<script>
function Promise(func) {
const that = this;
//给Promise对象设置属性status，用于管理状态，初始值为Pending
that.status = "Pending";
//给promise对象指定一个用于存储结果数据的属性
that.data = undefined;
//每个元素结构:{onResolved(){}, onRejected(){}}
that.callbacks = [];
function resolve(value) {
//如果当前状态不是Pending，直接结束
if (that.status !== 'pending') {
return;
}
//将状态改为resolved
that.status = 'resolved';
//保存value数据
that.data = value;
//如果有待执行callback函数，立即异步执行回调
if (that.callbacks.length > 0) {
//放入队列中执行所有成功的回调
setTimeout(() => {
that.callbacks.forEach(callbacksObj => {
callbacksObj.onResolved(value);
});
});
}
}
function reject(reason) {
//如果当前状态不是pending，直接结束
if (that.status !== 'pending') {
return;
}
//将状态改为rejected
that.status = 'rejected';
//保存reason数据
that.data = reason;
//如果有待执行callback函数，立即异步执行回调
if (that.callbacks.length > 0) {
//放入队列中执行所有成功的回调
setTimeout(() => {
that.callbacks.forEach(callbacksObj => {
callbacksObj.onRejected(reason);
});
});
}
}
try {
func(resolve, reject);
} catch (error) {
//如果执行器抛出异常，promise对象变为rejected状态
reject(error);
}
}
Promise.prototype.then = function (onResolved, onRejected) {
//向后传递成功的value
onResolved = typeof onResolved === 'function' ? onResolved : value => value;
//指定默认的失败的回调（实现错误/异常穿透的关键点），向后传递失败的reason
onRejected =
typeof onRejected === 'function' ?
onRejected :
reason => {
throw reason;
};
const that = this;
//返回一个新的promise对象
return new Promise((resolve, reject) => {
/*指定回调函数处理，根据执行结果，改变return的promise的状态*/
function handle(callback) {
/*1. 如果抛出异常，retrun的promise就会失败，reason就是error
2. 如果回调函数返回不是promise，return的promise就会成功，value就是返回的值
3. 如果回调函数返回的是promise，return的promise结果就是这个promise的结果*/
try {
const result = callback(that.data);
if (result instanceof Promise) {
result.then(
//当result成功时，让return的promise也成功
//value => resolve(value),
//当result失败时，让return的promise也失败
//reason => reject(reason)
result.then(resolve, reject)
);
} else {
resolve(result);
}
} catch (error) {
reject(error);
}
}
if (that.status === 'pending') {
//假设当前状态还是PENDING状态，将回调函数保存起来
that.callbacks.push({
onResolved(value) {
handle(onResolved);
},
onRejected(reason) {
handle(onRejected);
}
});
} else if (that.status === 'resolved') {
//如果当前是resolved状态，异步执行onResolve并改变return的promise状态
setTimeout(() => {
handle(onResolved);
});
} else {
//'rejected'
//如果当前是rejected状态，异步执行onRejected并改变return的promise状态
setTimeout(() => {
handle(onRejected);
});
}
});
};
Promise.prototype.catch = function (onRejected) {
return this.then(undefined, onRejected);
};
</script>
