The whole board spins first. Then only the middle reel respins — the rest freeze in place. Every spin hits the same two methods: spin() and setResult().

The one-liner

setResult(grid) is the truth. If a reel’s target column already matches what it’s showing, the reel has nothing to do and just holds still.

That’s the whole trick. You don’t need a “hold this reel” API. Build the target grid with the frozen columns unchanged, and the library figures it out.

async function respinReel(reelIndex: number) {
  const grid: string[][] = [];
  for (let r = 0; r < reelSet.reels.length; r++) {
    if (r === reelIndex) {
      // New target for the one reel that is respinning.
      grid.push(await server.rollReel(r));
    } else {
      // Snapshot the visible column — the reel won't move.
      const reel = reelSet.reels[r];
      const col: string[] = [];
      for (let row = 0; row < reel.getVisibleSymbols().length; row++) {
        col.push(reel.getSymbolAt(row).symbolId);
      }
      grid.push(col);
    }
  }

  const p = reelSet.spin();
  reelSet.setResult(grid);
  await p;
}

Why it works for stacked symbols too

A “stacked” symbol is one that fills an entire reel column in the final grid — think a wild that covers all three rows of reel 3. When you want a follow-up respin that keeps that stacked wild in place, you use the exact same code path. Put the stacked column into grid[3] and the reel doesn’t move. No special API, no flag on the reel.

// Mark the stacked reel as held and respin the rest.
grid[reelWithStack] = ['wild', 'wild', 'wild'];   // snapshot of what's there
grid[otherReel]    = await server.rollReel(otherReel);

Variations

Respin-on-demand button — give the player a “respin reel 3” button that costs a fraction of the base bet.
Auto-nudge — if getSymbolAt(0) on a reel is the scatter symbol, silently respin the other reels to try for the remaining scatters.
Staggered hold & win — hold every locked coin, respin every free cell individually; see hold-and-win for the board-wide version.

Hold & Win — the board-wide respin, where every cell is either held or respinning
Sticky wild — wilds that stick across normal spins
Slam-stop — letting the player end a spin early

The one-liner

Why it works for stacked symbols too

Variations

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