'use strict';

var array = [
    { id: 1, selected: false },
    { id: 2, selected: true },
    { id: 3, selected: false },
    { id: 4, selected: true },
    { id: 5, selected: true },
    { id: 6, selected: false }
];

var filtered = array.filter(el => el.selected ? el : null)
    .map(el => el.id);

console.log('Ids of selected objects: ' + filtered);