infer

• 表示待推断类型

type ParamType<T> = T extends (...args: infer P) => any ? P : T;

• 如果T能赋值给(...args: infer P) => any, 结果是(...args: infer P) => any 否则返回T

interface User {
  name: string;
  age: number;
}

type Func = (user: User) => void;

type Param = ParamType<Func>; // Param = User
type AA = ParamType<string>; // string

in 遍历数组

• Q - 实现 TupleToObject

const tuple = ['tesla', 'model 3', 'model X', 'model Y'] as const

type result = TupleToObject<typeof tuple> // expected { tesla: 'tesla', 'model 3': 'model 3', 'model X': 'model X', 'model Y': 'model Y'}

• A - T[number]

type TupleToObject<T extends ReadonlyArray<string | number>> = {
    [p in T[number]]: p
}

First of Array

• Q

type arr1 = ['a', 'b', 'c']
type arr2 = [3, 2, 1]

type head1 = First<arr1> // expected to be 'a'
type head2 = First<arr2> // expected to be 3

• A

type First<T extends any[]> = T extends [] ? never : T[0]
type First<T extends any[]> = T['length'] extends 0 ? never : T[0]
type First<T extends any[]> = T extends [infer P, ...infer Rest] ? P : never