According to a series of tweets from Microsoft's Head of Xbox Games Marketing Aaron Greenberg, the Xbox outsold the PS4 in the month of December (via gamesindustry.biz.) The Nintendo Switch still outsold both Xbox and PS4, and Greenberg is citing unreleased NPD group marketing data in his tweets as a reason for his claims.
— Aaron Greenberg 🙅🏼♂️💚U (@aarongreenberg) January 11, 2018
This is still reasonably significant for Microsoft since December falls in the holiday season, the time when most people are shopping for new consoles as gifts. The Xbox One X also likely contributed to this success and probably resonated with consumers since Microsoft marketed it as the most powerful console ever.
Greenberg, though, mentioned "Xbox One" in his tweet, so the Xbox One S and other Xbox One offerings likely also are considered as part of this data. As you may recall, the Xbox One S sold at the cheapest price ever this year, and Microsoft pushed it as a low-cost holiday alternative.