2.10JavaScript引用类型之期约(Promise)

1.Thenable接口

//then实例方法签名
then(onFulfilled, onRejected)

2.Promise类

Promise 类实现了 Thenable 接口。

//静态方法
Promise.all()
Promise.allSettled()
Promise.any()
Promise.reject()
Promise.resolve()
Promise.race()
Promise.try()
Promise.withResolvers()

//实例方法
Promise.prototype.then()
Promise.prototype.catch()
Promise.prototype.finally()

3.创建Promise对象

const p = new Promise(executor);

//executor回调函数签名
(resolve, reject) => {}

//resolve和reject回调函数签名
//value和reason可以为任何JavaScript数据类型
resolve(value)
reject(reason)

4.Promise.prototype.then()实例方法

//then实例方法签名
then(onFulfilled)
then(onFulfilled, onRejected)

//onFulfilled和onRejected回调函数签名
onFulfilled(value)
onRejected(reason)

5.Promise对象状态

待定(pending)状态是 Promise 对象的初始状态。在待定状态下，Promise 对象可以落定(settled)为兑现(fulfilled)状态或拒绝(rejected)状态。

Promise 对象的状态不一定非要脱离待定状态从而落定为兑现或拒绝状态。但是如果 Promise 对象的状态一旦落定为兑现或拒绝状态，Promise 对象的状态就不再改变，不可从兑现或拒绝状态逆转为待定状态。

let p = new Promise((resolve, reject) => setTimeout(resolve, 1000));
setTimeout(console.log, 0, p);

//输出
Promise { <state>: "pending" }

let p = new Promise((resolve, reject) => resolve("success"));
setTimeout(console.log, 0, p);

//输出
Promise { <state>: "fulfilled", <value>: "success" }

let p = new Promise((resolve, reject) => reject("fail"));
setTimeout(console.log, 0, p);

//输出
Promise { <state>: "rejected", <reason>: "fail" }
//报错
Uncaught (in promise) fail