Symbol

Symbols 特征:

Symbols 对于循环和其它地方不可见(比如：for-in object.keys)
Symbols 是唯一的
"全局 Symbols"

// 创建 symbols
const mySymbol = Symbol('object name');
const myObject = { firstName: 'raja', lastName: 'rao' };
const keys = Object.keys(myObject);

解决自定义方法与 api 冲突

const includes = Symbol('will store custom includes');
Array.prototype[includes] = () => {
  console.log('inside includes func');
};
const arr = [1, 2, 3];

//测试:
console.log(arr.includes(1));
console.log(arr['includes'](1));
arr[includes](1);