Promise/A+规范
- 三种状态 pending || fullfiled(resolved) || rejected
- 当处于pending状态的时候,可以转移到fullfiled(resolved)或者rejected状态
- 当处于fullfiled(resolved)状态或者rejected状态的时候,就不可变
- 必须有一个then异步执行方法,then接收2个参数并且必须返回一个promise:
- onFullfiled 用来接收promise成功的值
- onRejected 用语接收promise失败的原因
- promise.prototype.then = function(onFullfiled,onRejected){}
我的实现
function myPromise(constructor) { let self = this; self.status = 'pending'; self.value = undefined; self.reason = undefined; function resolve(value) { if (self.status === 'pending') { self.value = value; self.status = 'resolved' } } function reject(reason) { if (self.status === 'pending') { self.reason = reason; self.status = 'rejected' } } try { constructor(resolve, reject) } catch (e) { reject(e) } } myPromise.prototype.then = function(onFullfiled, onRejected) { let self = this; switch (self.status) { case 'resolved': onFullfiled(self.value) break; case 'rejected': onRejected(self.reason) break; default: } } var p = new myPromise((resolve, reject) => { resolve(1) }) p.then((x) => { console.log(x) })