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<title>理解Symbol: Object.getOwnPropertySymbols()</title>
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/*
symbol：唯一性、不能遍历；
保证每个属性的名字都是独一无二的，从根本上防止属性名的冲突；
*/
//例子1
const school = Symbol();
const student = Symbol();
console.log(typeof school);//symbol
console.log(typeof student);//symbol
console.log(school === student);//false
//例子2：注意两个nian
const classRoom = {
lily: { grade: 60, sex: 'Woman' },
nina: { grade: 67, sex: 'Woman' },
nina: { grade: 80, sex: 'Woman' },
}
//第一个nina没有打印出来
console.log(classRoom);
//现在你想把两个nian都打印出来
const classRoom1 = {
[Symbol('lily')]: { grade: 60, sex: 'Woman' },
[Symbol('nina')]: { grade: 67, sex: 'Woman' },
[Symbol('nina')]: { grade: 80, sex: 'Woman' },
}
console.log(classRoom1);
/*
现在你想获取classRoom1对象里面的属性值
小知识点：
Object.keys() 方法会返回一个由一个给定对象的自身可枚举属性组成的数组，
数组中属性名的排列顺序和使用 for...in循环遍历该对象时返回的顺序一致 。
Object.getOwnPropertyNames()方法返回一个由指定对象的所有自身属性的属性名
（包括不可枚举属性但不包括Symbol值作为名称的属性）组成的数组。
*/
for (let key in classRoom1) {
console.log(key);
}
console.log(Object.keys(classRoom1));//[]
console.log(Object.getOwnPropertyNames(classRoom1));//[]
let sym = Object.getOwnPropertySymbols(classRoom1);
console.log(sym);//(3) [Symbol(lily), Symbol(nina), Symbol(nina)]
let sym1 = Object.getOwnPropertySymbols(classRoom1).map(s => classRoom1[s]);
console.log(sym1);//[{…}, {…}, {…}]
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