Update typescript.md (#1863)

typescript.md

@@ -18,16 +18,25 @@ string
 null
 undefined
 
+bigint
+symbol
+
 string[]          /* or Array<string> */
 [string, number]  /* tuple */
 
 string | null | undefined   /* union */
 
 never  /* unreachable */
+unknown
 ```
 
 ```ts
-enum Color {Red, Green, Blue = 4}
+enum Color {
+  Red,
+  Green,
+  Blue = 4
+};
+
 let c: Color = Color.Green
 ```
 
@@ -97,8 +106,8 @@ function printLabel(options: LabelOptions) { ... }
 
 ```ts
 interface User {
-  name: string,
-  age?: number
+  name: string;
+  age?: number;
 }
 ```
 
@@ -124,6 +133,16 @@ interface User {
 type Name = string | string[]
 ```
 
+### Intersection
+
+```ts
+interface Colorful { ... }
+
+interface Circle { ... }
+ 
+type ColorfulCircle = Colorful & Circle;
+```
+
 ## Function types
 
 ```ts
@@ -204,10 +223,62 @@ export interface User { ... }
 ```ts
 interface Building {
   room: {
-    door: string,
-    walls: string[],
+    door: string;
+    walls: string[];
   };
 }
 
 type Walls = Building['room']['walls']; // string[]
 ```
+
+## Keyof Type Operator
+
+```ts
+type Point = { x: number; y: number };
+
+type P = keyof Point; // x | y
+```
+
+## Conditinal Types
+
+```ts
+// SomeType extends OtherType ? TrueType : FalseType;
+
+type ToArray<T> = T extends any ? T[] : never;
+
+type StrArrOrNumArr = ToArray<string | number>; // string[] | number[]
+```
+
+### Inferring
+
+```ts
+type GetReturnType<T> = T extends (...args: unknown[]) => infer R
+  ? R
+  : never;
+
+type Num = GetReturnType<() => number>; // number
+```
+
+```ts
+type First<T extends Array<any>> = T extends [infer F, ...infer Rest] ? F : never;
+
+type Str = First<['hello', 1, false]>; // 'hello'
+```
+
+## Literal Type
+
+```ts
+const point = { x: 4, y: 2 }; // { x: number, y: number }
+
+const literalPoint = { x: 4, y: 2 } as const; // { readonly x: 4, readonly y: 2 };
+```
+
+## Template Literal Types
+
+```ts
+type SpaceChar = ' ' | '\n' | '\t';
+
+type TrimLeft<S extends string> = S extends `${SpaceChar}${infer Rest}` ? TrimLeft<Rest> : S;
+
+type Str = TrimLeft<'    hello'>; // 'hello'
+```