类型体操

模式匹配做提取

type GetReturnType<Func extends Function> = 
    Func extends (...args: any[]) => infer ReturnType 
        ? ReturnType 
        : never;

重新构造做变换

type UppercaseKey<Obj extends Record<string, any>> = { 
    [Key in keyof Obj as Uppercase<Key & string>]: Obj[Key]
}

递归复用做循环

type StringToUnion<Str extends string> = 
    Str extends `${infer First}${infer Rest}`
        ? First | StringToUnion<Rest>
        : never;

数组长度做计数

type BuildArray<
    Length extends number, 
    Ele = unknown, 
    Arr extends unknown[] = []
> = Arr['length'] extends Length 
        ? Arr 
        : BuildArray<Length, Ele, [...Arr, Ele]>;

type Subtract<Num1 extends number, Num2 extends number> = 
    BuildArray<Num1> extends [...arr1: BuildArray<Num2>, ...arr2: infer Rest]
        ? Rest['length']
        : never;