Symbol

基本数据类型：
undefined
nul
boolean
string
number
object

ES6 引入了一种新的原始数据类型 Symbol，表示独一无二的值。

{
    // 声明
    let a1 = Symbol();
    let a2 = Symbol();
    console.log(a1 === a2); // false
    let a3 = Symbol.for("a3");
    let a4 = Symbol.for("a3");
    console.log(a3 === a4); // true
}
{
    const a5 = Symbol("a5");
    console.log(Symbol.keyFor(a5)); // undefined
  const a6 = Symbol.for("a5");
  console.log(Symbol.keyFor(a6)) // a5
}
{
    let a1 = Symbol.for("abc");
    let obj = {
        [a1]: "123",
        abc: 345,
        c: 456,
    };
    console.log("obj", obj); // obj { abc: 345, c: 456, [Symbol(abc)]: '123' }
    for (let [key, value] of Object.entries(obj)) {
        console.log("let of", key, value);
        // let of abc 345
        // let of c 456
    }
    Object.getOwnPropertySymbols(obj).forEach(function (item) {
        console.log(obj[item]); // 123
    });
    Reflect.ownKeys(obj).forEach(function (item) {
        console.log("ownkeys", item, obj[item]);
        // ownkeys abc 345
        // ownkeys c 456
    });
}