Find last matching key

JavaScript, Object · Oct 22, 2020

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

Use Object.keys() to get all the properties of the object.
Use Array.prototype.reverse() to reverse the order and Array.prototype.find() to test the provided function for each key-value pair.
The callback receives three arguments - the value, the key and the object.

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

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