1、函数表达式

function add(x:number,y:number):number{
    return x+y;
}
let myAdd=function(x:number,y:number):number{
    return x+y;
}
let myAdd1:(x:number,y:number)=>number=function(x:number,y:number):number{
    return x+y;
}

2、函数参数形式

(1)可选参数，参数名旁使用 ? ，可选参数必须跟在必须参数后面 。

function buildName(firstName: string, lastName?: string) {
    if (lastName)
        return firstName + " " + lastName;
    else
        return firstName;
}
let result1 = buildName("Bob");  // works correctly now
let result2 = buildName("Bob", "Adams", "Sr.");  // error, too many parameters
let result3 = buildName("Bob", "Adams");  // ah, just right

(2)默认参数，= ，可以传入 undefined值。

function buildName1(firstName: string, lastName = "Smith") {
    return firstName + " " + lastName;
}

let result4 = buildName1("Bob");                  // works correctly now, returns "Bob Smith"
let result5 = buildName1("Bob", undefined);       // still works, also returns "Bob Smith"
let result6 = buildName1("Bob", "Adams", "Sr.");  // error, too many parameters
let result7 = buildName1("Bob", "Adams");         // ah, just right

(3)剩余参数

function buildName2(firstName: string, ...restOfName: string[]) {
    return firstName + " " + restOfName.join(" ");
}

let employeeName = buildName2("Joseph", "Samuel", "Lucas", "MacKinzie");

3、函数中的this,与箭头函数。

//把函数表达式变为使用lambda表达式（ () => {} ）。 这样就会在函数创建的时候就指定了‘this’值，而不是在函数调用的时候。
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);

//this参数在回调函数里
interface UIElement {
    addClickListener(onclick: (this: void, e: Event) => void): void;
}
class Handler {
    info: string;
    onClickGood(this: void, e: Event) {
    // can't use this here because it's of type void!
        console.log('clicked!');
    }
}
//class Handler1 {
//    info: string;
//    onClickGood = (e: Event) => { this.info = e.message }
//}
let h = new Handler();
uiElement.addClickListener(h.onClickGood);

4、函数重载

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);