Promise的初级理解

var promise1 = new Promise(function(resolve, reject) {
    let a ={ab:'hhhhh'};
    let b;
    resolve(b.a);//b.a会发生错误
});

promise1.then(

function(value) {//当promise状态为resolve时执行
  console.log(value);
}, 

function(err){//当promise状态为rejected时执行

console.log(err+"  hhjhj")

});
结果为 "TypeError: Cannot read property 'a' of undefined hhjhj"说明打印的结果是错误处理函数执行的