The Maritime Self-Defense Force is upgrading its Izumo and Kaga destroyers into de facto aircraft carriers, equipping them to operate the F-35B stealth fighter, which features short takeoff and vertical landing capabilities.
Performance tests conducted in October and November last year, with cooperation from the U.S. Navy and Marine Corps, confirmed that the upgraded Kaga can accommodate the F-35B without issue.
You just said this, but reiterating from why I had heard. Japan designed these to be aircraft carriers and called them helicopter carriers with the intent of converting them when they were needed.