Workshop › Chess › Step 44 of 48

Alpha-beta: skip what cannot matter

What we are doing

Three moves deep is about 30 × 30 × 30 positions. Alpha-beta skips most of them. alpha is the score we are already sure of, beta the most the opponent will allow. As soon as a move scores beta or more, the opponent would never let us reach this position, so the rest of our moves here need not be looked at.

```js
// Negamax with alpha-beta: the best score the side to move can force, looking `depth` moves ahead.
function search(depth, alpha, beta) {
  const moves = legalMoves()
  if (moves.length === 0) return inCheck(turn) ? -100000 - depth : 0
  if (depth === 0) return evaluate() * (turn === 'w' ? 1 : -1)
  for (const m of moves) {
    const undo = makeMove(m)
    const score = -search(depth - 1, -beta, -alpha)
    undoMove(undo)
    if (score >= beta) return beta
    if (score > alpha) alpha = score
  }
  return alpha
}

    const score = -search(DEPTH - 1, -Infinity, Infinity) + Math.random() // a tiny random tie-break
```

What it means

alpha replaces best: it starts at what the caller already guarantees and grows with each better move.
For the opponent the window flips and swaps: -beta, -alpha, just like the score flips with -.
score >= beta returns at once (a cut). The result is the same move choice with far fewer positions.
computerMove starts with the widest window: -Infinity, Infinity.

Your turn

Change search as shown (its comment, its header, the loop and the last line), and pass -Infinity, Infinity in computerMove.

Skills: Functions

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