TECHF5VE暑假活动之Promise实现
TECHF5VE暑假活动之Promise实现
Promise对象用于表示一个异步操作的最终完成(或失败)及其结果值。
1、抽象表达：它是JS中进行异步编程的新的解决方案
2、具体表达：（1）从语法上说，Promise是一个构造函数
（2）从功能上说，promise对象用来封装一个异步操作并可以获取其结果
有三种状态：pending、resolved、rejected
状态的改变只有两种：pending=>resolved
pending=>rejected
一个promise对象的状态只能改变一次，无论成功失败，都会有一个结果数据，成功的结果数据一般称为value，失败的一般称为reason
1、指定回调函数的方法更灵活
旧的方法必须在启动异步任务之前指定回调函数
promise：启动异步任务=>返回promise对象=>给promise对象绑定回调函数（可在异步任务结束后指定）
2、支持链式调用，可以解决回调地狱问题
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