Default values for child fields (#34)

packages/roqueform/src/main/createField.ts

@@ -94,7 +94,7 @@ function getOrCreateField(
       setValue(field, field.value, false);
     },
 
-    at: key => getOrCreateField(field.valueAccessor, field, key, null, plugin),
+    at: (key, defaultValue) => getOrCreateField(field.valueAccessor, field, key, defaultValue, plugin),
 
     on: (type, subscriber) => {
       const subscribers = (field.subscribers[type] ||= []);
@@ -111,7 +111,12 @@ function getOrCreateField(
   field.rootField = field;
 
   if (parentField !== null) {
-    field.value = parentField.valueAccessor.get(parentField.value, key);
+    const derivedValue = parentField.valueAccessor.get(parentField.value, key);
+
+    if (derivedValue !== undefined) {
+      field.value = derivedValue;
+    }
+
     field.initialValue = parentField.valueAccessor.get(parentField.initialValue, key);
     field.rootField = parentField.rootField;
     (parentField.children ||= []).push(field);

packages/roqueform/src/main/types.ts

@@ -104,10 +104,11 @@ export interface BareField<Value = any, Plugin = any> {
    * {@link value the current value}.
    *
    * @param key The key in the value of this field.
+   * @param defaultValue The default value used if the value at given key is `undefined`.
    * @returns The child field instance.
    * @template Key The key in the value of this field.
    */
-  at<Key extends KeyOf<Value>>(key: Key): Field<ValueAt<Value, Key>, Plugin>;
+  at<Key extends KeyOf<Value>>(key: Key, defaultValue?: ValueAt<Value, Key>): Field<ValueAt<Value, Key>, Plugin>;
 
   /**
    * Subscribes to all events.

packages/roqueform/src/test/createField.test.ts

@@ -48,6 +48,20 @@ describe('createField', () => {
     expect(field.at('aaa')).toBe(field.at('aaa'));
   });
 
+  test('returns a field at key with the default value', () => {
+    const field = createField<{ aaa?: number }>({}).at('aaa', 111);
+
+    expect(field.value).toBe(111);
+    expect(field.initialValue).toBe(undefined);
+  });
+
+  test('default value does not override a defined value', () => {
+    const field = createField({ aaa: 222 }).at('aaa', 111);
+
+    expect(field.value).toBe(222);
+    expect(field.initialValue).toBe(222);
+  });
+
   test('sets a value to a root field', () => {
     const field = createField(111);