TS-12 类型体操

• 递归

type A = ['ji','ni','tai','mei']

type Reverse<Arr extends unknown[]> = 
  Arr extends [...infer Rest,infer Last] ? [Last, ...Reverse<Rest>] : Arr

type Result = Reverse<A> // ['mei','tai','ni','ji']

• infer可以理解为引用，也可以理解为声明变量，通常出现在条件类型，与 extends 一起使用
• 实践：

1. 元祖体操：模式匹配和infer

// 元祖扩展
type A = [1]
type B = [...A,2]

// 取元祖的最后一个元素，这种方式也叫 模式匹配
type A = ['ji','ni','tai','mei']
type Last<T extends unknown[]> = T extends [...infer _ , infer L] ? L : never
type Result = Last<A>  // 'mei'

2. 字符串体操

// 内置 Capitalize;Uppercase;Lowercase;Uncapitalize

// 模版字符串类型
type S = 'ji'
type R = `${S}` // 'ji'

// 取第一个字符，模式匹配
type S = "ji ni tai mei"
type First<T extends string> = T extends `${infer F} ${string}` ? F : never
type Result = First<S>

// 取最后一个单词
type S = "ji ni tai mei"
type LastWord<T extends string> = T extends `${infer F} ${infer L}` ? LastWord<L> : T
type Result = LastWord<S> // 'mei'

// 取最后一个字符
type S = "ji ni tai mei !"
type LastOfTuple<T extends unknown[]> = T extends [...infer Rest, infer F] ? F : never
type StringToTuple<T extends string> = 
T extends `${infer F}${infer Rest}` ? [F, ...StringToTuple<Rest>] : []
type LastOfString = LastOfTuple<StringToTuple<S>>  // '!'

// string 转 联合类型
type StringToUnion<T extends string> = 
  T extends `${infer F}${infer R}` ? F | StringToUnion<R> : never
type C = StringToUnion<'abc'>  // 'a' | 'b' | 'c'

• 函数常用工具类型：ReturnType、Parameters(返回一个元祖类型，因为参数是数组)

const fn = (a:number,b:number) => 'a + b'
const fn1 = ({name,age}: {name: string, age: number}) => {}

type F = typeof fn // (a:number, b:number) => string

type R = ReturnType<F> // string
type MyReturnType<T extends (...args:any)=>any> = T extends (...args:any)=> infer R ? R : any

type P = Parameters<F> // [a:number,b:number]
type P2 = Parameters<typeof fn1> // [{name:string, age:number}]