let cells, msg_con, msg; const winning = [ [0, 1, 2], [3, 4, 5], [6, 7, 8], [0, 3, 6], [1, 4, 7], [2, 5, 8], [0, 4, 8], [2, 4, 6] ]; const x = "x"; const o = "o"; const board = []; let finished = false; let turnCount = 0, starting = 0, playing = 0; const players = [0, 1]; // human = 0, bot = 1 onload = init; function place(idx) { if(board[idx] != -1 || finished) { return; } board[idx] = playing; cells[idx].innerHTML = playing == 0 ? x : o; ++turnCount; if(checkBoard(board, turnCount, true) == -1) { playing = (playing + 1) % 2; next(); } } function switchPlayers() { [players[0], players[1]] = [players[1], players[0]]; } function newGame() { msg_con.style.display = "none"; for(let i = 0; i < 9; ++i) { board[i] = -1; cells[i].innerHTML = ""; } turnCount = 0; finished = false; playing = starting; next(); } function gameOver(except) { for(let i = 0; i < 9; ++i) { const symbol = board[i]; if(!except.includes(i) && symbol != -1) { cells[i].innerHTML = "" + (symbol == 0 ? "x" : "o") + ""; } } finished = true; msg_con.style.display = "flex"; msg.innerHTML = except.length ? (playing == 0 ? x : o) + " won" : "It\'s draw"; setTimeout(() => { switchPlayers(); newGame(); }, 3000); } function checkBoard(board, turnCount, real = false) { for(const c of winning) { const symbol = board[c[0]]; if(symbol != -1 && symbol == board[c[1]] && symbol == board[c[2]]) { if(real) { gameOver(c); } return symbol; } } if(turnCount == 9) { if(real) { gameOver([]); } return -2; } return -1; } function onCellClick(idx) { if(finished || players[playing] == 1) { return; } place(idx); } function init() { const grid = document.getElementById("board"); cells = document.getElementsByClassName("board-cell"); msg_con = document.getElementById("msg_container"); msg = document.getElementById("msg"); for(let i = 0; i < 9; ++i) { const cell = document.createElement("div"); cell.classList.add("board-cell"); cell.onclick = () => onCellClick(i); grid.appendChild(cell); } newGame(); } function next() { if(players[playing] == 1) { playBot(); } } function playBot() { if(turnCount == 0) { place(Math.floor(Math.random() * 9)); } else { let idx; calc(turnCount, playing, true); place(idx); function calc(t, p, setValue = false) { const res = checkBoard(board, t); switch(res) { case 0: return 1; case 1: return -1; case -2: return 0; default: break; } let best, desired, other; if(p == 0) { best = -Infinity; desired = 1; other = 1; } else { best = Infinity; desired = -1; other = 0; } for (let i = 0; i < 9; ++i) { if (board[i] == -1) { board[i] = p; const temp = calc(t + 1, other, false); const val = p == 0 ? Math.max(temp, best) : Math.min(temp, best); board[i] = -1; if (val != best) { if (setValue) { idx = i; } if (val == desired) { return val; } best = val; } } } return best; } } }

Tic tac toe on 150 -> 140 lines !!