Firebase: pass value to variable in node.js

I'm trying to retrieve a value from a Firebase database and put the value in the variable "current_temp". I'm trying to get this done by using the code below (got some parts from the firebase docs), but It's not updating the variable "current_temp" outside the function. What am I doing wrong?

var TempRef = firebase.database().ref().child("temperature");
var current_temp=""

TempRef.on('value', function (datasnapshot){
  current_temp = datasnapshot.val();
});

via Luke Janissen