# Symbol type

## Symbols

symbol 代表唯一的值。

```javascript
// id is a new symbol
let id = Symbol();

// id is a symbol with the description "id" for debug purpose
let id = Symbol("id");

// same description but different value
let id1 = Symbol("id");
let id2 = Symbol("id");

alert(id1 == id2); // false
```

### Symbols don’t auto-convert to a string

 ​JavaScript 大多數的值可以自動轉換成 string 但 symbol 無法自動轉換。

```javascript
let id = Symbol("id");
alert(id); // TypeError: Cannot convert a Symbol value to a string

// convert to string
let id = Symbol("id");
alert(id.toString()); // Symbol(id), now it works

// only get description
let id = Symbol("id");
alert(id.description); // id
```

## “Hidden” properties

symbol 可以為物件創造隱藏的屬性，程式碼的其他部份不容易取得或改寫。

```javascript
let user = { name: "John" };
let id = Symbol("id");

user[id] = "ID Value";
alert( user[id] ); // we can access the data using the symbol as the key

// useing string would be a conflict 
let user = { name: "John" };

// our script uses "id" property
user.id = "ID Value";

// ...if later another script the uses "id" for its purposes...

user.id = "Their id value"
// boom! overwritten! it did not mean to harm the colleague, but did it!
```

### Symbols in a literal

使用 symbol 作為 key 值，要加上 \[ ] 因為它是一個變數。 

```javascript
let id = Symbol("id");

let user = {
  name: "John",
  [id]: 123 // not just "id: 123"
};
```

### Symbols are skipped by for…in

symbol 不算在物件迴圈內 `for ( in )`。

```javascript
let id = Symbol("id");
let user = {
  name: "John",
  age: 30,
  [id]: 123
};

for (let key in user) alert(key); // name, age (no symbols)

// the direct access by the symbol works
alert( "Direct: " + user[id] );

// Object.assign() 會複製 symbol
let id = Symbol("id");
let user = {
  [id]: 123
};

let clone = Object.assign({}, user);

alert( clone[id] ); // 123

// other type of key convert to string
let obj = {
  0: "test" // same as "0": "test"
};

// both alerts access the same property (the number 0 is converted to string "0")
alert( obj["0"] ); // test
alert( obj[0] ); // test (same property)
```

## Global symbols

symbol 通常是唯一的值，但如果想要同樣的 symbol 可以用 `Symbol.for(key)` 設定全域 symbol registry。

```javascript
// read from the global registry
let id = Symbol.for("id"); // if the symbol did not exist, it is created

// read it again
let idAgain = Symbol.for("id");

// the same symbol
alert( id === idAgain ); // true
```

### Symbol.keyFor

`Symbol.keyFor(sym)` 返回全域 symbol 的 key名稱。

```javascript
let sym = Symbol.for("name");
let sym2 = Symbol.for("id");

// get name from symbol
alert( Symbol.keyFor(sym) ); // name
alert( Symbol.keyFor(sym2) ); // id

// 只能使用在全域的 symbol
alert( Symbol.keyFor(Symbol.for("name")) ); // name, global symbol
alert( Symbol.keyFor(Symbol("name2")) ); // undefined, the argument isn't a global symbol
```

## System symbols

* `Symbol.hasInstance`
* `Symbol.isConcatSpreadable`
* `Symbol.iterator`
* `Symbol.toPrimitive`
* …and so on.


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