a == b // false, because they are different array objects. In all other examples Javascript interpreter will do type-convert: !object -> !true -> false -> 0 object -> empty array -> 0 which will lead to comparison 0 == 0, which gives true. In case if one of the array was not empty, for example var a = [1], then second and third comparison would be converted to 1 == 0, and that will result false.