SOURCE

function myPromise(constructor) {
    let self = this;
    self.status = "pending" //定义状态改变前的初始状态
    self.value = undefined;//定义状态为resolved的时候的状态
    self.reason = undefined;//定义状态为rejected的时候的状态
    function resolve(value) {
        //两个==="pending"，保证了状态的改变是不可逆的
        if (self.status === "pending") {
            self.value = value;
            self.status = "resolved";
        }
    }
    function reject(reason) {
        //两个==="pending"，保证了状态的改变是不可逆的
        if (self.status === "pending") {
            self.reason = reason;
            self.status = "rejected";
        }
    }
    //捕获构造异常
    try {
        constructor(resolve, reject);
    } catch (e) {
        reject(e);
    }
}
myPromise.prototype.then = function (onFullfilled, onRejected) {
    let self = this;
    switch (self.status) {
        case "resolved":
            onFullfilled(self.value);
            break;
        case "rejected":
            onRejected(self.reason);
            break;
        default:
    }
}

// test
let promise = new myPromise(function(resolve, reject){
  setTimeout(function(){
      resolve(11111);
  }, 1000);
});

promise.then(value => {
    console.log(value, 'value');
});