_.findIndex

Returns the index of the first element in the array that satisfies the provided testing function. Otherwise -1 is returned.

// Underscore/Lodash
var users = [
  { 'user': 'barney',  'age': 36, 'active': true },
  { 'user': 'fred',    'age': 40, 'active': false },
  { 'user': 'pebbles', 'age': 1,  'active': true }
]
 
var index = _.findIndex(users, function (o) { return o.age >= 40; })
console.log(index)
// output: 1
 
// Native
var users = [
  { 'user': 'barney',  'age': 36, 'active': true },
  { 'user': 'fred',    'age': 40, 'active': false },
  { 'user': 'pebbles', 'age': 1,  'active': true }
]
 
var index = users.findIndex(function (o) { return o.age >= 40; })
console.log(index)
// output: 1

Browser Support for Array.prototype.findIndex()

45.0 ✔	✔	25.0 ✔	✖	32.0 ✔	7.1 ✔