16. 手写一个Promise

参考答案：

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);
    }
}

//同时，需要在 myPromise的原型上定义链式调用的 then方法：
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:
    }
};

//测试一下：
var p = new myPromise(function(resolve, reject) {
    resolve(1);
});
p.then(function(x) {
    console.log(x);
});