TS學習之函數

定義函數類型（規定函數參數及函數返回值的類型，若函數沒有返回值，則返回類型為null）

function add(x: number, y: number): number {
    return x + y
}

推斷類型（ts自動識別類型（按上下文歸類））

function add(x: number, y: number) {
    return x + y
}
//ts會自動識別出返回類型為number

可選參數，默認參數，剩余參數

• 可選參數（參數名旁使用 ?實現可選參數的功能，可選參數放在必填參數之后）

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

let result1 = buildName("Bob");  // Bob
let result3 = buildName("Bob", "Adams");  // Bob Adams

• 默認參數（默認參數可傳值也可不傳，一般默認參數在必須參數之后，但也可以定義在默認參數之前，但此時必須明確傳入undefined以獲取默認值）

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

let result1 = buildName("Bob");  // Bob Smith
let result3 = buildName("Bob", "Adams");  // Bob Adams

function buildName(firstName: string = "Mr", lastName: string ) {
    return firstName + " " + lastName;
}

let result1 = buildName("Bob","Adams");  // Bob Adams
let result2 = buildName("Bob"); //error: Expected 2 arguments, but got 1.
let result3 = buildName(undefined,"Smith")  //Mr Smith

• 剩余參數（當參數個數不確定時）

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

let employeeName = buildName("Joseph", "Samuel", "Lucas", "MacKinzie");
console.log(employeeName)   //Joseph Samuel Lucas MacKinzie

this和箭頭函數（傳統函數的this作用域是在函數調用時確定，但箭頭函數的this作用域在創建時就已經確定）

• 傳統函數

let myobj = {
    person:["Bob","Fred","Smith"],
    myFn:function(){
        return this.person
    }
}

let test = myobj.myFn;
console.log(test()) //undefined

• 箭頭函數

let myobj = {
    person: ["Bob", "Fred", "Smith"],
    myFn: function () {
        return () => {
           return this.person
        }
    }
}

let test = myobj.myFn();
console.log(test()) //[ 'Bob', 'Fred', 'Smith' ]