var p1 = new Promise(function (resolve, reject) {
    setTimeout(() => reject("123"), 3000)
})

var p2 = new Promise(function (resolve, reject) {
    setTimeout(() => resolve(p1), 1000)
})

p2
    .then(result => console.log("error01",result),
        result => console.log("error02",result))
    .catch(error => console.log("error2",error))

1. Promise.resolve(promise);当resolve方法中传入一个promise对象时，则返回的状态会采用传入promise对象的返回状态，也是就是说这里p2的状态会采用p1返回的状态。

2. 因为p1中reject('123'),所以p2返回的状态自然为p1的reject('123'),即调用了.then中的第二个reject时候的回调函数即 result => console.log("error02",result)。

Returns a Promise object that is resolved with the given value. If the
value is a thenable (i.e. has a then method), the returned promise
will "follow" that thenable, adopting its eventual state;
－ https://developer.mozilla.org...

resolve的参数如果是个promise的话，它会使用那个promise的结束状态作为自己的结束状态

这里面主要有状态传递，即如果p2 resolve的是另一个Promise(p1)对象，则p2的状态由p1决定。

var p1 = new Promise(function(resolve, reject) {    
    setTimeout(() => reject("123"), 3000);            
});

var p2 = new Promise(function(resolve, reject) {
    setTimeout(() => resolve(p1), 1000);
    //0-1000,p1的状态为pending， p2的状态为pending；
    //1000s时，p2的要执行resolve，但是此时p1状态为peding，所以1000-3000会等待p1的状态改变
    //3000s时，p1变为reject，并且传递给p2，此时p2也变为reject
});

p2.then(result => console.log("error01", result),
    result => console.log("error02", result))
.catch(error => console.log("error3", error));