10. 类型计算入门（描述类型的小工具)

# 描述类型的小工具



观察所有Typescritp提供的类型下小工具，思考这个问题？

- 类型是可以被计算的吗？ 



内容

- Partial\- Required\- Readonly\- Record\- Pick\- Omit\- Exclude\- Extract\- NonNullable\- Parameters\- ConstructorParameters\- ReturnType\- InstanceType\- ThisParameterType\- OmitThisParameter\- ThisType\- Intrinsic String Manipulation Types
- Uppercase\- Lowercase\- Capitalize\- Uncapitalize\
## Keyof 操作符



```tsx
type Point = { x: number; y: number };
type P = keyof Point;

// type P = "x" | "y"

type Arrayish = { [n: number]: unknown };
type A = keyof Arrayish;

// type A = number

type Mapish = { [k: string]: boolean };
type M = keyof Mapish;

// type M = string | number

```



## Typeof



```tsx
console.log(typeof "xxx") // string

let s = "hello"
let n : typeof s
// n -- string
```





## Partial Type

```tsx
interface Todo {
  title: string;
  description: string;
}
 
function updateTodo(todo: Todo, fieldsToUpdate: Partial) {
  return { ...todo, ...fieldsToUpdate };
}
 

const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};
 
const todo2 = updateTodo(todo1, {
  description: "throw out trash",
})
```

源码怎么实现 Partial?

```tsx
type Partial = {
    [P in keyof T]?: T[P];
}
```



## Required

```tsx
interface Props {
  a?: number;
  b?: string;
}

 
const obj: Props = { a: 5 };
 
const obj2: Required = { a: 5 };

// Error : Property 'b' is missing in type '{ a: number; }' but required in type 'Required'

```

源码如何实现？

```tsx
type Required = {
    [P in keyof T]-?: T[P];
};

```



## Readonly



```tsx
interface Todo {
  title: string;
}
 
const todo: Readonly = {
  title: "Delete inactive users",
};
 
todo.title = "Hello";
// Error : Cannot assign to 'title' because it is a read-only property.
```

源码怎么实现？

```tsx
type Readonly = {
    readonly [P in keyof T]: T[P];
};
```



## Record



```tsx
interface CatInfo {
  age: number;
  breed: string;
}

 
type CatName = "miffy" | "boris" | "mordred";
 
const cats: Record = {
  miffy: { age: 10, breed: "Persian" },
  boris: { age: 5, breed: "Maine Coon" },
  mordred: { age: 16, breed: "British Shorthair" },
};
 
cats.boris;
 
const cats: Record
```

源码怎么实现？

```tsx
type Record = {
    [P in K]: T;
};

```





## Pick



```tsx
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}
 
type TodoPreview = Pick;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};
 
todo;
//const todo: TodoPreview
```

源码怎么实现？

```tsx
type Pick = {
    [P in K]: T[P];
};
```



## Exclude



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


```

源码怎么实现？

```tsx
type Exclude = T extends U ? never : T;
```



## Omit（英文：省略）

```tsx
interface Todo {
  title: string;
  description: string;
  completed: boolean;
  createdAt: number;
}
 
type TodoPreview = Omit;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
  createdAt: 1615544252770,
};
 
todo;
 
// const todo: TodoPreview


 
type TodoInfo = Omit;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};
 
todoInfo;
   
// const todoInfo: TodoInfo

```

源码怎么实现？

```tsx
type Omit = Pick>
```





## Extract

```tsx
type T0 = Extract<"a" | "b" | "c", "a" | "f">;
     
// type T0 = "a"
type T1 = Extract void), Function>;
     
// type T1 = () => void
```



源码怎么实现？

```tsx
type Extract = T extends U ? T : never;
```





## NonNullable

```tsx
type T0 = NonNullable;
     
// type T0 = string | number
type T1 = NonNullable;
     
// type T1 = string[]
```

源码怎么实现？

```tsx
type NonNullable = T extends null | undefined ? never : T;
```



## Parameters

```tsx
declare function f1(arg: { a: number; b: string }): void;
 
type T0 = Parameters<() => string>;
     
//type T0 = []
type T1 = Parameters<(s: string) => void>;
     
//type T1 = [s: string]
type T2 = Parameters<(arg: T) => T>;
     
//type T2 = [arg: unknown]

type T3 = Parameters
     
//type T3 = [arg: {
//    a: number;
//    b: string;
//}]
                     

```

源码怎么实现？

```tsx
type Parameters any> = T extends (...args: infer P) => any ? P : never;
```



## ConstructorParameters

```tsx
type T0 = ConstructorParameters;
     
//type T0 = [message?: string]
type T1 = ConstructorParameters;
     
//type T1 = string[]
type T2 = ConstructorParameters;
     
//type T2 = [pattern: string | RegExp, flags?: string]
type T3 = ConstructorParameters;
     
//type T3 = unknown[]
type T4 = ConstructorParameters;
// Error : Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
//  Type 'Function' provides no match for the signature 'new (...args: any): any'.     
//type T4 = never
```

```tsx
interface ErrorConstructor {
    new(message?: string): Error;
    (message?: string): Error;
    readonly prototype: Error;
}
    
interface FunctionConstructor {
    /**
     * Creates a new function.
     * @param args A list of arguments the function accepts.
     */
    new(...args: string[]): Function;
    (...args: string[]): Function;
    readonly prototype: Function;
}

    
interface RegExpConstructor {
    new(pattern: RegExp | string): RegExp;
    new(pattern: string, flags?: string): RegExp;
    (pattern: RegExp | string): RegExp;
    (pattern: string, flags?: string): RegExp;
    readonly prototype: RegExp;

    // Non-standard extensions
    $1: string;
    $2: string;
    $3: string;
    $4: string;
    $5: string;
    $6: string;
    $7: string;
    $8: string;
    $9: string;
    lastMatch: string;
}

```

源码怎么实现？

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



## ReturnType

```tsx
declare function f1(): { a: number; b: string };
 
type T0 = ReturnType<() => string>;
     
//type T0 = string
type T1 = ReturnType<(s: string) => void>;
     
//type T1 = void
type T2 = ReturnType<() => T>;
     
//type T2 = unknown
type T3 = ReturnType<() => T>;
     
//type T3 = number[]
type T4 = ReturnType;

// type T4 = { a: number;  b: string; }
type T5 = ReturnType;
     
// type T5 = any
type T6 = ReturnType;
     
// type T6 = never
type T7 = ReturnType;
// Type 'string' does not satisfy the constraint '(...args: any) => any'.    
//type T7 = any
                     
type T8 = ReturnType;
// Type 'Function' does not satisfy the constraint '(...args: any) => any'.
// Type 'Function' provides no match for the signature '(...args: any): any'.
     
// type T8 = any
```

源码怎么实现？

```tsx
type ReturnType any> = T extends (...args: any) => infer R ? R : any;
```



## InstanceType

```tsx

class C {
  x = 0;
  y = 0;
}
 
type T0 = InstanceType;

// type T0 = C
                       
type T1 = InstanceType;
     
// type T1 = any
type T2 = InstanceType;
     
// type T2 = never
type T3 = InstanceType;
// Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.     
//type T3 = any
                       
type T4 = InstanceType;
// Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
//  Type 'Function' provides no match for the signature 'new (...args: any): any'.
     
// type T4 = any
```

源码如何实现？

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



## ThisParameterType



```tsx
function toHex(this: Number) {
  return this.toString(16);
}
 
function numberToString(n: ThisParameterType) {
  return toHex.apply(n);
}
```



源码如何实现？

```tsx
type ThisParameterType = T extends (this: infer U, ...args: any[]) => any ? U : unknown;
```



## OmitThisParameter



```tsx
function toHex(this: Number) {
  return this.toString(16);
}
 
const fiveToHex: OmitThisParameter = toHex.bind(5);
                                   
// const fiveToHex = () => string
 
console.log(fiveToHex());
```



源码如何实现？

```tsx
type OmitThisParameter = unknown extends ThisParameterType ? T : T extends (...args: infer A) => infer R ? (...args: A) => R : T;
```





## ThisType



```tsx
type ObjectDescriptor = {
  data?: D;
  methods?: M; // Type of 'this' in methods is D & M
};
 
function makeObject(desc: ObjectDescriptor): D & M {
  let data: object = desc.data || {};
  let methods: object = desc.methods || {};
  return { ...data, ...methods } as D & M;
}


let obj = makeObject({
  data: { x: 0, y: 0 },
  methods: {
    moveBy(dx: number, dy: number) {
      this.x += dx; // Strongly typed this
      this.y += dy; // Strongly typed this
    },
  },
});
 
obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);
```



源码如何实现？

```tsx
interface ThisType { }
```

Why？



## Uppercase / Lowercase



uppercase的例子：

```tsx

type Greeting = "Hello, world"
type ShoutyGreeting = Uppercase
           
// type ShoutyGreeting = "HELLO, WORLD"
 
type ASCIICacheKey = `ID-${Uppercase}`
type MainID = ASCIICacheKey<"my_app">

// type MainID = "ID-MY_APP"

```

lowercase的例子：

```tsx
type Greeting = "Hello, world"
type QuietGreeting = Lowercase
          
// type QuietGreeting = "hello, world"
 
type ASCIICacheKey = `id-${Lowercase}`
type MainID = ASCIICacheKey<"MY_APP">
       
// type MainID = "id-my_app"
```

源码如何实现？

```tsx

/**
 * Convert string literal type to uppercase
 */
type Uppercase = intrinsic;

/**
 * Convert string literal type to lowercase
 */
type Lowercase = intrinsic;

/**
 * Convert first character of string literal type to uppercase
 */
type Capitalize = intrinsic;

/**
 * Convert first character of string literal type to lowercase
 */
type Uncapitalize = intrinsic;


```





## 总结



- 类型是可以计算的吗？ 
  - & 
  - -
  - ？
  - infer
  - ……