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工具类型实现

内置类型​

Partial<T>​

// example
interface Todo {
  title: string;
  description: string;
}

function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
  return { ...todo, ...fieldsToUpdate };
}

const todo1 = {
  title: "organize desk",
  description: "clear clutter"
};

const todo2 = updateTodo(todo1, {
  description: "throw out trash"
});

// implement
type Partial<T> = {
    [P in keyof T]?: T[P]
}

Required<T>​

type Required<T> = {
    [P in keyof T]-?: T[P]
}

Readonly<T>​

type Readonly<T> = {
    readonly [P in keyof T]: T[P]
}

Pick<T, K>​

// example
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Pick<Todo, "title" | "completed">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false
};

// implement
type Pick<T, K extends keyof T> = {
    [P in K]: T[P]
}

Omit<T, K>​

// example
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Omit<Todo, "description">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false
};

// implement

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
// or
type Omit<T, K> = {
    [P in Exclude<keyof T, K>]: T[P]
}

Extract<T, U>​

type Extract<T, K> = T extends K ? T : never

Exclude<T, U>​

// example
type T0 = Exclude<"a" | "b" | "c", "a">; // "b" | "c"
type T1 = Exclude<"a" | "b" | "c", "a" | "b">; // "c"
type T2 = Exclude<string | number | (() => void), Function>; // string | number

// implement
// extends搭配 ?: 运算符使用
type Exclude<T, U> = T extends U ? never : T;

// 相当于: type A = 'a' 
type A = Exclude<'x' | 'a', 'x' | 'y' | 'z'>

Record<K, T>​

type Record<K extends keyof any, T> = {
    [P in K]: T
}

NonNullable<T>​

type NonNullable<T extends keyof any> = T extends null | undefined ? never : T

Parameters<T>​

// infer 关键字
type Parameters<T extends (...args: any) => any> = T extends (...args: infer P) => any ? P : never;

ReturnType<T>​

// infer 关键字
type ReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : never

ConstructorParameters<T>​

type ConstructorParameters<T extends new (...args: any) => any> = T extends new (...args: infer P) => any ? P : never;

InstanceType<T>​

type InstanceType<T extends new (...args: any) => any> = T extends new (...args: any) => infer R ? R : any;

Awaited<T>​

type MyAwaited<T> = T extends Promise<infer T> ? MyAwaited<T> : T;

其他​

  • Uppercase，全体大写
  • Lowercase，全体小些
  • Capitalize，首字母大写
  • Uncapitalize，首字母小写

Equal​

非内置工具类型，用于比较两个类型完全相等，因为其高泛用性和实现的特殊性所以特别拎出来讨论。社区主流的实现如下：

export type Equal<X, Y> =
    (
      (<T>() => (T extends X ? 1 : 2)) extends
      (<T>() => (T extends Y ? 1 : 2)) 
    )
        ? true 
        : false;

Type Challenge​

开始做题

简单题​

Tuple To Object​

问题的本质是元祖如何转化为联合类型

type TupleToObject<T extends readonly any[]> = {
  [k in T[number]]: k
}

First Of Array​

type First<T extends any[]> = T extends [infer F, ...infer R] ? F : never;

Length Of Tuple​

需要注意的是需要用readonly any[]，否则元祖会走到never分支

type Length<T> = T extends readonly any[] ? T['length'] : never;

Concat​

type Concat<T extends readonly any[], U extends readonly any[]> = [...T, ...U]

Include​

参考，需要借助额外实现的Equal类型

// 迭代写法
type IterativeIncludes<T extends readonly any[], U> = {
  [K in keyof T]: Equal<T[K], U>;
}[number] extends false
  ? false
  : true;

// 递归写法
type RecursiveIncludes<T extends readonly any[], U> = T extends [
  infer F,
  ...infer R
]
  ? Equal<F, U> extends true
    ? true
    : RecursiveIncludes<R, U>
  : false;

中等题​

Readonly2​

type MyReadonly2<T, K extends keyof T = keyof T> = {
  [k in Exclude<keyof T, K>]: T[k]
} & {
  readonly [k in K]: T[k]
}

Deep Readonly​

type DeepReadonly<T> = {
  readonly [K in keyof T]: keyof T[K] extends never ? T[K] : DeepReadonly<T[K]> 
}

Tuple To Union​

type TupleToUnion<T extends readonly any[]> = T[number]

Chainable Options​

type Chainable<T = {}> = {
  option<K extends string, V extends any>(key: K, value: Exclude<V, K extends keyof T ? T[K] : never>): Chainable<{
    [k in (keyof T) | K]: k extends keyof T ? K extends keyof T ? V : T[k] : V
  }>
  get(): T
}

注意点

  1. option的返回值中需要进行字段的覆盖，不用单纯用&，除了这里写的方法也可以先Omit再&
  2. 需要对option的参数value的类型V进行约束，比如在某些情况下V的类型不能是string

扩展阅读

Exclude<T, string> = 'hi'，一元方程式求解，自动推导出T的类型为'hi'，arg为never

const fn = <T extends any>(arg: Exclude<T, string>) => arg;
fn('hi') // const fn: <"hi">(arg: never) => never

Promise.all​

type Util<T extends readonly any[]> = T extends readonly [infer F, ...infer L] ? [Awaited<F>, ...Util<L>] : []
declare function PromiseAll<T extends any[]>(values: readonly [...T]): Promise<Util<typeof values>>

注意点

  1. 这里通过extends [infer F]来递归获取元组的参数，但是数组不会跑到这个分支，所以我们需要一种手段把数组转化为等效的元组，通过values: readonly [...T]实现

Type Lookup​

type LookUp<U, T> = U extends {
  type: T
} ? U : never;

Trim Left​

type TrimLeft<S extends string> = S extends `${' ' | '\n' | '\t'}${infer R}` ? TrimLeft<R> : S;

Capitalize​

type MyCapitalize<S extends string> = S extends `${infer T}${infer R}` ?  `${Uppercase<T>}${R}`: S

Replace​

type Replace<S extends string, From extends string, To extends string> = S extends `${infer L}${Exclude<From, ''>}${infer R}` ? `${L}${To}${R}` : S;

ReplaceAll​

type ReplaceAll<S extends string, From extends string, To extends string> = S extends `${infer L}${Exclude<From, ''>}${infer R}` ? `${L}${To}${ReplaceAll<R, From, To>}` : S

Amend Argument​

type AppendArgument<Fn extends (...args: any[]) => any, A> = (...args: [...Parameters<Fn>, A]) => ReturnType<Fn>

Permutation​

type Permutation<T, K = T> = [T] extends [never] ? [] : T extends any ? [T, ...Permutation<Exclude<K, T>>] : never;

注意点

  1. 触发类型分发后，为了仍然能够获取完整的联合类型，我们额外声明了一个类型参数K = T

Length Of String​

type LengthOfString<S extends string, Len extends any[] = []> = S extends `${infer L}${infer R}` ? LengthOfString<R, [...Len, L]> : Len['length']

思路：字符串转元组

Flatten​

type Flatten<T> = T extends any[] ? T extends [infer L, ...infer R] ? [...Flatten<L>, ...Flatten<R>] : [] : [T]

Amend To Object​

type AppendToObject<T, U extends keyof any, V> = {
  [K in (keyof T | U)]: K extends keyof T ? T[K] : V
}

Absolute​

type Absolute<T extends number | string | bigint> = T extends string ? T extends `-${infer N}` ? N : T: Absolute<`${T}`>

思路：数字转字符串

String To Union​

type StringToUnion<T extends string> = T extends `${infer L}${infer R}` ?  L | StringToUnion<R>:  never;

Merge​

type Merge<F, S> = {
  [K in keyof F | keyof S]: K extends keyof S ? S[K] : K extends keyof F ? F[K] : never;
}

Diff​

type Diff<T, U> = Pick<{
  [K in keyof T | keyof U]: K extends keyof T ? T[K] : K extends keyof U ? U[K] : never;
}, Exclude<keyof T | keyof U, keyof T & keyof U>>

IsNever​

type IsNever<T> = [T] extends [never] ? true : false;

// 以下是错误答案
type IsNever2<T> = T extends never ? true : false 
type Test = IsNever2<never> // never

注意点：

  1. never不能触发类型分发，否则结果是never。

isUnion​

type IsUnion<T, Copy = T> = [T] extends [never] ? false : T extends Copy ? [Copy] extends [T] ? false : true : never

思路：先对T进行类型分发，然后再用T的完整拷贝U和T进行比较来判断是否为联合类型

Remove Index Signature​

type RemoveIndexSignature<T> = {
  [K in keyof T as string extends K ? never : number extends K ? never : symbol extends K ? never : K]: T[K]
}

思路

  1. 问题的本质是对象的过滤。我们通过as子句的特性来实现该功能，当as成never后就会自动过滤该键

DropChar​

type DropChar<S, C> = S extends `${infer L}${infer R}` ?  L extends C ?  DropChar<R, C> : `${L}${DropChar<R, C>}` : S;

MinusOne​

TODO

PickByType​

type PickByType<T, U> = {
  [K in keyof T as T[K] extends U ? K : never]: T[K]
}

StartsWith​

type StartsWith<T extends string, U extends string> = T extends `${U}${infer R}` ? true : false;

type EndsWith<T extends string, U extends string> = T extends `${infer L}${U}` ? L : false;

PartialByKeys​

type Merge<T> = {
  [K in keyof T]: T[K]
}
type PartialByKeys<T, K extends keyof any = keyof T> = Merge<Omit<T, K> & Partial<Pick<T, K & keyof T>>>

注意点

  1. 通过Merge实现{ name: string } & { age: number}到{ name: string; age: number}的转化
  2. 此处的T类型为string | number | symbol，为了让它满足keyof T有两种思路，一是通过T extends keyof T ? T : never，一种是这里的T & keyof T

ObjectEntries​

type ObjectEntries<T> = {
  [K in keyof T]-?: [K, T[K] extends infer R | undefined ? R : T[K]]
}[keyof T]

Tuple to Nested Object​

type TupleToNestedObject<T, U> = T extends [infer L, ...infer R]  ? {
  [K in L & keyof any]: TupleToNestedObject<R, U>
} : U;

Reverse​

type Reverse<T, Result extends readonly any[] = []> = T extends [infer L, ...infer R] ? Reverse<R, [L, ...Result]> : Result;

Flip Arguments​

type FlipArguments<T extends (...args: any[]) => any> = T extends (...args: infer P) => infer R ? (...args: Reverse<P>) => R : never;

BEM style string​

type BEM<B extends string, E extends string[], M extends string[]> = `${B}${E extends [] ? '' : E[number] extends infer T ? `__${T & string}`: never}${M extends [] ? '' : M[number] extends infer R ? `--${R & string}`: never}`

InorderTraversal​

type InorderTraversal<T extends TreeNode | null> = [T] extends [TreeNode] ? (
  [
    ...InorderTraversal<T['left']>,
    T['val'],
    ...InorderTraversal<T['right']>,
  ]
) : []

Filp​

type Flip<T> = {
  [K in keyof T as `${T[K] & (number | string | boolean) }` ]: K
}