Use the indexOf method with the object you are looking for.
let index = aaa.indexOf(bbb);
So full script would be...
const aaa = [
{id: 10, val: "xx"},
{id: 27, val: "tr"},
{id: 13, val: "ut"}
]
const bbb = aaa.find(obj => (obj.id === 27) );
let index = aaa.indexOf(bbb);
console.log(index);