Взгляните на этот пример . Этот запрос должен работать:
var leftFinal = from left in lefts
join right in rights on left equals right.Left into leftRights
from leftRight in leftRights.DefaultIfEmpty()
select new { LeftId = left.Id, RightId = left.Key==leftRight.Key ? leftRight.Id : 0 };
const move = (from, to, ...a) =>from === to ? a : (a.splice(to, 0, ...a.splice(from, 1)), a);
const moved = move(0, 2, ...['a', 'b', 'c']);
console.log(moved)