函数

可选参数

函数可选参数 加?,只能写在必填参数的后面

function buildName(firstName: string, lastName?: string): string {
  return firstName + lastName;
}

默认参数

function buildName(firstName: string, lastName: string = '翠花'): string {
  return firstName + lastName;
}

编译后：

function buildName(firstName, lastName) {
  if (lastName === void 0) {
    lastName = '翠花';
  }
  return firstName + lastName;
}

剩余参数

// 剩余参数:编译后其实是用了arguments
function buildName3(firstName: string, ...resOfName: string[]): string {
  let name = '';
  resOfName.forEach((item) => {
    name += item;
  });
  return firstName + name;
}
console.log(buildName3('1', '2', '多少都可以'));

编译后其实是用了 arguments

// 编译后：
function buildName3(firstName) {
  var resOfName = [];
  for (var _i = 1; _i < arguments.length; _i++) {
    resOfName[_i - 1] = arguments[_i];
  }
  var name = '';
  resOfName.forEach(function (item) {
    name += item;
  });
  return firstName + name;
}
console.log(buildName3('1', '2', '多少都可以'));

this

this :函数被调用的上下文，箭头函数的 this 指向声明位置的上下文

interface Card {
  suit: string;
  card: number;
}
interface Deck {
  suits: string[];
  cards: number[];
  // 指定this类型,如果哪里有写错this的引用，会智能提示
  createCardPicker(this: Deck): () => Card;
}
let deck: Deck = {
  suits: ['黑桃', '红桃', '梅花', '尖尖'],
  cards: Array(52),
  createCardPicker: function () {
    // 箭头函数，this指向函数声明时的this
    return () => {
      let pickedCard = Math.floor(Math.random() * 52);
      let pickedSuit = Math.floor(pickedCard / 13);
      return {
        suit: this.suits[pickedSuit],
        card: pickedCard % 13,
      };
    };
  },
};

重载

函数的参数类型、个数不同，匹配对应的签名

function run(x: number): number;
function run(x: string, y: string): string;
function run(x: number | string, y?: string) {
  if (typeof x === 'number') {
    return x;
  } else if (typeof y === 'string') {
    return y;
  }
}