promise2 = promise1.then(onFulfilled, onRejected);

If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x).

 let x = onfulfilled(this.value);// x是普通值还是promise 如果是普通值 直接调用promise2的resolve// 如果是promise  那应该让x这个promise执行 x.thenresolvePromise(promise2,x,resolve,reject);

 then(onfulfilled,onrejected){ let promise2;promise2 = new Promise((resolve,reject)=>{if(this.status === 'fulfilled'){setTimeout(()=>{ try{let x = onfulfilled(this.value);// x是普通值还是promise 如果是普通值 直接调用promise2的resolve// 如果是promise  那应该让x这个promise执行 x.then
                        resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0);}if(this.status === 'rejected'){setTimeout(()=>{try{let x = onrejected(this.reason);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)}if(this.status === 'pending'){this.resolveCallbacks.push(()=>{setTimeout(()=>{try{let x = onfulfilled(this.value);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)});this.rejectCallbacks.push(()=>{setTimeout(()=>{try{let x = onrejected(this.reason);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)})}});return promise2}

let  resolvePromise = (promise2,x,resolve,reject) => {if(promise2 === x){return reject(new TypeError('循环引用'))}
}

   if(typeof x === 'function' || (typeof x === 'object' && x !== null)){}

  let then = x.then; // 如果取then方法出错 那就用错误拒绝promise2

 if(typeof then === 'function'){ // 我就认为他是一个promisethen.call(x,y=>{ // 让当前的promise执行 ，不用多次取then方法了// y 有可能还是一个promise , 继续调用resolvePromise方法，直到解析出一个常量为止，最终把常量传递下去if(called) return; // 放置此方法多次被调用called = true;resolvePromise(promise2,y,resolve,reject);},r=>{if(called) return;called = true;reject(r); // 让当前的promise变成失败态即可
                })}else{// x就是一个普通的对象 并没有then方法
                resolve(x);}

let  resolvePromise = (promise2,x,resolve,reject) => {// 判断x的类型 来处理promise2是成功还是失败// 所有的promise都遵循这个规范，不同的人写的promise可能会混用// 尽可能考虑的周全 要考虑别人promise可能出错的情况if(promise2 === x){return reject(new TypeError('循环引用'))}// 判断x是不是一个promise  ,这个x 可能不是自己的promise 所以为了安全 需要在进行校验，放置调一起用成功和失败 if(typeof x === 'function' || (typeof x === 'object' && x !== null)){// 尝试取当前x的then方法, 这个then方法可能别人定义的时候 用的Object.defineProperty
        let called;try{let then = x.then; // 如果取then方法出错 那就用错误拒绝promise2if(typeof then === 'function'){ // 我就认为他是一个promisethen.call(x,y=>{ // 让当前的promise执行 ，不用多次取then方法了// y 有可能还是一个promise , 继续调用resolvePromise方法，直到解析出一个常量为止，最终把常量传递下去if(called) return; // 放置此方法多次被调用called = true;resolvePromise(promise2,y,resolve,reject);},r=>{if(called) return;called = true;reject(r); // 让当前的promise变成失败态即可
                })}else{// x就是一个普通的对象 并没有then方法
                resolve(x);}}catch(e){if(called) return;called = true;reject(e);}}else{// x肯定一个常量
        resolve(x);}
}class Promise{constructor(executor){this.status = 'pending'; this.value; this.reason;this.resolveCallbacks = []; // 当then是pending 我希望吧成功的方法都放到数组中this.rejectCallbacks = [];let resolve = (value)=>{if(this.status == 'pending'){this.status = 'fulfilled';this.value = value;this.resolveCallbacks.forEach(fn=>fn()); // 发布
            }}let reject = (reason)=>{if(this.status === 'pending'){this.status = 'rejected';this.reason = reason;this.rejectCallbacks.forEach(fn=>fn())}}try{executor(resolve,reject);}catch(e){reject(e);}}then(onfulfilled,onrejected){ let promise2;promise2 = new Promise((resolve,reject)=>{if(this.status === 'fulfilled'){setTimeout(()=>{ try{let x = onfulfilled(this.value);// x是普通值还是promise 如果是普通值 直接调用promise2的resolve// 如果是promise  那应该让x这个promise执行 x.then
                        resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0);}if(this.status === 'rejected'){setTimeout(()=>{try{let x = onrejected(this.reason);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)}if(this.status === 'pending'){this.resolveCallbacks.push(()=>{setTimeout(()=>{try{let x = onfulfilled(this.value);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)});this.rejectCallbacks.push(()=>{setTimeout(()=>{try{let x = onrejected(this.reason);resolvePromise(promise2,x,resolve,reject);}catch(e){reject(e);}},0)})}});return promise2}
}
module.exports = Promise;