Add support and test for function default properties

CHANGELOG.md

@@ -241,6 +241,7 @@ realm.write(() => {
     ```
 ### Enhancements
 * Small improvement to performance by caching JSI property String object [#4863](https://github.com/realm/realm-js/pull/4863)
+* Add support for using functions as default property values, in order to allow dynamic defaults [#5001](https://github.com/realm/realm-js/pull/5001), [#2393](https://github.com/realm/realm-js/issues/2393)
 
 ### Compatibility
 * React Native >= v0.70.0

integration-tests/tests/src/tests/index.ts

@@ -36,3 +36,4 @@ import "./sync/flexible";
 import "./sync/asymmetric";
 import "./sync/sync-as-local";
 import "./transaction";
+import "./schema";

integration-tests/tests/src/tests/schema.ts

@@ -0,0 +1,50 @@
+////////////////////////////////////////////////////////////////////////////
+//
+// Copyright 2022 Realm Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+////////////////////////////////////////////////////////////////////////////
+
+import { expect } from "chai";
+import Realm from "realm";
+
+describe("Realm schema", () => {
+  describe("Default property values", () => {
+    it("can take a function as a default property value", () => {
+      interface Test {
+        dynamic?: number;
+      }
+
+      const realm = new Realm({
+        schema: [
+          {
+            name: "Test",
+            properties: {
+              dynamic: {
+                type: "int",
+                default: () => 42,
+              },
+            },
+          },
+        ],
+      });
+
+      const test = realm.write(() => {
+        return realm.create<Test>("Test", {});
+      });
+
+      expect(test.dynamic).to.equal(42);
+    });
+  });
+});

src/js_object_accessor.hpp

@@ -27,6 +27,8 @@
 #include "js_set.hpp"
 #include "js_realm_object.hpp"
 #include "js_schema.hpp"
+#include "js_types.hpp"
+#include "realm/util/optional.hpp"
 
 #if REALM_ENABLE_SYNC
 #include <realm/util/base64.hpp>
@@ -119,9 +121,20 @@ class NativeAccessor {
 
     OptionalValue default_value_for_property(const ObjectSchema& object_schema, const Property& prop)
     {
-        auto defaults = get_delegate<JSEngine>(m_realm.get())->m_defaults[object_schema.name];
+        const auto& defaults = get_delegate<JSEngine>(m_realm.get())->m_defaults[object_schema.name];
         auto it = defaults.find(prop.name);
-        return it != defaults.end() ? util::make_optional(ValueType(it->second)) : std::nullopt;
+
+        if (it == defaults.end()) {
+            return std::nullopt;
+        }
+        else {
+            auto value = ValueType(it->second);
+            // Call functions passed as the default value to allow dynamic defaults
+            if (Value::is_function(m_ctx, value)) {
+                value = Function<JSEngine>::call(m_ctx, Value::validated_to_function(m_ctx, value), 0, {});
+            }
+            return util::make_optional(value);
+        }
     }
 
     template <typename T>