如何利用JavaScript的Symbol特性扩展内建对象行为，以及它如何避免与未来语言特性的冲突？

Symbol()

Array.prototype.processingStatus = 'pending'

processingStatus

const PROCESSING_STATUS = Symbol('processing status'); // 创建一个独一无二的Symbol

Array.prototype[PROCESSING_STATUS] = 'idle'; // 为所有数组原型添加一个Symbol属性

const myArray = [1, 2, 3];
console.log(myArray[PROCESSING_STATUS]); // 输出: idle

myArray[PROCESSING_STATUS] = 'processing';
console.log(myArray[PROCESSING_STATUS]); // 输出: processing

// 关键是，这个属性不会被常规的Object.keys()或for...in循环枚举到
console.log(Object.keys(myArray)); // 输出: []
for (let key in myArray) {
    console.log(key); // 不会打印PROCESSING_STATUS
}

// 只有通过Object.getOwnPropertySymbols才能发现
console.log(Object.getOwnPropertySymbols(myArray)); // 输出: [Symbol(processing status)]

Array.prototype

Symbol()

const s1 = Symbol('my-id');
const s2 = Symbol('my-id');
console.log(s1 === s2); // false

'status'

'status'

Array.prototype

String.prototype

Array.prototype.customMap = ...

Array.prototype.customMap

Symbol.iterator

Symbol.toStringTag

[Symbol('some-internal-state')]

'__someInternalState__'

[Symbol.iterator]

iterator

Object.keys()

for...in

Object.getOwnPropertySymbols()

Map

creationTimestamp

myMap.__creationTimestamp = Date.now()

myMap

// 使用Symbol作为内部元数据键
const CREATION_TIMESTAMP = Symbol('creation timestamp');

function createTrackedMap() {
    const map = new Map();
    map[CREATION_TIMESTAMP] = new Date(); // 添加内部元数据
    return map;
}

const myMap = createTrackedMap();

// 外部代码无法轻易发现或修改CREATION_TIMESTAMP
console.log(myMap.size); // 0
console.log(Object.keys(myMap)); // []
console.log(Object.getOwnPropertySymbols(myMap)); // [Symbol(creation timestamp)]

// 只有模块内部可以通过CREATION_TIMESTAMP访问
function getMapAge(map) {
    if (map[CREATION_TIMESTAMP]) {
        return Date.now() - map[CREATION_TIMESTAMP].getTime();
    }
    return null;
}

console.log(`Map age: ${getMapAge(myMap)}ms`);

Object.getOwnPropertySymbols()

#privateField

Symbol.for()

Symbol.for(keyString)

keyString

Symbol.for()

Symbol.for()

Symbol()

JSON.stringify()

toJSON

JSON.stringify()

Symbol('my-feature-id')

Symbol()

Array.prototype

String.prototype

Object.defineProperty(Array.prototype, mySymbol, { value: '...', enumerable: false })