TS中几种函数重载

函数重载

函数重载，先要了解什么是函数签名：
函数签名 = 函数名称+函数参数+函数参数类型+返回值类型四者合成，包含了实现签名和重载签名
function funa(value: number, type: string): string 这就是函数签名（重载签名）

type MessageType = "order" | "pay" | string; //微信消息类型
type Message = {
  id: number;
  type: MessageType;
  msg: string;
};

let messages: Message[] = [
  {
    id: 1,
    type: "order",
    msg: "订单金额错误",
  },
  {
    id: 2,
    type: "pay",
    msg: "支付失败",
  },
  {
    id: 3,
    type: "order",
    msg: "订单无效",
  },
  {
    id: 4,
    type: "pay",
    msg: "支付回调错误",
  },
];

函数重载由一个实现签名 + 一个或多个重载签名合成

function getMessage(value: number): Message; //重载签名1
function getMessage(value: MessageType, count: number): Message[]; //重载签名2
//实现签名↓
//参数类型也可以定义为： any 或 unknown（和any一样都是顶级类型）
function getMessage(value: number | MessageType, count: number = 1) {
  //count: number = 1 必须要加这个默认值，因为签名1没有这个参数，函数体中却有，会报错
  if (typeof value === "number") {
    return messages.find((msg) => {
      return value === msg.id;
    });
  } else {
    //过滤后得到多条，在splice确定返回条数
    return messages.filter((msg) => value === msg.type).splice(0, count);
  }
}

const detail = getMessage(1);
const list = getMessage("pay", 2); //签名定义了必须2个参数
console.log("1:", detail.msg); //这样直接点即可操作，不需要在(<>)转换了

list.forEach((item) => {
  console.log("2:", item.msg);
});

构造函数重载

type people = {
  name: string;
  age: number;
  sex?: string;
};
const san: people = {
  name: "山姆",
  age: 18,
  sex: "男",
};
console.log("type类型：", typeof san); //object

class Love {
  public name: string;
  public age: number;
  public sex: string;

  constructor(name_: string, age_: number);
  constructor(param: people);
  //由于构造函数本身不返回任何值，默认this且隐藏，所以重载和实现签名都不返回
  constructor(nameOrParam: any, age_: number = 0) {
    if (typeof nameOrParam === "object") {
      this.name = nameOrParam.name;
      this.age = nameOrParam.age;
      this.sex = nameOrParam.sex;
    } else {
      this.name = nameOrParam;
      this.age = age_;
      this.sex = "未知";
    }
  }

  public show(): string {
    let str = `这是一位${this.age}的${this.sex},名叫${this.name}`;
    console.log(str);
    return str;
  }
}

const a = new Love("小丽", 18);
const b = new Love(san);
a.show(); //这是一位18的未知,名叫小丽
b.show(); //这是一位18的男,名叫山姆

export {};