fix: also allow setting nested field to undefined re: #14205

lib/document.js

@@ -1069,7 +1069,7 @@ Document.prototype.$set = function $set(path, val, type, options) {
           } else {
             throw new StrictModeError(key);
           }
-        } else if (pathtype === 'nested' && path[key] === null) {
+        } else if (pathtype === 'nested' && path[key] == null) {
           this.$set(pathName, path[key], constructing, options);
         }
       } else if (path[key] !== void 0) {

test/document.test.js

@@ -10567,4 +10567,23 @@ describe('document', function() {
     doc.set({ nested: null });
     assert.strictEqual(doc.toObject().nested, null);
   });
+
+  it('handles setting nested path to undefined (gh-14205)', function() {
+    const schema = new mongoose.Schema({
+      nested: {
+        key1: String,
+        key2: String
+      }
+    });
+
+    const Model = db.model('Test', schema);
+
+    const doc = new Model();
+    doc.init({
+      nested: { key1: 'foo', key2: 'bar' }
+    });
+
+    doc.set({ nested: void 0 });
+    assert.strictEqual(doc.toObject().nested, void 0);
+  });
 });