diagnolcheck.js

function diagnolWinner() {
    var color;
    for (var i = 0; i <= board.length - 4; i++) {
        if ((color = genericDiagnol(i, 1)) !== 0) {
            return color;
        }
    }
    for (var i = board.length - 1; i >= board.length - 4; i--) {
        if ((color = genericDiagnol(i, -1)) !== 0) {
            return color;
        }
    }
    return 0;
}

/**
 * Keeps code DRY
 *
 * @param i column to look in
 * @param direction offset type thing
 * @return winners within the
 */
function genericDiagnol(i, direction) {
    for (var j = 0; j <= board[0].length - 4; j++) {
        var current = board[i][j];
        if (current !== 0) {
            var success = true;
            for (int k = 1; k < 4; k++) {
                if (board[i + k * direction][j + k] !== current) {
                    success = false;
                    break;
                }
            }
            if (success) {
                return current;
            }
        }
    }
    return 0;
}