ES 6個人メモ——Symbol

ES5            ，           
  ，              ，
              Cmixin   〉，
                   。 

ES6             Symbol，        
        
1.Number
2.String
3.Undefined
4.Null
5.Boolean
6.Object

Symbol    Symbol    ，
       Symbol  ，       ，
              

let s1 = Symbol();
console.log(typeof s1);

PS:Symbol      new  ，
	     Symbol         ，    
	   Symbol    ，        
	   ，               

// Symbol    ，    
let s2 = Symbol('foo');
let s3 = Symbol('bar');

console.log(s2,s3);

// eg1
let mySymbol = Symbol();
let mySymbol2 = Symbol();
let mySymbol3 = Symbol();

//      
let a = {
     };
a = {
     
	[mySymbol2] : 'world'
}

//      
a[mySymbol] = 'hello';

//      	   
Object.defineProperty(a,mySymbol3,{
     value : 'wjy'});

console.log(a)

/*
log.levels = {
	DEBUG : Symbol('debug'),
	INFO : Symbol('info'),
	WARN : Symbol('warn')
};
log(log.levels.DEBUG,'debug message');
*/

//   ：       
function getArea(shape,options){
     
	let area = 0;

	switch(shape){
     
		case 'Triangle': 	//     
			area = .5 * options.width * options.height;
			break;
		default:
			area = "false";
			break;
	}
	return area;
}

console.log(getArea('Triangle',{
     width : 100,height : 200}));

// change
function getAreaChange(shape,options){
     
	let area = 0;
	let {
      PI } = Math;
	switch(shape){
     
		case shapeType.triangle:
			area = .5 * options.width * options.height;
			break;
		case shapeType.circle:
			area = PI * options.radius ** 2;
			break;
		default:
			area = "false";
			break;
	}
	return area;
}

let shapeType = {
     
	// triangle : 'Triangle'
	/*
		      ，shapeType.triangle        
		         shapeType        
	 */
	triangle : Symbol(),
	circle : Symbol()
};

console.log(getAreaChange(shapeType.triangle,{
      width : 200, height : 3}));
console.log(getAreaChange(shapeType.circle,{
      radius : 2 }));

let {
       getOwnPropertyNames , getOwnPropertySymbols , defineProperty } = Object;
let objectSymbols1 = getOwnPropertySymbols(a);
console.log(objectSymbols1)

// another example
let obj3 = {
     };
let foo3 = Symbol("foo");
defineProperty(obj3,foo3,{
     
	value : "foobar",
});

for(let i in obj3){
     
	console.log(i);		//   
}

  getOwnPropertyNames     Symbol   
    getOwnPropertySymbols  

console.log(getOwnPropertyNames(obj3));
console.log(getOwnPropertySymbols(obj3));

    Reflect.ownKeys()           
       Symbol  

const {
      ownKeys } = Reflect;

let obj4 = {
     
	[Symbol('my_key')]: 1,
	enum : 2,
	nonEnum : 3
};

console.log(ownKeys(obj4));

 Symbol                   。
                               。

let size = Symbol('size');

class Collection{
     
	constructor(){
     
		this[size] = 0;
	}

	add(item){
     
		this[this[size]] = item;
		this[size]++;
	}

	static sizeOf(instance){
     
		return instance[size];
	}
}

let x = new Collection();
console.log(Collection.sizeOf(x));
x.add('foo');
console.log(Collection.sizeOf(x))

const {
      keys , getOwnPropertySymbols , getOwnPropertyNames } = Object;
console.log(keys(x),getOwnPropertyNames(x),getOwnPropertySymbols(x));