This method is like _.find except that it returns the index of the first element predicate returns truthy for instead of the element itself.

var users = [
  { 'user': 'barney',  'active': false },
  { 'user': 'fred',    'active': false },
  { 'user': 'pebbles', 'active': true }
];

_.findIndex(users, function(o) { return o.user == 'barney'; });
// => 0

// using the `_.matches` callback shorthand
_.findIndex(users, { 'user': 'fred', 'active': false });
// => 1

// using the `_.matchesProperty` callback shorthand
_.findIndex(users, ['active', false]);
// => 0

// using the `_.property` callback shorthand
_.findIndex(users, 'active');
// => 2