feat: react-like state resolver to use it in stateful hooks;

src/__tests__/resolveHookState.ts

@@ -0,0 +1,26 @@
+import { resolveHookState } from '../util/resolveHookState';
+
+describe('resolveHookState', () => {
+  it('should defined', () => {
+    expect(resolveHookState).toBeDefined();
+  });
+
+  it(`should return value as is if it's not a function`, () => {
+    expect(resolveHookState(1)).toBe(1);
+    expect(resolveHookState('HI!')).toBe('HI!');
+    expect(resolveHookState(undefined)).toBe(undefined);
+  });
+
+  it('should call passed function', () => {
+    const spy = jest.fn();
+    resolveHookState(spy);
+    expect(spy).toHaveBeenCalled();
+  });
+
+  it('should pass 2nd parameter to function', () => {
+    const spy = jest.fn();
+    resolveHookState(spy, 123);
+    expect(spy).toHaveBeenCalled();
+    expect(spy.mock.calls[0][0]).toBe(123);
+  });
+});

src/util/resolveHookState.ts

@@ -0,0 +1,18 @@
+export type StateSetter<S> = (prevState: S) => S;
+export type InitialStateSetter<S> = () => S;
+
+export type InitialHookState<S> = S | InitialStateSetter<S>;
+export type HookState<S> = S | StateSetter<S>;
+export type ResolvableHookState<S> = S | StateSetter<S> | InitialStateSetter<S>;
+
+export function resolveHookState<S>(newState: S | InitialStateSetter<S>): S;
+export function resolveHookState<S>(newState: Exclude<HookState<any>, StateSetter<any>>, currentState: S): S;
+// tslint:disable-next-line:unified-signatures
+export function resolveHookState<S>(newState: StateSetter<S>, currentState: S): S;
+export function resolveHookState<S>(newState: ResolvableHookState<S>, currentState?: S): S {
+  if (typeof newState === 'function') {
+    return (newState as Function)(currentState);
+  }
+
+  return newState as S;
+}