add utility type and mapped type example code

Author: joshua1988

example/10_utility-types.ts

@@ -0,0 +1,36 @@
+interface Product {
+  id: number;
+  name: string;
+  price: number;
+  brand: string;
+  stock: number;
+}
+
+let products: Product[] = [
+  { id: 1, name: '참치김밥', price: 3000, brand: '김가네', stock: 3 },
+];
+
+function displayProduct(productInfo: { id: 1; name: '참치김밥'; price: 3000 }) {
+  // ...
+}
+
+// #1 - Partial
+type Subset<T> = {
+  [K in keyof T]?: T[K];
+};
+
+const productDetail: Product = {
+  id: 1,
+};
+
+// #2 - Pick
+type PickFewThings<T, K extends keyof T> = {
+  [P in K]: T[P];
+}
+
+const productName: PickFewThings<Product, 'name'> = {
+
+}
+const productNameWithPrice: PickFewThings<Product, 'name' | 'price'> = {
+  
+}

example/11_mapped-types.ts

@@ -0,0 +1,6 @@
+// mapped types
+type Heroes = 'Hulk' | 'Thor' | 'Capt';
+type HeroAges = { [K in Heroes]: number };
+const ages: HeroAges = {
+  Hulk: 'a', // hulk's age must be number
+};