ES6 class实现一个简易Promise

根据promise A+规范，实现一个简易promise需要考虑如下的问题：

1.状态：pending，resolve（fulfilled），rejected

2.then方法注册回调（onFulfilled,onRejected），参数可选，参数类型是函数

3.对微任务，宏任务的调用处理（setTimeout,then回调）

4.最终值

//index.html


  
    
    
  
  
    

  
  
  

//promise.js
(function(w){
    //不可变的常量状态
    const PENDING = 'pending'
    const RESOLVED = 'resolved'
    const REJECTED = 'rejected'
    class MyPromise{
        constructor(fn){
            this.value = null;
            this.state = PENDING;
            this.resolvedCallbacks = [];  //异步任务队列
            this.rejectedCallbacks = [];
            try {
                fn(this.resolve,this.reject);
            } catch(e){
                this.reject(e);
            }
        }
        then = (onFulfilled,onRejected) => {
            onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
            onRejected =
                typeof onRejected === 'function'
                ? onRejected
                : r => {
                    throw r
                };
            if (this.state === PENDING) {
                this.resolvedCallbacks.push(onFulfilled);
                this.rejectedCallbacks.push(onRejected);
            }
            if (this.state === RESOLVED) {
                onFulfilled(this.value);
            }
            if (this.state === REJECTED) {
                onRejected(this.value);
            }
        }     
        //resolve的参数是promise对象的话，该promise对象的最终状态由then方法执行决议，否则状态直接返回fulfilled
        //所有要判断value的类型
        resolve = (value) => {
            if(value instanceof MyPromise){
                return value.then(this.resolve,this.reject);
            }
            //通过setTimeout控制执行顺序
            setTimeout(() => {
                if(this.state === PENDING){
                    this.state = RESOLVED;
                    this.value = value;
                    this.resolvedCallbacks.map(cb => cb(this.value));
                }
            },0);
        }
        reject = (value) => {
            setTimeout(() => {
                if(this.state === PENDING){
                    this.state = REJECTED;
                    this.value = value;
                    this.rejectedCallbacks.map(cb => cb(this.value));
                }
            },0);
        }     
    }
    window.MyPromise = window.MyPromise || MyPromise;
})(window);

目前暂时实现这样，后续继续更新。。。

参考：

http://www.ituring.com.cn/article/66566

https://juejin.im/book/5bdc715fe51d454e755f75ef