class Promise{
constructor(excutorCallBack){
this.status = 'pending'
this.value = undefined;
this.fulfilledAry = []; //管控，必须得then执行后才能执行resolveFn方法，成功要做的方法
this.rejectedAry = []; //失败要做的方法
// => 执行EXCUTOP
let resolveFn = (result) =>{
let timer = setTimeout(()=>{
if(this.status !== 'pending') return;
clearTimeout(timer)
this.status = 'fulfilled';
this.value = result;
this.fulfilledAry.forEach(item=>item(this.value))
},0)
};
let rejectFn = (reason) =>{
let timer = setTimeout(()=>{
clearTimeout(timer)
if(this.status !== 'pending') return;
this.status = 'rejected';
this.value = reason;
this.rejectedAry.forEach((item)=>item(this.value))
},0)
}
// => 执行EXCUTOP（异常捕获）
try{
excutorCallBack(resolveFn,rejectFn);
} catch (err){
// 有异常信息按照rejected状态信息处理
rejectFn(err);
}
}
then (fulfilledCallBack,rejectedCallBack) {
// fulfilledCallBack不传递的情况
typeof fulfilledCallBack !== 'function'? fulfilledCallBack = result =>{
return result
} : null;
typeof rejectedCallBack !== 'function'? rejectedCallBack = reason =>{
throw new Error(reason.message)
} : null
// 返回一个新的Promise实例
return new Promise((resolve,reject)=>{
this.fulfilledAry.push(()=>{
try {
let x = fulfilledCallBack(this.value);
if(x instanceof Promise){
x.then(resolve,reject);
return;
}
resolve(x);
}catch(err){
reject(err)
}
});
this.rejectedAry.push(()=>{
try{
let x = rejectedCallBack(this.value);
if(x instanceof Promise){
x.then(resolve,reject);
return;
}
resolve(x)
}catch(err){
reject(x)
}
})
});
}
catch(rejectedCallBack){
return this.then(null,rejectedCallBack)
}
}
module.exports = Promise;
