手写原理 | Promise

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';

function myPromise(fn) {
    // 保存初始的状态
    const self = this;
    
    // 定义初始的状态为pending
    this.state = PENDING;
    
    // 用于保存resolve reject传入的值
    this.value = [];
    // 用于保存resolve reject的回调函数
    this.resolveCallbacks = [];
    this.rejectCallbacks = [];
    
    // 状态转变为resolve的操作
    function resolve(value) {
        // 判断传入元素是否为 Promise 值，如果是，则状态改变必须等待前一个状态改变后再进行改变
        if (value instanceof myPromise) {
            return value.then(resolve, reject);
        }
        
        // 保证代码的执行顺序为本轮事件循环的末尾
        setTimeout(() => {
            if (this.state === PENDING) {
                this.state = FULFILLED;
                this.value = value;
                resolveCallbacks.forEach(callback=>callback(value));
            }
        }, 0);
    }
    
    // 状态转变为reject的操作
    function reject(reason) {
         setTimeout(() => {
            if (this.state === PENDING) {
                this.state = REJECTED;
                this.value = reason;
                resolveCallbacks.forEach(callback=>callback(reason));
            }
        }, 0);
    }
    
    // 把两个函数传入fn中
    try{
        fn(resolve, reject);
    } catch (err) {
        reject(err);
    }
}