题目描述
创建一个Length泛型，这个泛型接受一个只读的元组，返回这个元组的长度。
例如：
type tesla = ['tesla', 'model 3', 'model X', 'model Y']
type spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT']
type teslaLength = Length<tesla> // expected 4
type spaceXLength = Length<spaceX> // expected 5
题解
// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'
const tesla = ['tesla', 'model 3', 'model X', 'model Y'] as const
const spaceX = ['FALCON 9', 'FALCON HEAVY', 'DRAGON', 'STARSHIP', 'HUMAN SPACEFLIGHT'] as const
type cases = [
Expect<Equal<Length<typeof tesla>, 4>>,
Expect<Equal<Length<typeof spaceX>, 5>>,
// @ts-expect-error
Length<5>,
// @ts-expect-error
Length<'hello world'>,
]
// ============= Your Code Here =============
type Length<T extends readonly unknown[]> = T['length']
1、T extends readonly unknown[]约束传入的T必须是一个只读的数组类型
2、T['length']：元组实际上是一种特殊的数组类型，它继承了数组的很多特性，其中就包括length属性。length属性用于表示元组中元素的个数
