函数类型声明

定义函数中各部分的类型。包括函数类型、形参类型、返回值类型、类型、剩余参数

定义变量为函数类型

let test: Function; //函数类型首字母大写
test = () => {};

定义函数形参类型

function test(one: string, two: boolean) {
  return one;
}

function test2(one: string, two: boolean, three?: number) {
  //使用 ? 进行定义非必填参数，不传时，该属性为undefined
  return one;
}

function test3(one: string, two: boolean, three: number = 100) {
  //设置形参默认值，不传时，使用默认值
  return one;
}

定义函数返回值类型

function test(one: string, two: boolean): string | boolean {
  return one;
}

let test2 = (one: string, two: boolean): string | boolean => one;

类型参数声明

type A = { one: string; two: boolean };

let test = (b: A): void => b.one;

指定变量完整函数类型

let test: (a: string, b: number) => void;
test = (x: string, b: number): void => {};

type userType = { name: string; age: number };
type addUserFunc = (user: userType) => boolean;
let addUser: addUserFunc = (u: userType): boolean => {
  return true;
};

剩余参数

function test ( one: string  ,  ...args: any[ ] ): void => {  }   //指定args 数据类型

函数重载

//重载签名
function test(a: string): string;
function test(a: number): number;

//函数实现
function test(a: unknown): unknown {
  //正确
  return a;
}

function test(a: unknown): string {
  //报错，不能将类型“unknown”分配给类型“string”
  return a;
}