How do I get the currently logged-in user details like avatar, id and Name using Javascript?

There's a JavaScript object available called LITHIUM.CommunityJsonObject which has many details related to the current user and the page that they're on. For example:

LITHIUM.CommunityJsonObject = {
  "User" : {
    "login" : "AdamN"
  },
  "Community" : {
    "id" : "lithium",
    "title" : "Lithosphere Community",
    "shortTitle" : "Lithosphere"
  },
  "Activity" : {
    "Results" : [ ]
  },
  "WebTracking" : {
    "path" : "Community:Lithosphere/Board:ask a question/Message:How do I get the currently logged-in user details like avatar, id and Name using Javascript?",
    "Activities" : { }
  },
  "CoreNode" : {
    "id" : "public-qa",
    "title" : "ask a question",
    "shortTitle" : "ask a question",
    "ancestors" : [
      {
        "id" : "lithium",
        "title" : "Lithosphere Community",
        "shortTitle" : "Lithosphere",
        "nodeType" : "Community"
      }
    ],
    "nodeType" : "Board"
  }
};

If this object doesn't have all the details that you need, you could always write out your own JavaScript object populated using a combination of FreeMarker and our REST API.

I hope this helps!