feat: improve defaultFor with rest return value

__tests__/unit/env.picker.spec.ts

@@ -40,14 +40,17 @@ describe('EnvPicker', () => {
       ).toBe(undefined);
 
       expect(new EnvPicker(false as boolean | undefined, 'test').defaultFor({ test: true }).value()).toBe(false);
-      expect(new EnvPicker('', 'test').defaultFor({ test: '123' }).value()).toBe('');
+      expect(new EnvPicker('' as string | undefined, 'test').defaultFor({ test: '123' }).value()).toBe('');
     });
   });
 
   describe('map', () => {
     it('maps state', () => {
       expect(new EnvPicker('123', 'test').map(Number).value()).toBe(123);
       expect(new EnvPicker(undefined as string | undefined, 'test').default('123').map(Number).value()).toBe(123);
+      expect(
+        new EnvPicker(undefined as string | undefined, 'test').defaultFor({ rest: '123' }).map(Number).value(),
+      ).toBe(123);
       expect(new EnvPicker('false', 'test').default('123').map(JSON.parse).value()).toBe(false);
       expect(new EnvPicker(undefined as string | undefined, 'test').map((state) => !!state).value()).toBe(false);
     });

src/env/env.picker.ts

@@ -31,10 +31,12 @@ export class EnvPicker<NodeEnv extends string, State> {
     return this as unknown as EnvPicker<NodeEnv, NewState | NonNullable<State>>;
   }
 
-  public defaultFor(envRecord: EnvRecord<NodeEnv, State>) {
-    this.state ??= envRecord[this.nodeEnv] ?? (envRecord.rest as State);
+  public defaultFor<NewState extends State, EnvOptions extends EnvRecord<NodeEnv, NewState>>(envRecord: EnvOptions) {
+    this.state ??= (envRecord[this.nodeEnv] ?? envRecord.rest) as NewState;
 
-    return this;
+    return this as unknown as EnvOptions extends { rest: NewState }
+      ? EnvPicker<NodeEnv, NonNullable<NewState>>
+      : EnvPicker<NodeEnv, NewState>;
   }
 
   public map<Result>(mapper: (state: State) => Result) {