.findIndex()

Returns the index of the first element in the array that satisfies the provided testing function.

Examples:

let numbers = [5, 12, 8, 130, 44];
let largeNumIndex = numbers.findIndex(num => num > 100);
console.log(largeNumIndex); // 3

let people = [{ name: 'Jack', age: 20 }, { name: 'Laura', age: 18 }];
let adultIndex = people.findIndex(person => person.age >= 18);
console.log(adultIndex); // 0