//创建一个 Promise 的类
class Promise {
constructor(executer) {//构造函数 constructor 里面是个执行器
this.status = 'pending';//默认的状态 pending
this.value = undefined//成功的值默认 undefined
this.reason = undefined//失败的值默认 undefined
//状态只有在 pending 时候才能改变
let resolveFn = value => {
//判断只有等待时才能 resolve 成功
if (this.status == pending) {
this.status = 'resolve';
this.value = value;
}
}
//判断只有等待时才能 reject 失败
let rejectFn = reason => {
if (this.status == pending) {
this.status = 'reject';
this.reason = reason;
}
}
try {
//把 resolve 和 reject 两个函数传给执行器 executer
executer(resolve, reject);
} catch (e) {
reject(e);//失败的话进 catch
}
}
then(onFufilled, onReject) {
//如果状态成功调用 onFufilled
if (this.status = 'resolve') {
onFufilled(this.value);
}
//如果状态失败调用 onReject
if (this.status = 'reject') {
onReject(this.reason);
}
}
}
