【TypeScript】Conditional Types

conditional types

// type Exclude<T, U> = T extends U ? never : T;
type MyExclude = 
( string extends string | number ? never : string )  // never
| ( number extends string | number ? never : number )  // never
| ( DebugType extends string | number ? never : DebugType )  // DebugType
type MyFunctionType = MyExclude

infer

type MyReturnType<T extends (...args: any) => any> = T extends (
  ...args: any
) => infer R 
? R 
: any;
// infer = 条件の一部としてジェネリクス型（<T extends (...args: any) => any>）を宣言することができる
// infer R = <T extends (...args: any) => any> 後ろのanyを取得
// (...args: any) => any = 関数を意味する → Tは関数