Простой reduce
в объект сделал бы это:
class QueryFilter {
}
let filter1 = new QueryFilter();
filter1.filterName = "SpamScore";
filter1.filterValue = 5;
let filter2 = new QueryFilter();
filter2.filterName = "Pages";
filter2.filterValue = 50;
let filters = [filter1, filter2];
console.log(
filters.reduce((a, { filterName, filterValue }) => (
Object.assign(a, { [filterName]: filterValue })
), {})
);