Jamieson, Fausset and Brown on 1 John 5:15

15. hear--Greek, "that He heareth us."

we have the petitions that we desired of him--We have, as present possessions, everything whatsoever we desired (asked) from Him. Not one of our past prayers offered in faith, according to His will, is lost. Like Hannah, we can rejoice over them as granted even before the event; and can recognize the event when it comes to pass, as not from chance, but obtained by our past prayers. Compare also Jehoshaphat's believing confidence in the issue of his prayers, so much so that he appointed singers to praise the Lord beforehand.