var dict = {
  "x": 1,
  "y": 6,
  "z": 9,
  "a": 5,
  "b": 7,
  "c": 11,
  "d": 17,
  "t": 3
};

// Create items array
var items = Object.keys(dict).map(function(key) {
  return [key, dict[key]];
});

// Sort the array based on the second element
items.sort(function(first, second) {
  return second[1] - first[1];
});

// Create a new array with only the first 5 items
console.log(items.slice(0, 5));