findKey

Finds the first key that satisfies the provided testing function. Otherwise undefined is returned.

• Use Object.keys(obj) to get all the properties of the object, Array.prototype.find() to test each key-value pair using fn.
• The callback receives three arguments - the value, the key and the object.

const findKey = (obj, fn) => 
  Object.keys(obj).find(key => fn(obj[key], key, obj));

findKey(
  {
    barney: { age: 36, active: true },
    fred: { age: 40, active: false },
    pebbles: { age: 1, active: true }
  },
  x => x['active']
); // 'barney'