// 实现一个简易版的promise

//先创建三个常量用于表示状态
const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'


function MyPromise(fn) {
    const that = this
    that.state = PENDING //一开始Promise转态应该是pedding
    that.value = null //value用于保存resolved或者rejected传入的值
    //以下两个属性用于保存then中的回调，因为当执行完Promise时转态可能还在pending状态中，这时候应该把then中的回调保存起来用于状态改变时使用
    that.resolvedCallbacks = [] 
    that.rejectedCallbacks = []

    function resolved(value) {
        if(that.state === PENDING){
            that.state = RESOLVED
            that.value = value
            that.resolvedCallbacks.map(cb => cb(that.value))
        }
    }
    function rejected(value) {
        if(that.state === REJECTED){
            that.state = REJECTED
            that.value = value
            that.rejectedCallbacks.map(cb => cb(that.value))
        }
    }
    try {
        fn(resolved,rejected)
    } catch (e) {
        rejected(e)
    }
}

//实现then函数
MyPromise.prototype.then = function(onFullfied,onRejected) {
    const that = this
    onFullfied = typeof onFullfied === 'function'?onFullfied:v=>v
    onRejected = typeof onRejected === 'function'?onRejected:r=>{
        throw r
    }
    if(that.state === PENDING){
        that.resolvedCallbacks.push(onFullfied)
        that.rejectedCallbacks.push(onRejected)
    }
    if(that.state === RESOLVED){
        onFullfied(that.value)
    }
    if(that.state === REJECTED){
        onRejected(that.value)
    }
}

new MyPromise((resolved,rejected)=>{
    setTimeout(()=>{
        resolved(1)
    },0)
}).then(value=>{
    console.log(value); // 1
    
})