function Promise(executor) {
    // 在promise内部定义一个状态，当前promise的状态
    let self = this;
    self.status = 'pending';    // 默认promise的状态
    self.value = undefined;
    self.reason = undefined;

    function resolve(value) {
        if(self.status === 'pending'){
            self.value = value;
            self.status = 'resolved';
        }
    }

    function reject(reason) {
        if(self.status === 'pending'){
            self.reason = reason;
            self.status = 'rejected';
        }
    }

    executor(resolve, reject);
}

Promise.prototype.then = function (onFulfilled, onRejected) {
    let self = this;
    if(self.status === 'resolved'){
        onFulfilled(self.value);
    }
    if(self.status === 'rejected'){
        onRejected(self.reason);
    }
};

let p = new Promise(function (resolve, reject) {
    resolve('成功啦')
});

p.then((value)=>{
    console.log('very good');
    console.log(value);
});

 

promise更多资料查询 https://promisesaplus.com