findKey.js

const assertEqual = function(actual, expected) {
  if (actual === expected) {
    console.log(`✅✅✅ Assertion Passed: ${actual} === ${expected}`);
  } else {
    console.log(`🛑🛑🛑 Assertion Failed: ${actual} !== ${expected}`);
  }
};

const findkey = function(object, callback) {
  for (let key in object) {
    let value = object[key];
    if (callback(value)) {
      return key;
    }
  }
};

assertEqual(findkey({
  "Blue Hill": { stars: 1 },
  "Akaleri":   { stars: 3 },
  "noma":      { stars: 2 },
  "elBulli":   { stars: 3 },
  "Ora":       { stars: 2 },
  "Akelarre":  { stars: 3 }
}, x => x.stars === 2), "noma");

assertEqual(findkey({
  "Blue Hill": { stars: 1 },
  "Akaleri":   { stars: 3 },
  "noma":      { stars: 2 },
  "elBulli":   { stars: 3 },
  "Ora":       { stars: 2 },
  "Akelarre":  { stars: 3 }
}, x => x.stars === 4), undefined);

assertEqual(findkey({
  "Blue Hill": { stars: 1 },
  "Akaleri":   { stars: 3 },
  "noma":      { stars: 2 },
  "elBulli":   { stars: 3 },
  "Ora":       { stars: 2 },
  "Akelarre":  { stars: 3 }
}, x => x.stars === 3), "Akaleri");