// 表示狀態(tài)，便于后期維護
const PENDING = 'pending';
const RESOLVED = 'resolved';
const REJECTED = 'rejected';

function MyPromise(fn) {
    // 因代碼可能會異步執(zhí)行，用于獲取正確的this對象
    const that = this;
    // 初始狀態(tài)為pending
    that.state = PENDING;
    // 用于保存resolve 或者 reject 中傳入的值
    that.value = null;
    // 用于保存then中的回調(diào)，因為當執(zhí)行完Promise時狀態(tài)可能還是等待中，
    // 這時候應該把then中的回調(diào)保存起來用戶狀態(tài)改變時使用
    that.resolvedCallbacks = [];
    that.rejectedCallbacks = [];

    // 首先兩個函數(shù)都得判斷當前狀態(tài)是否是等待中，因為只有等待狀態(tài)可以改變狀態(tài)
    // 將當前狀態(tài)更改為對應狀態(tài)，并且將傳入的值賦值給value
    // 遍歷回調(diào)數(shù)組并執(zhí)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));
        }
    }

    // 執(zhí)行傳入的參數(shù)并且將之前的兩個函數(shù)當做參數(shù)傳進去
    // 注意： 可能執(zhí)行函數(shù)過程中會遇到錯誤，需要捕獲錯誤并執(zhí)行reject函數(shù)
    try{
        fn(resolve, reject)
    } catch (e) {
        reject(e);
    }
}


MyPromise.prototype.then = function(onFulfilled, onRejected) {
    const that = this;
    // 首先判斷兩個參數(shù)是否為函數(shù)類型，因為這兩個參數(shù)時可選參數(shù)
    // 當參數(shù)不是函數(shù)類型時，需要創(chuàng)建一個函數(shù)賦值給對應的參數(shù)，同時也實現(xiàn)了透傳
    // 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; };

    // 判斷狀態(tài)
    // 當狀態(tài)不是等待態(tài)時，就去執(zhí)行相應的函數(shù)。
    // 如果狀態(tài)是等待態(tài)的話，就往回調(diào)函數(shù)中push函數(shù)
    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)
 });