# call by ... 家族

## 資料型別介紹
資料型別(data type)主要分成兩大類，原始型別(primitive type)和物件(Object)

原始型別:
* Boolean
* Null
* Undefined
* Number
* BigInt
* String
* Symbol (ES6才新增的)

補充:

    BigInt->當number大於2的53次方後會失準所以要使用bigint來解決數值過大的問題
    Symbol->在object裡使用，每一個symbol都是獨一無二的，解決object容易出現重複名稱的問題

物件:
* array
* function
* map
* object
* ...

重點:
**primitive type會是call by value，而object則是call by reference**

## call by ... 介紹

### Call By Value

    1. 傳值呼叫
    2. 記憶體位置不共用
    3. 只傳值不傳址

實例

function fn (a, b) {
  var temp = 100;
  a = temp;
  b = a;
}

var x = 10;
var y = 20;

fn(x, y);
    
console.log(x, y); // 10, 20

### Call By Reference

    1. 傳參數呼叫
    2. 記憶體位置共用
    3. 傳值也傳址

實例

function fn (a) {
  var temp = 100;
  a.x = temp;
  a.y = a.x;
}

var obj = {
  x: 10,
  y: 20,
}

fn(obj);

console.log(obj); // { x: 100, y: 100 }

### Call By Sharing

    1. 傳共享物件呼叫
    2. 記憶體位置不共用
    3. 不傳值也不傳址

實例

function fn (a) {
  a = {
    b: 100,
    c: 50
  }
}

var obj = {
  x: 10,
  y: 20,
}

fn(obj);

console.log(obj); { x: 10, y: 20 }

示範: https://playcode.io/javascript

function changeAge(person) {
  person.age = 25;
  person = {
    name: 'John',
    age: 50
  }
  return person
}
var personObj1 = {
  name: 'Charles',
  age: 30
};
var personObj2 = changeAge(personObj1);
console.log(personObj1);
console.log(personObj2);

上面是第一種講法
下面有第二種

參考資料
1. https://ithelp.ithome.com.tw/m/articles/1027046
2. https://pjchender.dev/javascript/js-symbols/
3. https://israynotarray.com/javascript/20200904/1772972600/
4. https://medium.com/@mengchiang000/js%E5%9F%BA%E6%9C%AC%E8%A7%80%E5%BF%B5-call-by-value-%E9%82%84%E6%98%AFreference-%E5%8F%88%E6%88%96%E6%98%AF-sharing-22a87ca478fc
5. https://blog.techbridge.cc/2018/06/23/javascript-call-by-value-or-reference/