What is the significance of Nora slamming the door at the end of Act 3?

Ibsen manipulates the audience with several intimations of a happy ending: when Krogstad and Mrs. Linde’s love is revealed, when Krogstad promises that he will take back his letter, when he returns Helmer’s IOU, and when Nora and Torvald discuss the possibility of a “miracle of miracles.” But the outside door slams as the curtain comes down. It is not a happy ending but a sad one, particularly for Torvald. There is only a remote possibility of the redemption of the Helmers’ marriage. As for Nora, it is an open ending. It is an opening out of possibilities for Nora, a new journey which, as much as possible, she will take alone.