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<meta charset="UTF-8" />
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<title>promise A+ 规范</title>
</head>
<body>
<script>
let concept = `1：每个异步操作都看作一个任务，任务都有两个阶段和三个状态，分别是 settled 已决阶段 和 unsettled 未决阶段，分别是 pending 等待态
fulfilled 成功态 rejected 失败态
2：状态之间不可逆转，只能由 pending 态转 fulfilled态 或者 pending转 rejected态；
3：任务由未决状态转已决状态，其中由pending态转fulfilled态的动作称之为 resolve，由pending态转rejected态的动作称之为 rejected态；
4：任务都提供一个then方法，这个方法有两个回调参数，分别是 onFulfilled onRejected`;
const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
class myPromise {
constructor(executor) {
this.status = PENDING;
this.data = null;
this.reason = null;
this.dispatchers = [];
try {
executor(this.__resolve.bind(this), this.__reject.bind(this));
} catch (err) {
this.__reject(err);
}
}
__resolve(data) {
if (this.status !== PENDING) return;
this.status = FULFILLED;
this.data = data;
this.__dispatchers();
}
__reject(reason) {
if (this.status !== PENDING) return;
this.status = REJECTED;
this.reason = reason;
this.__dispatchers();
}
__dispatchers() {
if (this.status === PENDING) return;
for (const dispatcherConf of this.dispatchers) {
this.__dispatcher(dispatcherConf);
}
}
__dispatcher({ status, dispatcher, resolve, reject }) {
if (this.status !== status) return;
if (typeof dispatcher !== 'function') {
this.status === FULFILLED ? resolve(this.data) : reject(this.data);
return;
}
try {
const result = dispatcher(this.status === FULFILLED ? this.data : this.reason);
resolve(result);
} catch (err) {
reject(err);
}
}
then(onFulfilled, onRejected) {
return new myPromise((resolve, reject) => {
this.dispatchers.push(
{
status: FULFILLED,
dispatcher: onFulfilled,
resolve,
reject
},
{
status: REJECTED,
dispatcher: onRejected,
resolve,
reject
}
);
this.__dispatchers();
});
}
catch(err) {
this.then(null, err);
}
}
const myPro = new myPromise((resolve, reject) => {
setTimeout(function () {
resolve(1);
}, 2000);
});
myPro.then(r => {
console.log(r);
});
</script>
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