defineReactive 函数的第一行

function defineReactive(obj, key, val, cb) { let val = val Object.defineProperty(obj, key, { enumerable: true, configurable: true, get: () => { /* ... 依赖收集等 ... / / GitHub: https://GitHub.com/answershuto / return val }, set: newVal => { val = newVal; cb(); / 订阅者收到消息的回调 */ } }) }