js promise中如何取到[[PromiseValue]]

返回的值
Promise {[[PromiseStatus]]: "resolved", 
        [[PromiseValue]]: "http://dl.stream.qqmusic.qq.com/M8000046HRBd0FvKLm…C380C8F140044403EDC0124&guid=489780640&fromtag=30"
        }

取其中的PromiseValue

var a = Promise.resolve('xx')
// Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: "xx"}
a.then(function (result) { console.log(result) })