// JavaScript Demo: Symbol - Constructor
const symbol1 = Symbol();
const symbol2 = Symbol(42);
const symbol3 = Symbol('foo');
console.log(typeof symbol1);
// expected output: "symbol"
console.log(symbol3.toString());
// expected output: "Symbol(foo)"
console.log(Symbol('foo') === Symbol('foo'));
// expected output: false
Symbol([description])
description 可选
var sym1 = Symbol();
var sym2 = Symbol('foo');
var sym3 = Symbol('foo');
Symbol('foo') === Symbol('foo'); // false
var sym = new Symbol(); // TypeError
var sym = Symbol('foo');
typeof sym; // "symbol"
var symObj = Object(sym);
typeof symObj; // "object"