简易版Promise实现

// 表示状态，便于后期维护
const PENDING = 'pending';
const RESOLVED = 'resolved';
const REJECTED = 'rejected';

function MyPromise(fn) {
    // 因代码可能会异步执行，用于获取正确的this对象
    const that = this;
    // 初始状态为pending
    that.state = PENDING;
    // 用于保存resolve 或者 reject 中传入的值
    that.value = null;
    // 用于保存then中的回调，因为当执行完Promise时状态可能还是等待中，
    // 这时候应该把then中的回调保存起来用户状态改变时使用
    that.resolvedCallbacks = [];
    that.rejectedCallbacks = [];

    // 首先两个函数都得判断当前状态是否是等待中，因为只有等待状态可以改变状态
    // 将当前状态更改为对应状态，并且将传入的值赋值给value
    // 遍历回调数组并执行
    function resolve(value) {
        if(that.state === PENDING) {
            that.state = RESOLVED;
            that.value = value;
            that.resolvedCallbacks.map(cb => cb(that.value));
        }
    }

    function reject(value) {
        if(that.state === PENDING) {
            that.state = REJECTED;
            that.value = value;
            that.rejectedCallbacks.map(cb => cb(that.value));
        }
    }

    // 执行传入的参数并且将之前的两个函数当做参数传进去
    // 注意： 可能执行函数过程中会遇到错误，需要捕获错误并执行reject函数
    try{
        fn(resolve, reject)
    } catch (e) {
        reject(e);
    }
}


MyPromise.prototype.then = function(onFulfilled, onRejected) {
    const that = this;
    // 首先判断两个参数是否为函数类型，因为这两个参数时可选参数
    // 当参数不是函数类型时，需要创建一个函数赋值给对应的参数，同时也实现了透传
    // eg: Promise.resolve(4).then().then(value => console.log(value))
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
    onRejected = typeof onRejected === 'function' ? onRejected : r => { throw r; };

    // 判断状态
    // 当状态不是等待态时，就去执行相应的函数。
    // 如果状态是等待态的话，就往回调函数中push函数
    if(that.state === PENDING) {
        that.resolvedCallbacks.push(onFulfilled);
        that.rejectedCallbacks.push(onRejected);
    }

    if(that.state === RESOLVED) {
        onFulfilled(that.value);
    }

    if(that.state === REJECTED) {
        onRejected(that.value)
    }
};

eg:

new MyPromise((resolve, reject) => {
    setTimeout(() => {
        resolve(1);
    }, 0)
 }).then(value => {
    console.log(value)
 });