[FIX] Fix nested array isolation in Observer constructor

src/observer.ts

@@ -1,6 +1,6 @@
 import { Events } from './events';
 import { ObserverHistory } from './observer-history';
-import { arrayEquals } from './utils';
+import { arrayEquals, deepCopyArray } from './utils';
 
 /**
  * The ObserverSync class is used to construct an interface for synchronizing changes from Observer
@@ -202,7 +202,7 @@ class Observer extends Events {
             for (i = 0; i < target._data[key].length; i++) {
                 if (typeof target._data[key][i] === 'object' && target._data[key][i] !== null) {
                     if (target._data[key][i] instanceof Array) {
-                        target._data[key][i].slice(0);
+                        target._data[key][i] = deepCopyArray(target._data[key][i]);
                     } else {
                         target._data[key][i] = new Observer(target._data[key][i], {
                             parent: this,

src/utils.ts

@@ -38,4 +38,27 @@ const arrayEquals = (a: any[], b: any[]) => {
     return true;
 };
 
-export { arrayEquals };
+/**
+ * Creates a deep copy of an array, recursively copying any nested arrays.
+ * Non-array objects within the array are not deep copied (they remain references).
+ *
+ * @param arr - The array to copy.
+ * @returns A deep copy of the array with all nested arrays also copied.
+ *
+ * @example
+ * const original = [[1, 2], [3, 4]];
+ * const copy = deepCopyArray(original);
+ * copy[0][0] = 99;
+ * console.log(original[0][0]); // 1 (unchanged)
+ */
+const deepCopyArray = (arr: any[]): any[] => {
+    const copy = arr.slice(0);
+    for (let i = 0; i < copy.length; i++) {
+        if (copy[i] instanceof Array) {
+            copy[i] = deepCopyArray(copy[i]);
+        }
+    }
+    return copy;
+};
+
+export { arrayEquals, deepCopyArray };

test/observer.test.mjs

@@ -238,4 +238,83 @@ describe('Observer', () => {
         observer.destroy();
     });
 
+    describe('nested array isolation', () => {
+        it('isolates nested arrays from source data modifications', () => {
+            // Simulates the ShareDB scenario: source data is modified externally
+            const sourceData = {
+                vectors: [[1, 2, 3], [4, 5, 6]]
+            };
+
+            const observer = new Observer(sourceData);
+
+            // Verify initial values
+            expect(observer.get('vectors.0')).to.deep.equal([1, 2, 3]);
+            expect(observer.get('vectors.0.0')).to.equal(1);
+
+            // Simulate external modification (like ShareDB updating its document)
+            sourceData.vectors[0][0] = 999;
+
+            // Observer's data should NOT be affected - it should have its own copy
+            expect(observer.get('vectors.0.0')).to.equal(1);
+            expect(observer.get('vectors.0')).to.deep.equal([1, 2, 3]);
+
+            observer.destroy();
+        });
+
+        it('fires set event when updating nested array element after source modification', () => {
+            // This is the exact bug scenario from GitHub issue #684
+            const sourceData = {
+                arrayVec3: [[1, 0, 0], [0, 1, 0]]
+            };
+
+            const observer = new Observer(sourceData);
+
+            // Simulate external modification (ShareDB updates its document)
+            sourceData.arrayVec3[0][0] = -1.94;
+
+            // Track if set event fires
+            let setEventFired = false;
+            let eventPath = null;
+            let eventValue = null;
+
+            observer.on('*:set', (path, value) => {
+                setEventFired = true;
+                eventPath = path;
+                eventValue = value;
+            });
+
+            // Now set the value through the observer (like sync.write does)
+            observer.set('arrayVec3.0.0', -1.94);
+
+            // The set event SHOULD fire because the observer's internal data
+            // should still be 1, not -1.94
+            expect(setEventFired).to.be.true;
+            expect(eventPath).to.equal('arrayVec3.0.0');
+            expect(eventValue).to.equal(-1.94);
+
+            observer.destroy();
+        });
+
+        it('maintains independent copies of deeply nested arrays', () => {
+            const sourceData = {
+                matrix: [
+                    [[1, 2], [3, 4]],
+                    [[5, 6], [7, 8]]
+                ]
+            };
+
+            const observer = new Observer(sourceData);
+
+            // Modify source at various levels
+            sourceData.matrix[0][0][0] = 100;
+            sourceData.matrix[1][1] = [99, 99];
+
+            // Observer should have independent data
+            expect(observer.get('matrix.0.0.0')).to.equal(1);
+            expect(observer.get('matrix.1.1')).to.deep.equal([7, 8]);
+
+            observer.destroy();
+        });
+    });
+
 });