keysWhere.js

function keysWhere (obj, predicate) {
  return Object.keys(obj).reduce(function (acc, key) {
    if (predicate(key, obj[key])) {
      acc.push(key);
    }
    
    return acc;
  }, []);
}

keysWhere({a: 1, b: 2, c: 1}, function (key, value) { return value === 1; }); // ['a', 'c']