dynamodb get item call using aws sdk

I am using the following code for Node.js to get a item from dynamodb

var params = {
    AttributesToGet: [
    "password"
  ],
  TableName : 'foo',
  Key : { 
   "username" : {
    "S" : "bar"
  },
 }
}

db.getItem(params, function(err, data) {
 if (err) {
   console.log(err); // an error occurred
   } 
 else {
   console.log(data); // successful response
   res.send(data);
   }
 return next();
});

but there are cases where I do not know the key value for fetching the items. I want to know whether I can fetch items based on attribute values. something like the following :

var params = {
 AttributesToGet: [
   "password"
],
TableName : 'foo',
Attribute : { 
  "userlocation" : {
    "S" : "EUROPE"
  },
 }
}