feat: add lazy (#406)

packages/utilities/src/index.ts

@@ -26,6 +26,7 @@ export * from './lib/isNumber';
 export * from './lib/isObject';
 export * from './lib/isPrimitive';
 export * from './lib/isThenable';
+export * from './lib/lazy';
 export * from './lib/makeObject';
 export * from './lib/mergeDefault';
 export * from './lib/mergeObjects';

packages/utilities/src/lib/lazy.ts

@@ -0,0 +1,10 @@
+/**
+ * Lazily creates a constant or load a module and caches it internally
+ * @param cb The callback to lazily run
+ * @returns The value returned by the callback, or the cached value if it was already initialised once.
+ */
+export function lazy<T>(cb: () => T) {
+	let defaultValue: T;
+
+	return () => (defaultValue ??= cb());
+}

packages/utilities/tests/lazy.test.ts

@@ -0,0 +1,23 @@
+import { lazy } from '../src';
+
+describe('lazy', () => {
+	test('GIVEN string callback THEN returns the same', () => {
+		const callback = vi.fn(() => 'Lorem Ipsum');
+
+		const lazyStoredValue = lazy(callback);
+
+		expect(lazyStoredValue()).toEqual('Lorem Ipsum');
+	});
+
+	test('GIVEN string callback with cached value THEN returns the same', () => {
+		const callback = vi.fn(() => 'Lorem Ipsum');
+
+		const lazyStoredValue = lazy(callback);
+
+		lazyStoredValue();
+		const cachedValue = lazyStoredValue();
+
+		expect(callback).toHaveBeenCalledOnce();
+		expect(cachedValue).toEqual('Lorem Ipsum');
+	});
+});