Includes
题目
在类型系统里实现 JavaScript 的 Array.includes 方法，这个类型接受两个参数，返回的类型要么是 true 要么是 false。
例如：
type isPillarMen = Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // expected to be `false`解题思路
第一次尝试是这样的：
type Includes<T extends readonly any[], U> = T extends [infer First, ...infer Rest]
? U extends First
? true
: Includes<Rest, U>
: false但是存在一个问题，即 {} 和 { 1, 2 } 是同类型的，并非相等。故需要引入 Equal 进行相等判断。
// 利用函数进行延迟求值
type MyEqual<T, U> = (
<P>() => P extends T ? 1 : 2
) extends (
<P>() => P extends U ? 1 : 2
) ? true : false答案
type MyEqual<T, U> = (
<P>() => P extends T ? 1 : 2
) extends (
<P>() => P extends U ? 1 : 2
) ? true : false
type Includes<T extends readonly any[], U> = T extends [infer First, ...infer Rest]
? MyEqual<U, First> extends true
? true
: Includes<Rest, U>
: false验证
type cases = [
Expect<Equal<Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Kars'>, true>>,
Expect<Equal<Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'>, false>>,
Expect<Equal<Includes<[1, 2, 3, 5, 6, 7], 7>, true>>,
Expect<Equal<Includes<[1, 2, 3, 5, 6, 7], 4>, false>>,
Expect<Equal<Includes<[1, 2, 3], 2>, true>>,
Expect<Equal<Includes<[1, 2, 3], 1>, true>>,
Expect<Equal<Includes<[{}], { a: 'A' }>, false>>,
Expect<Equal<Includes<[boolean, 2, 3, 5, 6, 7], false>, false>>,
Expect<Equal<Includes<[true, 2, 3, 5, 6, 7], boolean>, false>>,
Expect<Equal<Includes<[false, 2, 3, 5, 6, 7], false>, true>>,
Expect<Equal<Includes<[{ a: 'A' }], { readonly a: 'A' }>, false>>,
Expect<Equal<Includes<[{ readonly a: 'A' }], { a: 'A' }>, false>>,
Expect<Equal<Includes<[1], 1 | 2>, false>>,
Expect<Equal<Includes<[1 | 2], 1>, false>>,
Expect<Equal<Includes<[null], undefined>, false>>,
Expect<Equal<Includes<[undefined], null>, false>>,
]