pengembalian indeks array dengan fungsi di array sudut

const a = [
{ firstName: "Adam", LastName: "Howard" },
{ firstName: "Ben", LastName: "Skeet" },
{ firstName: "Joseph", LastName: "Skeet" }];

let index = a.findIndex(x => x.LastName === "Skeet");
console.log(index);
Mysterious Mantis