let s1 = Symbol('a');
let s2 = Symbol('a');
console.log(s1); //Symbol(a)
console.log(typeof s1); //symbol
console.log(s1 == s2); //false

var s1 = Symbol();
var s2 = Symbol();
var s3 = Symbol();
var obj = {
  [s1]: 'hi'
};
obj[s2] = 'ES6';
Object.defineProperty(obj, s3, {
  value: 'ES2015'
});
console.log(obj); //Object {Symbol(): "hi", Symbol(): "ES6", Symbol(): "ES2015"}
console.log(obj.s1); //undefined -> 所以当用Symbol作为属性名时候，不能用.运算符访问属性
console.log(obj[s1]); //hi
console.log(obj['s1']); //undefined

let s1 = Symbol('a');
let s2 = Symbol('a');
let s3 = Symbol.for('b');
let s4 = Symbol.for('b');
let name1 = Symbol.keyFor(s1);
let name3 = Symbol.keyFor(s3);
console.log(s1 == s2); //false
console.log(s1 == s3); //false
console.log(s2 == s3); //false
console.log(s3 == s4); //true
console.log(name1); //undefined
console.log(name3); //b