Promise

Promise 是一个异步操作返回的对象，用来传递异步操作的消息。

根据自己对 Promise 的理解，实现一个Promise :

Promise 有三种状态：Pending 初始态； Fulfilled 成功态； Rejected 失败态。

function Promise(executor) {
    let self = this;
    self.status = 'pending'; //等待态
    self.value = undefined;  //成功的返回值
    self.reason = undefined; //失败的原因

    function resolve(value){
        if(self.status === 'pending'){
            self.status = 'resolved';
            self.value = value;
        }
    }
    function reject(reason) {
        if(self.status === 'pending') {
            self.status = 'rejected';
            self.reason = reason;
        }
    }
    try{
        executor(resolve, reject);
    }catch(e){
        reject(e);// 捕获时发生异常，就直接失败
    }
}
//onFufiled 成功的回调
//onRejected 失败的回调
Promise.prototype.then = function (onFufiled, onRejected) {
    let self = this;
    if(self.status === 'resolved'){
        onFufiled(self.value);
    }
    if(self.status === 'rejected'){
        onRejected(self.reason);
    }
}
module.exports = Promise;