How to read a particular value from the json.stringify

ionic-v1
1

From the post i have got a res as

.subscribe( res => {
let result = JSON.stringify(res);
alert(result)
})

& as the response of alert(result) I’m getting
{"access_token": "hjhdshJHUHYI67HUjhhk98BJ_BJHBBHbnbhbhbjh_jmnhghmghgfffgxcgfcf98hujh","expires_in":"518399"}
Here I want to read the access_token value & save it in a variable how Can I do it?