JavaScript function not returning variable

I'm trying to get the ID of a signed in user in Google Chrome using a separate function, however the gaiaId variable always seems to be 'undefined' even though userId contains the ID (I checked).

How do I retu the id from the function that's called?

function getGaia(){
   chrome.identity.getProfileUserInfo(function(userInfo) {
      var userId = userInfo.id;
      alert(userId);
      retu userId;
   });
}

var gaiaId = getGaia();
alert(gaiaId);