typescript使用笔记_代码007(未授权)

本文介绍: typescript已经成为前端日常开发中常用的工具之一，本文结合自己的使用做一下笔记，方便以后查阅。

typescript已经成为前端日常开发中常用的工具之一，本文结合自己的使用做一下笔记，方便以后查阅。
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Pa rt i al可以将某个类型中定义的属性变成可选的.
下面的定义了Book类型，创建实力时如果不设置id属性就会提示错误。

type Book = {
  id: string;
  name: string;
};

const book1: Book = {
    id: "1",
    name: "book1"
}

// 提示错误: Property 'id' is missing in type '{ name: string; }' but required in type 'Book'.ts(2741)
const book2: Book = {
    name: "book2"
}

可以通过Partial设置成部分属性

const book2: Partial&lt;Book&gt; = {
    name: "book2"
}

is用来指定数据的类型。

下面isNumber函数用来判断输入为数字，如果是就用调用setValue，但是提示value的值可能为undefined不能赋值给number类型。

const isNumber = (taregt: unknown): boolean =&gt; typeof taregt === "number" &amp;&amp; !Number.isNaN(taregt)

  const setValue = (newValue: number) => {
    // ....
  };

const reset = (value?: number) => {
    if (isNumber(value)) {
    // 错误提示: Argument of type 'number | undefined' is not assignable to parameter of type 'number'. Type 'undefined' is not assignable to type 'number'.ts(2345)
      setValue(value);
    }
};

isNumber函数已经判断 value位数字了，但是编译器还是报错，那怎样才能让编译器认为value就是nmber，没错is可能达到目的。将isNumber的返回类型使用is指定为number.

const isNumber = (taregt: unknown): taregt is number => typeof taregt === "number" &amp;&amp; !Number.isNaN(taregt)

infer用来推断数据的类型，typescript可以根据条件来确定类型比如官网的例子:

interface Animal {
  live(): void;
}
interface Dog extends Animal {
  woof(): void;
}

type Example1 = Dog extends Animal ? number : string;
// type Example1 = number

type Example2 = RegExp extends Animal ? number : string;
// type Example2 = string

表示如果T是一个无参数的函数，则返回T函数的返回值，返回返回T本身，

type func = () => number;
type GetReturnType<T> = T extends () => infer R ? R : T;
type FuncReturnType = GetReturnType<func>; // number

type Foo = string;
type StringReturnType = GetReturnType<Foo>; // string

可以将上面的type func = () => number改成有参数的type func = (a: string) => number，可以看返回了func函数本身。

type InferType<T> = T extends {name: infer V, age: infer V} ? V : never;
type Foo1 = InferType<{name: string; age: string}>;// type Foo1 = string
type Foo2 = InferType<{name: string; age: number}>;// type Foo = string | number

type InferType<T> = T extends (infer R)[] ? R : never;
type Foo3 = InferType<[string, number, boolean]>; // type Foo3 = string | number | boolean

type Book = {
    id: string;
    name: string;
    time: number;
}

const book1: Book = { // Type '{ name: string; }' is missing the following properties from type 'Book': id, timets(2739)
    name: "book1",
}

通过Omit可以忽略id、time属性，只要name属性。

const book1: Omit<Book, "id" | "time"> = {
    name: "book1",
}

type Foo = "a" | "b" | "c";
type Bar = Exclude<Foo, "a">; // type Foo = "a" | "b" | "c";

下面从Foo中提取"a"、"d"，最后Bar为"a"。

type Foo = "a" | "b" | "c";
type Bar = Extract<Foo, "a" | "d">; // type Bar = "a"

type T1 = Extract<string | number | (() => void), Function>; //type T1 = () => void
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T2 = Extract<Shape, { kind: "circle" }>
// type T2 = {
//     kind: "circle";
//     radius: number;
// }

排除联合类型中undefined、null

下面的例子类型为string | number。

type T = NonNullable<string | number | undefined | null>; //type T = string | number

type A = Awaited<Promise<string>>; // type A = string
type B = Awaited<Promise<Promise<number>>>; // type B = number
type C = Awaited<boolean | Promise<number>>; // type C = number | boolean

下面的Props的属性都是可选的，但是使用Required后就必填了

interface Props {
  a?: number;
  b?: string;
}
 
const obj: Props = { a: 5 };
 
//error: Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.ts(2741)
const obj2: Required<Props> = { a: 5 };

interface Todo {
  title: string;
}
 
const todo: Readonly<Todo> = {
  title: "Delete inactive users",
};
 
todo.title = "Hello"; // Cannot assign to 'title' because it is a read-only property.ts(2540)