#00898 include

[pkc918]

题目

type isPillarMen = Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // expected to be `false`

// test
Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Kars'> // true
Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'> // false
Includes<[1, 2, 3, 5, 6, 7], 7> // true
Includes<[1, 2, 3, 5, 6, 7], 4> // false
Includes<[1, 2, 3], 2> // true
Includes<[1, 2, 3], 1> // true
Includes<[{}], { a: 'A' }> // false
Includes<[boolean, 2, 3, 5, 6, 7], false> // false
Includes<[true, 2, 3, 5, 6, 7], boolean> // false
Includes<[false, 2, 3, 5, 6, 7], false> // true
Includes<[{ a: 'A' }], { readonly a: 'A' }> // false
Includes<[{ readonly a: 'A' }], { a: 'A' }> // false
Includes<[1], 1 | 2> // false
Includes<[1 | 2], 1> // false
Includes<[null], undefined> // false
Includes<[undefined], null> // false

实现 js 中的 includes 的功能，返回boolean

答案

type IsEqual<T, U> =
    (<G>() => G extends T ? 1 : 2) extends (<G>() => G extends U ? 1 : 2) ? true : false

type Includes<T extends readonly any[], U> = T extends [infer First, ...infer Rest] ? IsEqual<First, U> extends true ? true : Includes<Rest, U> : false

讲解

先不用看 IsEqual，我们可以看到，我们首先是利用了每次取第一个值，判断是否和第一个值相等。用这样的方式进行递归，直到为空，IsEqual 实现以及讲解(可以看看官方的探讨)[https://github.com/microsoft/TypeScript/issues/27024]