From 957e05f38606b5ef987736dc549d5f370ceac15b Mon Sep 17 00:00:00 2001
From: Evan Freethy <efreethy@gmail.com>
Date: Tue, 3 Feb 2026 13:31:29 -0800
Subject: [PATCH] Adds zoom as an optional third parameter to the reduce
 callback

---
 index.js     |  4 ++--
 test/test.js | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/index.js b/index.js
index 4dce8d36..e47c734c 100644
--- a/index.js
+++ b/index.js
@@ -14,7 +14,7 @@ const defaultOptions = {
     generateId: false,
 
     // a reduce function for calculating custom cluster properties
-    reduce: null, // (accumulated, props) => { accumulated.sum += props.sum; }
+    reduce: null, // (accumulated, props, zoom) => { accumulated.sum += props.sum; }
 
     // properties to use for individual points when running the reducer
     map: props => props // props => ({sum: props.my_value})
@@ -331,7 +331,7 @@ export default class Supercluster {
                             clusterPropIndex = this.clusterProps.length;
                             this.clusterProps.push(clusterProperties);
                         }
-                        reduce(clusterProperties, this._map(data, k));
+                        reduce(clusterProperties, this._map(data, k), zoom);
                     }
                 }
 
diff --git a/test/test.js b/test/test.js
index 5fb05107..1ea4db24 100644
--- a/test/test.js
+++ b/test/test.js
@@ -95,6 +95,31 @@ test('aggregates cluster properties with reduce', () => {
         [298, 122, 12, 36, 98, 7, 24, 8, 125, 98, 125, 12, 36, 8]);
 });
 
+test('passes zoom level to reduce function', () => {
+    const zoomsReceived = new Set();
+    const index = new Supercluster({
+        map: props => ({sum: props.scalerank}),
+        reduce: (a, b, zoom) => {
+            a.sum += b.sum;
+            zoomsReceived.add(zoom);
+        },
+        radius: 100,
+        minZoom: 0,
+        maxZoom: 5
+    }).load(places.features);
+
+    // Trigger clustering by getting clusters
+    index.getClusters([-180, -85, 180, 85], 0);
+
+    // Verify zoom values were passed (should be integers from minZoom to maxZoom)
+    assert.ok(zoomsReceived.size > 0, 'reduce should have been called with zoom values');
+    for (const zoom of zoomsReceived) {
+        assert.equal(typeof zoom, 'number', 'zoom should be a number');
+        assert.ok(Number.isInteger(zoom), 'zoom should be an integer');
+        assert.ok(zoom >= 0 && zoom <= 5, 'zoom should be within minZoom/maxZoom range');
+    }
+});
+
 test('returns clusters when query crosses international dateline', () => {
     const index = new Supercluster().load([
         {