详解JavaScript中原始数据类型Symbol的使用

let s = Symbol()
console.log(typeof s);  // symbol

let s2 = Symbol('desc')
let s3 = Symbol('desc2')
console.log(s2);  // Symbol(desc)
console.log(s3);  // Symbol(desc2)

let obj = {
       name : '东方不败'
  }
let s4 = Symbol(JSON.stringify(obj)) 
console.log(s4); // Symbol({"name":"东方不败"})

let s5 = Symbol(obj) 
console.log(s5);// Symbol([object Object])

let sy = Symbol()
let sy2 = Symbol()
console.log(sy === s2); // false

let sy3 = Symbol('a')
let sy4 = Symbol('a')
console.log(sy3 === sy4); // false

let a = Symbol('hello')
console.log(a + 'world');  // 报错 Cannot convert a Symbol value to a string

let a2 = Symbol('hello')
console.log(String(a2)); // Symbol(hello)

let a2 = Symbol('hello')
console.log(a2.description); // hello

let a2 = Symbol('hello')
console.log(Boolean(a2));  // true
console.log(Boolean(!a2)); // false

let n = Symbol()
// 方式一
let obj2 = {
      [n] : '东方不败'
   }
console.log(obj2);  // {Symbol(): '东方不败'}
console.log(obj2[n]);  // 东方不败

// 方式二
obj2[n] = '东方求败'
console.log(obj2[n]);  // 东方求败

// 方式三
let obj3 = {}
let back = Object.defineProperty(obj3,n,{value : '艺术概论'})
console.log(obj3[n]); // 艺术概论

let n2 = Symbol()
let obj4 = {}
console.log(obj4.n2 = '中国工艺美术史');  // 中国工艺美术史
console.log(obj4[n2]);  // undefined
console.log(obj4);  // {n2: '中国工艺美术史'}

let m = Symbol('a')
let f = {
    [m]:'东方不败',
    name:'西方求败',
    name2: '光合作用'
}

// 西方求败 、 光合作用
for(k in f){
   console.log(f[k]);
}

console.log(Object.keys(f)); // ['name','name2']
console.log(JSON.stringify(f));  // {"name":"西方求败","name2":"光合作用"}

console.log(Reflect.ownKeys(f)); //  ['name', 'name2', Symbol(a)] 

console.log(Object.getOwnPropertySymbols(f)); // [Symbol(a)]

let r = Symbol.for('a')
let r2 = Symbol.for('a')
console.log(r === r2);  // true

let r3 = Symbol.for('b')
let r4 = Symbol('c')
console.log(Symbol.keyFor(r3));  // b
console.log(Symbol.keyFor(r4));  // undefined

class myClass {
     static [Symbol.hasInstance](val){
            return typeof val === 'number'
     }
     // static [Symbol.hasInstance](val){
     //     return typeof val === 'boolean'
     // }
 }
console.log(100 instanceof myClass); // true
console.log('100' instanceof myClass); // false

let arr1 = [1,2]
let arr2 = [3,4]
console.log(arr1[Symbol.isConcatSpreadable]);  // undefined
console.log(arr1.concat(arr2));  // [1,2,3,4]

console.log(arr1[Symbol.isConcatSpreadable] = false)
console.log(arr1.concat(arr2)); // [[1,2],3,4]

// 这里继承了Array的原型
class MyArray extends Array { }
let a = new MyArray(1,2,3)
let b = a.map(el => el + 1)
console.log(b);  // constructor : class MyArray

class MyArray extends Array {
      static get [Symbol.species]() { return Array }
}

let a = new MyArray(1,2,3)
let b = a.map(el => el + 1)
let c = a.filter(el => el == 2)

console.log(a,b,c);  // 1,2,3    2,3,4   2
console.log(b instanceof MyArray); // false
console.log(b);  // constructor : class MyArray

class MyMatch {
      [Symbol.match](val){
         return 'hello world'.indexOf(val)
      }
}

// match字符串方法，可以在字符串内检索指定的值并返回
console.log('e'.match(new MyMatch()));  // 1