话不多说直接上代码

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

    // 存储回调函数
    self.onFulfilledCallbacks = [];
    self.onRejectedCallbacks = [];

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

            self.onFulfilledCallbacks.forEach(function(fulfilledCallback) {
                fulfilledCallback();
            });
        }
    }

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

            self.onRejectedCallbacks.forEach(function(rejectedCallback) {
                rejectedCallback();
            });
        }
    }

    // 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;
        case 'pending':
            self.onFulfilledCallbacks.push(() => {
                onFullfilled(self.value);
            });
            self.onRejectedCallbacks.push(() => {
                onRejected(self.reason);
            });
            break;
        default:
    }
}

new myPromise((resolve) => {
    console.log(1);
    setTimeout(() => {
        console.log(2);
        resolve(3)
    }, 2000);
}).then(value => console.log(value))

new myPromise((resolve) => {
    console.log(4);
    resolve(5)
}).then(value => console.log(value))