Includes

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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`

解答

type Includes<T extends readonly any[], U> = U extends T[number] ? true : false

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/**
Returns a boolean for whether given two types are equal.
@link https://github.com/microsoft/TypeScript/issues/27024#issuecomment-421529650
*/
type IsEqual<T, U> =
	(<G>() => G extends T ? 1 : 2) extends
	(<G>() => G extends U ? 1 : 2)
		? true
		: false;

type Includes<Value extends any[], Item> =
	IsEqual<Value[0], Item> extends true
		? true
		: Value extends [Value[0], ...infer rest]
			? Includes<rest, Item>
			: false;