type-challenge刷题(easy部分)示例详解

type MyPick<T, K extends keyof T> = {[k in K]: T[k]};
type MyReadonly<T> = {readonly [k in keyof T]: T[k]};
type TupleToObject<T extends readonly (string|number|symbol)[]> = {[k in T[number]]: k};
type First<T extends any[]> = T extends [infer Head, ...unknown[]] ? Head : never;
type Length<T extends readonly unknown[]> = T["length"]; // 题目要求readonly数组
type If<C extends boolean, T, F> = C extends true ? T : F;
type Concat<T extends unknown[], U extends unknown[]> = [...T, ...U];
type Push<T extends unknown[], U> = [...T, U];
type Unshift<T extends unknown[], U> = [U, ...T];
type MyParameters<T extends (...args: any[]) => any> = T extends (...args: infer U) => any ? U : never;

type MyExclude<T, U> = T extends U ? never : T;

MyExclude<'a'|'b'|'c', 'a'|'b'> = 
  ('a' extends 'a'|'b' ? never : 'a') |
  ('b' extends 'a'|'b' ? never : 'b') |
  ('c' extends 'a'|'b' ? never : 'c')

type PromiseLike<T = unknown> = {then: (cb: (arg: T)=>unknown) => unknown};
type MyAwaited<T extends PromiseLike> = T extends PromiseLike<infer U> ? (U extends PromiseLike ? MyAwaited<U> : U) : never;

type Eq<X, Y> =
  (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? true : false
type Includes<T extends readonly any[], U> =
  T extends [infer H, ...infer Rest]
  ? (Eq<H, U> extends false ? Includes<Rest, U>: true)
  : false;

type Eq<X, Y> = X extends Y ? (Y extends X ? true : false) : false;
// 这个是错的，测试用例如下
type check = Eq<boolean, true> // boolean

type check = Eq<boolean, true> // boolean
// ⬇️
// boolean -> true|false
// ====> Eq<true, true>|Eq<false, true> -> true|false -> boolean

type Eq<X, Y> =
    // 这里没有直接进行X和Y的比较，那样会触发分配条件类型
    // 因此借助范型的变量T作为桥梁进行比较
  (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? true : false