What Is Prophecy in the New Covenant?

Have the gifts of prophecies stopped now that we have the whole Bible (Old and New Testament)?
John Piper explains these passages that are so controversial and relevant today:

"Do not despise prophecies, but test everything; hold fast what is good."
1 Thessalonians 1:20-21

"Follow the way of love and eagerly desire gifts of the Spirit, especially prophecy. For anyone who speaks in a tongue does not speak to people but to God. Indeed, no one understands them; they utter mysteries by the Spirit. But the one who prophesies speaks to people for their strengthening, encouraging and comfort."
1 Corinthians 14:1-3