# JavaScript - 字串型別與轉換
###### tags: `javascript`

## 宣告變數與印出值

let a = 1;
let b = 2;
console.log(a);
console.log(b);
console.log(a, b);
alert(a);

## 宣告字串

let hello = "你好嗎?";
console.log(hello);
let 去玩 = "let's go!";
console.log(去玩);

## 字串相加

let friendName = 'Joey'; //string
let content = 'How do you doin?';
let total = friendName + ' ' + content;
console.log(total);
let age = 25; //number
let ageString = "25"; //string

## 透過 typeof 瞭解當前變數型別

console.log(typeof friendName); //string
console.log(typeof age); //number
console.log(typeof ageString); //string

## 數字與字串相加

let myName = 'Ross'; //string
let myAge = 28; //number
console.log(typeof myName);
console.log(typeof myAge);

// JavaScript 有些情況會貼心自動轉型
let myTotal = myName + myAge; 
console.log(typeof myTotal); //string
console.log("Hi 我是 " + myName + "，我今年 " + myAge + " 歲");

## NaN 非數字(但型別屬number)

let herName = 'Racheal';
//string
let herAge = 26;
//number
let herTotal = herName * herAge;
console.log(herTotal);
//NaN
console.log(typeof herTotal);
//number

## 字串轉成數字 parseInt()
**parseInt()**

// 字串
// let weight = '50';

// 字串轉成數字
let weight = parseInt('50');
console.log(typeof weight);
//number

// 字串內容無法傳成數字 => NaN
let hi = parseInt('hello');
console.log(hi);
//NaN
console.log(typeof hi);
//number

## 字元長度查詢 .length
**.length**

let car = 'Porsche';
car.length;
//7

let z = "你好嗎";
z.length;
//3

## 取出內容"濾掉空白字元" .trim()
**.trim()**

let myCar = ' KIA ';
myCar.length;
//5
myCar;
//' KIA '
myCar.trim();
//'KIA'

let email = "  [e-mail]";
email;
//'  [e-mail]'
email.trim();
//'[e-mail]'

## 變數記憶體指向，儲存值需「賦予」=

// 變數記憶體指向
let myEmail = '  [e-mail]';
console.log(myEmail); // '  [e-mail]'
myEmail.length;
console.log(myEmail.length); // 17

// 濾掉空白字元
console.log(myEmail.trim()); // '[e-mail]'
console.log(myEmail); // '  [e-mail]'

// 「賦予」新的變數以儲存處理後的結果
let updateEmail = myEmail.trim();
console.log(myEmail); // '  [e-mail]'
console.log(updateEmail); // '[e-mail]'
console.log(updateEmail.length); // 15

// 儲存值需「賦予」值
// 宣告變數
let emailLength = myEmail.length;
console.log(emailLength); // 17

## ES6新語法 - 樣版字面值(Template literals)

let myName = 'Ross';
let myAge = 28;

// let text = "您好，我是" + myName + "我今年" + myAge + "歲";
let text = `您好，我是${myName}，我今年${myAge}歲`; // ES6新語法

console.log(text);
// 您好，我是Ross，我今年28歲

## 原始型別： 字串 / 數字 / 布林 / undefined / null
### boolean 布林值 - true / false

let isVip = true;
console.log(isVip); //true
console.log(typeof isVip); //boolean

console.log(1 > 2);

### undefined 尚未被賦值

let y;
// y = 1;
console.log(y); //undefined
console.log(typeof y); //undefined

### null 有被賦予值，是告知為空值
* 原本有值，被清掉 => 空值 (釋放記憶體空間)

let cart1 = 1;
let cart2 = 1234;
cart2 = null;
console.log(cart1, cart2); // 1 null

## 型別轉換
### 字串轉成數字型別的方法 parseInt()
**parseInt()**

let mac = "1";
mac = parseInt(mac); // 轉型別
console.log(mac);
console.log(mac + 1);

### 轉型別內容非數字 => 轉型別失敗 => NaN

mac = parseInt("小明"); // 非數字轉型別
console.log(mac); // NaN
console.log(mac + 1); // NaN

* 從文字欄位取出的內容都是字串，因此如果資料需進行數字運算，就須先轉換型別！

### 數字轉成字串型別的方法 .toString()
**.toString()**

let phone = 123;
phone = phone.toString();
console.log(phone);
console.log(phone + 1);

* 字串相加的使用情境：大樂透號碼、電話區碼"07"+"23456789"

## 【觀念整理】字串相加與相乘
>當使用加法運算子（"+"）時，只要有一個字串出現，就會將其餘的部分一併轉為"字串"再做運算
所以 "1" + "1" 會等於 "11"，也就是兩個字串會合併

let a = "1" + "1";
'11'

>但相乘就不一樣，相乘是為了做"數學運算"使用的，所以它會先轉成"數字"再做相乘

let a = "1" * "1";
1