SOURCE

var p1 = new Promise((resolve, reject) => {

    setTimeout(() => {

        resolve(1)

    }, 2000);
})
p1
    .then(res => {
        console.log(res,'res1')
        // ---------两种返回失败触发catch回调-------
        // return Promise.reject('失败')
        //throw new Error('失败')
        // ---返回undefined,null,数字文本,成功promise对象then回调--
        return res
    })
    .then(res => console.log(res,'res2'))
    .catch(err => console.log(err,'err'))

<button id="b">aa</button>