If you have an array of values, and you want to know what index a certain value has, you can use this:

const fruits = ["apple","banana","orange","grapes"];
console.log(fruits.indexOf("orange")); //returns 2, as it is the 3rd item in array but array index starts at 0

If you want to search with a predicate function, you can use findIndex which will return the index of the first item which the predicate returns true:

const people = [
  { nationality: 'British', name: 'Fred', },
  { nationality: 'French', name: 'Aimée',}
];

console.log(people.findIndex(person => person.nationality === 'French'));
//  { nationality: 'French', name: 'Aimée', }