es Symbol 一种新的原始类型

一种新的原始数据类型Symbol，表示独一无二的值

基本用法

• Symbol.for() 方法和 Symbol.keyFor() 方法从全局的symbol注册表设置和取得symbol
• 在对象中查找 Symbol 属性：Object.getOwnPropertySymbols()

let s1 = Symbol('foo');
let s2 = Symbol('foo');
let s3 = Symbol.for('foo1');
let s4 = Symbol.for('foo1');


console.log(s1) //Symbol(foo)
console.log(s2) //Symbol(foo)
console.log(s1 === s2) //false
console.log(s3 === s4) //true
console.log(Symbol.keyFor(s1)) //undefined
console.log(Symbol.keyFor(s3)) //foo1

应用

1. 消除魔术字符串

const shapeType = {
    triangle: Symbol(),
    circle: Symbol(),
}

function getArea(shape) {
    let area = 0;
    switch (shape) {
        case shapeType.triangle:
            area = 1;
            break;
        case shapeType.circle:
            area = 2;
            break;
        default:
            area = 0
            break;
    }
    return area
}
console.log(getArea(shapeType.triangle))  // 1
console.log(getArea(shapeType.circle)) // 2