Workshop › Tic-tac-toe › Step 31 of 33

Look ahead: minimax

What we are doing

For a game still going, score tries every free cell for the player to move, scores each result with itself, undoes the move, and assumes both sides play their best: O takes the highest score, X the lowest. This is minimax.

```js
  if (end === 'draw') return 0
  const scores = []
  for (let index = 0; index < 9; index++) {
    if (cells[index] !== '') continue
    cells[index] = turn
    scores.push(score(cells, turn === 'O' ? 'X' : 'O'))
    cells[index] = ''
  }
  return turn === 'O' ? Math.max(...scores) : Math.min(...scores)
}
```

What it means

The loop tries every free cell: continue skips taken ones.
score(...) calls itself with the other player to move: a function that calls itself is recursive. It stops at finished games (the three returns above), and each call fills one more cell, so it always gets there.
cells[index] = '' undoes the move (backtracking), so the board is left as it was.
Math.max(...scores) is the largest score, Math.min the smallest.

Your turn

In score, under if (end === 'draw') return 0, write the new lines, above the function's closing }.

Skills: Functions, Loops

← Score a finished game A computer that never loses →

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