js手写系列promise

1. 三种状态 pending || fullfiled(resolved) || rejected
2. 当处于pending状态的时候，可以转移到fullfiled(resolved)或者rejected状态
3. 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：

1. onFullfiled 用来接收promise成功的值
2. onRejected 用语接收promise失败的原因
3. promise.prototype.then = function(onFullfiled,onRejected){}

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)
})