Fix: Don't call merge() when key doesn't exist

Author: nzakas

src/object-schema.js

@@ -112,12 +112,16 @@ class ObjectSchema {
         }
 
         return objects.reduce((result, object) => {
+            
             this.validate(object);
+            
             for (const [key, strategy] of this[strategies]) {
                 try {
-                    const value = strategy.merge.call(this, result[key], object[key]);
-                    if (value !== undefined) {
-                        result[key] = value;
+                    if (key in result || key in object) {
+                        const value = strategy.merge.call(this, result[key], object[key]);
+                        if (value !== undefined) {
+                            result[key] = value;
+                        }
                     }
                 } catch (ex) {
                     ex.message = `Key "${key}": ` + ex.message;

tests/object-schema.js

@@ -102,6 +102,23 @@ describe("ObjectSchema", () => {
             assert.propertyVal(result, "foo", "bar");
         });
 
+        it("should not call the merge() strategy when both objects don't contain the key", () => {
+            
+            let called = false;
+
+            schema = new ObjectSchema({
+                foo: {
+                    merge() {
+                        called = true;
+                    },
+                    validate() {}
+                }
+            });
+
+            const result = schema.merge({}, {});
+            assert.isFalse(called, "The merge() strategy should not have been called.");
+        });
+
         it("should omit returning the key when the merge() strategy returns undefined", () => {
             schema = new ObjectSchema({
                 foo: {