grid[i + 2][j - 2] == grid[i + 3][j - 3])
                    return grid[i][j];

    return '.'; //No one won;
};

function placeToken(row) {
    var line = 0;
    while (line < grid.length - 1 && grid[line + 1][row] == '.') {
        line++;
    }
    grid[line][row] = currentPlayer;
    swapPlayer();
    displayGrid();
};

function swapPlayer() {
    if (currentPlayer == 'X')
        currentPlayer = 'O';
    else
        currentPlayer = 'X';
}

function displayGrid() {
    for (var i = 0; i < grid.length; i++)
        console.log(grid[i]);
}

while (checkWin() == '.') {
    placeToken(prompt('EnterRow'));
    console.log(checkWin());
}

alert(checkWin());