how to find index of array in angular 6 code example

Example: return index of array with function in array angular

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);