Optional and Default Parameters

• 如果默認參數被用作尾參數，那么它起的作用和尾參數是可選的作用是一樣的：

function buildName(firstName: string, lastName?: string) {
    // ...
}
function buildName(firstName: string, lastName = "Smith") {
    // ...
}
(firstName: string, lastName?: string) => string

• 可選參數如果有，那么只能放在尾參數
• 默認參數位置任意，其類型由默認值決定，也就是說如果賦值了，那么值只能是設定的默認值的類型或者undefined

function buildName(firstName = "sss", lastName: string) {
    return firstName + " " + lastName;
}
let result3 = buildName(undefined | string , "ssss");
console.log(result3);

Rest Parameters

• ...用于無窮個可選參數，用于類型里面定義rest參數

function buildName(firstName: string, ...restOfName: string[]) {
    return firstName + " " + restOfName.join(" ");
}
let buildNameFun: (fname: string, ...rest: string[]) => string = buildName;

this

this是在函數調用時被設定的變量

let deck = {
    suits: ["hearts", "spades", "clubs", "diamonds"],
    cards: Array(52),
    createCardPicker: function() {
        return function() {
            let pickedCard = Math.floor(Math.random() * 52);
            let pickedSuit = Math.floor(pickedCard / 13);
            return {suit: this.suits[pickedSuit], card: pickedCard % 13};
        }
    }
}
let cardPicker = deck.createCardPicker();
let pickedCard = cardPicker();
//出錯，this.suits[pickedSuit] this被綁定到了 window | undefined
alert("card: " + pickedCard.card + " of " + pickedCard.suit);

this和arrow function

• 通過使用arrow function綁定函數的上下文
• 可以通過執行加上--noImplicitThis來幫助提醒可能存在的綁定上下文錯誤

let deck = {
    suits: ["hearts", "spades", "clubs", "diamonds"],
    cards: Array(52),
    createCardPicker: function() {
        // NOTE: the line below is now an arrow function, allowing us to capture 'this' right here
        return () => {
            let pickedCard = Math.floor(Math.random() * 52);
            let pickedSuit = Math.floor(pickedCard / 13);

            return {suit: this.suits[pickedSuit], card: pickedCard % 13};
        }
    }
}
let cardPicker = deck.createCardPicker();
let pickedCard = cardPicker();
alert("card: " + pickedCard.card + " of " + pickedCard.suit);

this parameter

• 然而，即使arrow function綁定了上下文,但是，this.suits[pickedSuit]的類型依然是any,因為this來源于對象字面量內部的函數表達式
• 所以TypeScript提供一個this parameter來修復這個bug

interface Card {
    suit: string;
    card: number;
}
interface Deck {
    suits: string[];
    cards: number[];
    createCardPicker(this: Deck): () => Card;
}
let deck: Deck = {
    suits: ["hearts", "spades", "clubs", "diamonds"],
    cards: Array(52),
    // NOTE: The function now explicitly specifies that its callee must be of type Deck
    createCardPicker: function(this: Deck) {
        return () => {
            let pickedCard = Math.floor(Math.random() * 52);
            let pickedSuit = Math.floor(pickedCard / 13);

            return {suit: this.suits[pickedSuit], card: pickedCard % 13};
        }
    }
}
let cardPicker = deck.createCardPicker();
let pickedCard = cardPicker();
alert("card: " + pickedCard.card + " of " + pickedCard.suit);

這樣就修復了bug，TypeScript知道了this的類型是Deck，就沒有any錯誤了

• this parameter in callbacks
  當函數作為參數傳入一個一個庫中的函數中的時候，這些庫會把傳入的函數當做普通的函數處理，因此這個函數捕獲的this會變成undefined，所以在這個函數內涉及到this的會報錯。
  通過以下幾步解決錯誤，而且可以使用this
• 首先通過fake parameter this，確保傳入的參數函數是不會捕獲this，第一層檢查。

interface UIElement {
    addClickListener(onclick: (this: void, e: Event) => void): void;
}

• 然后通過箭頭函數來幫忙

class Handler {
    info: string;
    onClickGood = (e: Event) => { this.info = e.message }
}

以上可以會有缺點。那就是每次傳建一個Handler類型的對象都會創建一個新的arrow function，造成浪費，解決辦法是在Handler的原型上創建公用的方法

Overloads

JavaScript函數對于不同的參數可能返回不同類型的對象等等，所以需要如下措施來幫助做type-check

• 通過overloads和調用函數相同名字的函數組，來檢查每一個可能產生的結果。

let suits = ["hearts", "spades", "clubs", "diamonds"];
function pickCard(x: {suit: string; card: number; }[]): number;
function pickCard(x: number): {suit: string; card: number; };
function pickCard(x): any {
    // Check to see if we're working with an object/array
    // if so, they gave us the deck and we'll pick the card
    if (typeof x == "object") {
        let pickedCard = Math.floor(Math.random() * x.length);
        return pickedCard;
    }
    // Otherwise just let them pick the card
    else if (typeof x == "number") {
        let pickedSuit = Math.floor(x / 13);
        return { suit: suits[pickedSuit], card: x % 13 };
    }
}
let myDeck = [{ suit: "diamonds", card: 2 }, { suit: "spades", card: 10 }, { suit: "hearts", card: 4 }];
let pickedCard1 = myDeck[pickCard(myDeck)];
alert("card: " + pickedCard1.card + " of " + pickedCard1.suit);
let pickedCard2 = pickCard(15);
alert("card: " + pickedCard2.card + " of " + pickedCard2.suit);