Scatter triggers Free Spins

Why this page exists

The “3 scatters anywhere” rule is the most common FS trigger in the industry. The interesting part isn’t the detection — it’s producing it on demand so you can polish the transition without playing 100 spins.

Setup

1. Register a scatter symbol

builder.symbols((r) => {
  r.register('scatter', BlockSymbol, { colors: { scatter: 0xffd24c } });
});
builder.weights({ scatter: 2 });   // low natural frequency

2. Detect on completion

reelSet.events.on('spin:complete', ({ symbols }) => {
  const count = symbols.flat().filter((s) => s === 'scatter').length;
  if (count >= 3) enterFreeSpins(count * 5);
});

3. Anticipation on near-miss

If the server response says “2 scatters visible and reel 5 is spinning,” call setAnticipation before setResult to slow the last reel.

const promise = reelSet.spin();
setTimeout(() => {
  reelSet.setAnticipation([4]);       // slow reel 5
  reelSet.setResult(response.symbols);
}, 200);

4. Test it

The forceScatters(3, 'scatter') cheat makes this deterministic:

const { reelSet, spinAndLand } = createTestReelSet({
  reels: 5, visibleRows: 3, symbolIds: ['a','b','c','scatter'],
});
const engine = new CheatEngine({ reelCount: 5, visibleRows: 3, symbolIds: ['a','b','c','scatter'], seed: 1 });
engine.register({ id: 's', label: 's', enabled: true, cheat: forceScatters(3, 'scatter') });

await spinAndLand(engine.next().symbols);
expect(countSymbol(reelSet, 'scatter')).toBeGreaterThanOrEqual(3);

Cheats on this page

• Force 3 scatters (default on) — guarantees the trigger.
• Force 4 scatters — retrigger territory.
• Near-miss on reel 5 — 2 scatters on reels 1–4, reel 5 blanks with anticipation.