Get Array Index using Few Object Properties

var list = [ { id : 0, firstName : 'Chairul', lastName : 'Umam' }, { id : 1, firstName : 'Steven', lastName : 'Gerrard' } ];

var i = list.findIndex(function(x) {
    return JSON.stringify({ id : x.id, lastName : x.lastName }) == JSON.stringify({ id : 1, lastName : 'Gerrard' });
});

console.log(i);